Evaluating the use of HILIC in large-scale, multi dimensional proteomics: Horses for courses?

Peptide retention time prediction for hydrophilic interaction liquid chromatography at acidic pH in formic-acid based eluents

Peptide separation selectivity was evaluated for hydrophilic interaction liquid chromatography (HILIC) ZIC-HILIC, ZIC-cHILIC, and XBridge Amide sorbents using formic acid as eluent additive (pH 2.7). Sequence-specific retention prediction algorithms were trained using retention datasets of ∼30,000 peptides for each column. Our retention models were able to attain ∼0.98 R2-value and yielded retention coefficients that can be probed to understand peptide-stationary phase interaction. Overall, the hydrophilicity for these columns decreased when the mobile phase changed pH from 4.5 to 2.7, when using 0.1 % formic acid in the mobile phase. The acidic residues became protonated, and the resultant hydrophilic interaction is dampened at the lower pH, leaving only the basic residues as the primary hydrophilic interactors. Hence, peptides of increasing charge have higher retention. In this comparison between the three columns, ZIC-HILIC has the highest chromatographic resolution between groups of peptides of different charge. From the position-dependent retention coefficients for ZIC-HILIC at pH 2.7, we found that the amino acids at the terminal positions of the peptide modulate the basicity of the N-terminal amino group or the C-terminal Arg/Lys for tryptic peptides. With respect to the separation orthogonality between HILIC and acidic pH RPLC for two dimensional separations, the orthogonality values were lower at pH 2.7 than operating HILIC at pH 4.5 for the first dimension. We also demonstrate that ZIC-HILIC was able to distinguish citrullinated and deamidated peptides based on predicted retention values.

An HPLC-ESI-QTOF method to analyze polar heteroatomic species in aviation turbine fuel via hydrophilic interaction chromatography

Aviation turbine fuel is a complex mixture of thousands of compounds. An analytical method using hydrophilic interaction liquid chromatography (HILIC) coupled with electrospray ionization and quadrupole time-of-flight mass spectrometry (ESI-QTOF) was developed for the identification of heteroatomic, polar compounds in aviation turbine fuel. Although compounds containing oxygen, nitrogen, and sulfur functional groups are each found at low levels (<0.1 % by mass) in fuels, their presence can generate significant effects on fuel properties. The HILIC-ESI-QTOF method is a combined separation and detection technique that possesses many advantages including a fast and simple sample preparation-requiring no extraction step therefore ensuring no loss of compounds of interest-and the ability to acquire high-fidelity compound data for chemometric analysis of heteroatomic species in aviation turbine fuel. In the development of the method, it was found that the chromatographic conditions and nature of the injection sample had a significant effect on separation efficiency and repeatability. For a sample dataset optimized using a singular aviation turbine fuel, retention time shift was able to be reduced from 0.4 min to 2.0 % relative standard deviation (RSD) to approximately 0.1 min with RSD of 0.4 % using the newly developed method. In addition, a high number of untargeted molecular features (944) and targeted amines (121) were able to be identified when utilizing optimal method conditions. The specific benefits and limitations of utilizing HILIC techniques with HPLC-ESI-QTOF are also discussed herein. This new method is currently being expanded to include analysis of all heteroatoms and is being applied to real fuel sets. The results of these studies are forthcoming.

Targeting the ERK mitogen-activated protein kinase cascade for the treatment of KRAS-mutant pancreatic cancer

Mutational activation of the KRAS oncogene is found in ~95% of pancreatic ductal adenocarcinoma (PDAC), the major form of pancreatic cancer. With substantial experimental evidence that continued aberrant KRAS function is essential for the maintenance of PDAC tumorigenic growth, the National Cancer Institute has identified the development of effective anti-KRAS therapies as one of four major initiatives for pancreatic cancer research. The recent clinical success in the development of an anti-KRAS therapy targeting one specific KRAS mutant (G12C) supports the significant potential impact of anti-KRAS therapies. However, KRAS^G12C mutations comprise only 2% of KRAS mutations in PDAC. Thus, there remains a dire need for additional therapeutic approaches for targeting the majority of KRAS-mutant PDAC. Among the different directions currently being pursued for anti-KRAS drug development, one of the most promising involves inhibitors of the key KRAS effector pathway, the three-tiered RAF-MEK-ERK mitogen-activated protein kinase (MAPK) cascade. We address the promises and challenges of targeting ERK MAPK signaling as an anti-KRAS therapy for PDAC. In particular, we also summarize the key role of the MYC transcription factor and oncoprotein in supporting ERK-dependent growth of KRAS-mutant PDAC.

