Reversed-Phase High-Performance Liquid Chromatographic Prefractionation of Immunodepleted Human Serum Proteins to Enhance Mass Spectrometry Identification of Lower-Abundant Proteins

Revisiting Protein Reversed-Phase Chromatography for Bottom-Up Proteomics

We revisited protein reversed-phase chromatography (RP), using state-of-the-art RP columns developed for biopharmaceuticals, such as monoclonal antibodies, in order to evaluate the suitability of this methodology as a prefractionation step for bottom-up proteomics. The protein RP prefractionation (Prot-RP) method was compared with two other widely used prefractionation methods, SDS-PAGE and high-pH peptide RP (Pept-RP) by using cell lysates as samples. The overlap between fractions of Prot-RP was comparable to that of SDS-PAGE, and the protein recovery was approximately 2-fold higher. On the other hand, the overlap between fractions of Prot-RP was slightly larger than that of Pept-RP, but Prot-RP was able to identify more protein termini and more isoform-specific peptides than Pept-RP. Our results indicate that the combination of highly efficient protein prefractionation with modern mass spectrometers is particularly effective for proteoform profiling from cellular samples.

Isolation of extracellular fluids reveals novel secreted bioactive proteins from muscle and fat tissues

Proteins are secreted from cells to send information to neighboring cells or distant tissues. Because of the highly integrated nature of energy balance systems, there has been particular interest in myokines and adipokines. These are challenging to study through proteomics because serum or plasma contains highly abundant proteins that limit the detection of proteins with lower abundance. We show here that extracellular fluid (EF) from muscle and fat tissues of mice shows a different protein composition than either serum or tissues. Mass spectrometry analyses of EFs from mice with physiological perturbations, like exercise or cold exposure, allowed the quantification of many potentially novel myokines and adipokines. Using this approach, we identify prosaposin as a secreted product of muscle and fat. Prosaposin expression stimulates thermogenic gene expression and induces mitochondrial respiration in primary fat cells. These studies together illustrate the utility of EF isolation as a discovery tool for adipokines and myokines.

Plasma Proteomics for Epidemiology: Increasing Throughput With Standard-Flow Rates

BACKGROUND

Mass spectrometry is selective and sensitive, permitting routine quantification of multiple plasma proteins. However, commonly used nanoflow liquid chromatography (LC) approaches hamper sample throughput, reproducibility, and robustness. For this reason, most publications using plasma proteomics to date are small in study size.

METHODS AND RESULTS

Here, we tested a standard-flow LC mass spectrometry (MS) method using multiple reaction monitoring for the application to large epidemiological cohorts. We have reduced the LC-MS run time to almost a third of the nanoflow LC-MS approach. On the basis of a comparison of the quantification of 100 plasma proteins in >1500 LC-MS runs, the SD range of the retention time during continuous operation was substantially lower with the standard-flow LC-MS (<0.05 minutes) compared with the nanoflow LC-MS method (0.26-0.44 minutes). In addition, the standard-flow LC method also offered less variation in protein measurements. However, 5× more sample volume was required to achieve similar sensitivity. Two different commercial multiple reaction monitoring kits and an antibody-based multiplexing kit were used to compare the apolipoprotein measurements in a subset of samples. In general, good agreement was observed between the 2 multiple reaction monitoring kits, but some of the multiple reaction monitoring-based measurements differed from antibody-based assays.

CONCLUSIONS

The multiplexing capability of LC-MS combined with a standard-flow method increases throughput and reduces the costs of large-scale protein measurements in epidemiological cohorts, but protein rather than peptide standards will be required for defined absolute proteoform quantification.

Performance of Optimized Wide Pore Superficially Porous Particles for Separation of Proteins and Immunoglobulin G Antibodies

In this study, we studied the chromatographic performance of this newly developed wide pore superficially porous particles (SPPs) with 3.5 μm particle size and 450 Å pore size, for the separation of proteins and Immunoglobulin G antibodies. We studied the selectivity of different phases (C4, SB-C18 and Diphenyl), the effect of temperature, column carryover and column chemical lifetime. We also compared our SPPs with other wide pore SPPs in similar particle sizes and sub 2 µ wide pore totally porous particles by van Deemter studies and gradient separations of proteins and immunoglobulin G antibodies. The results showed that the SPPs containing larger pore size gave better chromatographic performance.

Preparation and Immunoaffinity Depletion of Fresh Frozen Tissue Homogenates for Mass Spectrometry-Based Proteomics in the Context of Drug Target/Biomarker Discovery

The discovery of novel drug targets and biomarkers via mass spectrometry (MS)-based proteomic analysis of clinical specimens has proven to be challenging. The wide dynamic range of protein concentration in clinical specimens and the high background/noise originating from highly abundant proteins in tissue homogenates and serum/plasma encompass two major analytical obstacles. Immunoaffinity depletion of highly abundant blood-derived proteins from serum/plasma is a well-established approach adopted by numerous researchers; however, the utilization of this technique for immunodepletion of tissue homogenates obtained from fresh frozen clinical specimens is lacking. We first developed immunoaffinity depletion of highly abundant blood-derived proteins from tissue homogenates, using renal cell carcinoma as a model disease, and followed this study by applying it to different tissue types. Tissue homogenate immunoaffinity depletion of highly abundant proteins may be equally important as is the recognized need for depletion of serum/plasma, enabling more sensitive MS-based discovery of novel drug targets, and/or clinical biomarkers from complex clinical samples. Provided is a detailed protocol designed to guide the researcher through the preparation and immunoaffinity depletion of fresh frozen tissue homogenates for two-dimensional liquid chromatography, tandem mass spectrometry (2D-LC-MS/MS)-based molecular profiling of tissue specimens in the context of drug target and/or biomarker discovery.

Methodology and Applications of Disease Biomarker Identification in Human Serum

Biomarkers are biomolecules that serve as indicators of biological and pathological processes, or physiological and pharmacological responses to a drug treatment. Because of the high abundance of albumin and heterogeneity of plasma lipoproteins and glycoproteins, biomarkers are difficult to identify in human serum. Due to the clinical significance the identification of disease biomarkers in serum holds great promise for personalized medicine, especially for disease diagnosis and prognosis. This review summarizes some common and emerging proteomics techniques utilized in the separation of serum samples and identification of disease signatures. The practical application of each protein separation or identification technique is analyzed using specific examples. Biomarkers of cancers of prostate, breast, ovary, and lung in human serum have been reviewed, as well as those of heart disease, arthritis, asthma, and cystic fibrosis. Despite the advancement of technology few biomarkers have been approved by the Food and Drug Administration for disease diagnosis and prognosis due to the complexity of structure and function of protein biomarkers and lack of high sensitivity, specificity, and reproducibility for those putative biomarkers. The combination of different types of technologies and statistical analysis may provide more effective methods to identify and validate new disease biomarkers in blood.

A Routine 'Top-Down' Approach to Analysis of the Human Serum Proteome

Serum provides a rich source of potential biomarker proteoforms. One of the major obstacles in analysing serum proteomes is detecting lower abundance proteins owing to the presence of hyper-abundant species (e.g., serum albumin and immunoglobulins). Although depletion methods have been used to address this, these can lead to the concomitant removal of non-targeted protein species, and thus raise issues of specificity, reproducibility, and the capacity for meaningful quantitative analyses. Altering the native stoichiometry of the proteome components may thus yield a more complex series of issues than dealing directly with the inherent complexity of the sample. Hence, here we targeted method refinements so as to ensure optimum resolution of serum proteomes via a top down two-dimensional gel electrophoresis (2DE) approach that enables the routine assessment