Dual-Functional Titanium(IV) Immobilized Metal Affinity Chromatography Approach for Enabling Large-Scale Profiling of Protein Mannose-6-Phosphate Glycosylation and Revealing Its Predominant Substrates

Mannose-6-phosphate (M6P) glycosylation is an important post-translational modification (PTM) and plays a crucial role in transferring lysosomal hydrolases to lysosome, and is involved in several other biological processes. Aberrant M6P modifications have been implicated in lysosomal storage diseases and numerous other disorders including Alzheimer's disease and cancer. Research on profiling of intact M6P glycopeptides remains challenging due to its extremely low stoichiometry. Here we propose a dual-mode affinity approach to enrich M6P glycopeptides by dual-functional titanium(IV) immobilized metal affinity chromatography [Ti(IV)-IMAC] materials. In combination with state-of-the-art mass spectrometry and database search engine, we profiled 237 intact M6P glycopeptides corresponding to 81 M6P glycoproteins in five types of tissues in mouse, representing the first large-scale profiling of M6P glycosylation in mouse samples. The analysis of M6P glycoforms revealed the predominant glycan substrates of this PTM. Gene ontology analysis showed that overrepresented M6P glycoproteins were lysosomal-associated proteins. However, there were still substantial M6P glycoproteins that possessed different subcellular locations and molecular functions. Deep mining of their roles implicated in lysosomal and nonlysosomal function can provide new insights into functional roles of this important yet poorly studied modification.

Improving deep proteome and PTMome coverage using tandem HILIC-HPRP peptide fractionation strategy

Despite being orthogonal to reverse-phase separation and valuable for posttranslational modification (PTM) pre-enrichment, hydrophilic interaction liquid chromatography (HILIC) has not been widely adopted for large-scale proteomic applications. Here, we first evaluated the performance of HILIC in comparison with the popular high-pH reverse-phase (HPRP) separation, as the first dimension for tryptic peptide fractionation in a shotgun workflow to characterize the complex 293T cell proteome. The data indicated that the complementary nature of HILIC and HPRP for peptide separation was mainly due to different hydrophobicity preferences. Realizing that uncaptured components from one mode can be resolved in the other mode, we then designed and compared two multidimensional separation schemes using HILIC and HPRP in tandem for peptide prefractionation, in terms of identification efficiency and coverage at peptide, protein, and PTM levels. A total of 22,604 and 23,566 peptides corresponding to 4481 and 4436 proteins from 293T cell lysate were detected using HILIC-HPRP- and HPRP-HILIC-based shotgun proteomics workflow, respectively. In addition, without assistance of enrichment techniques, the tandem fractionation methods aided to identify 46 different PTMs from over 10,000 of spectra using blind modification search algorithm. We concluded that HILIC is a valuable alternative option for peptide prefractionation in a large-scale proteomic study, but can be further augmented with the use of a secondary HPRP separation.

A polyacrylamide-based silica stationary phase for the separation of carbohydrates using alcohols as the weak eluent in hydrophilic interaction liquid chromatography

A hydrophilic interaction liquid chromatography (HILIC) stationary phase was prepared by a two-step synthesis method, immobilizing polyacrylamide on silica sphere particles. The stationary phase (named PA, 5μm dia) was evaluated using a mixture of carbohydrates in HILIC mode and the column efficiency reached 121,000Nm^-1. The retention behavior of carbohydrates on PA stationary phase was investigated with three different organic solvents (acetonitrile, ethanol and methanol) employed as the weak eluent. The strongest hydrophilicity of PA stationary phase was observed in both acetonitrile and methanol as the weak eluent, when compared with another two amide stationary phases. Attributing to its high hydrophilicity, three oligosaccharides (xylooligosaccharide, fructooligosaccharide and chitooligosaccharides) presented good retention on PA stationary phase using alcohols/water as mobile phase. Finally, PA stationary phase was successfully applied for the purification of galactooligosaccharides and saponins of Paris polyphylla. It is feasible to use safer and cheaper alcohols to replace acetonitrile as the weak eluent for green analysis and purification of polar compounds on PA stationary phase.

Understanding cell signaling in cancer stem cells for targeted therapy - can phosphoproteomics help to reveal the secrets?

Background

Cancer represents heterogeneous and aberrantly proliferative manifestations composed of (epi)genetically and phenotypically distinct cells with a common clonal origin. Cancer stem cells (CSC) make up a rare subpopulation with the remarkable capacity to initiate, propagate and spread a malignant disease. Furthermore, CSC show increased therapy resistance, thereby contributing to disease relapse. Elimination of CSC, therefore, is a crucial aim to design efficacious treatments for long-term survival of cancer patients. In this article, we highlight the nature of CSC and propose that phosphoproteomics based on unbiased high-performance liquid chromatography-mass spectrometry provides a powerful tool to decipher the molecular CSC programs. Detailed knowledge about the regulation of signaling processes in CSC is a prerequisite for the development of patient-tailored multi-modal treatments including the elimination of rare CSC.

Main body

Phosphorylation is a crucial post-translational modification regulating a plethora of both intra- and intercellular communication processes in normal and malignant cells. Small-molecule targeting of kinases has proven successful in the therapy, but the high rates of relapse and failure to stem malignant spread suggest that these kinase inhibitors largely spare CSC. Studying the kinetics of global phosphorylation patterns in an unbiased manner is, therefore, required to improve strategies and successful treatments within multi-modal therapeutic regimens by targeting the malignant behavior of CSC. The phosphoproteome comprises all phosphoproteins within a cell population that can be analyzed by phosphoproteomics, allowing the investigation of thousands of phosphorylation events. One major aspect is the perception of events underlying the activation and deactivation of kinases and phosphatases in oncogenic signaling pathways. Thus, not only can this tool be harnessed to better understand cellular processes such as those controlling CSC, but also applied to identify novel drug targets for targeted anti-CSC therapy.

Conclusion

State-of-the-art phosphoproteomics approaches focusing on single cell analysis have the potential to better understand oncogenic signaling in heterogeneous cell populations including rare, yet highly malignant CSC. By eliminating the influence of heterogeneity of populations, single-cell studies will reveal novel insights also into the inter- and intratumoral communication processes controlling malignant CSC and disease progression, laying the basis for improved rational combination treatments.

Development of liquid chromatography-tandem mass spectrometry methods for the quantitation of Anisakis simplex proteins in fish

The parasite Anisakis simplex is present in many marine fish species that are directly used as food or in processed products. The anisakid larvae infect mostly the gut and inner organs of fish but have also been shown to penetrate into the fillet. Thus, human health can be at risk, either by contracting anisakiasis through the consumption of raw or under-cooked fish, or by sensitisation to anisakid proteins in processed food. A number of different methods for the detection of A. simplex in fish and products thereof have been developed, including visual techniques and PCR for larvae tracing, and immunological assays for the determination of proteins. The recent identification of a number of anisakid proteins by mass spectrometry-based proteomics has laid the groundwork for the development of two quantitative liquid chromatography-tandem mass spectrometry methods for the detection of A. simplex in fish that are described in the present study. Both, the label-free semi-quantitative nLC-nESI-Orbitrap-MS/MS (MS1) and the heavy peptide-applying absolute-quantitative (AQUA) LC-TripleQ-MS/MS (MS2) use unique reporter peptides derived from anisakid hemoglobin and SXP/RAL-2 protein as analytes. Standard curves in buffer and in salmon matrix showed limits of detection at 1μg/mL and 10μg/mL for MS1 and 0.1μg/mL and 2μg/mL for MS2. Preliminary method validation included the assessment of sensitivity, repeatability, reproducibility, and applicability to incurred and naturally-contaminated samples for both assays. By further optimization and full validation in accordance with current recommendations the LC-MS/MS methods could be standardized and used generally as confirmative techniques for the detection of A. simplex protein in fish.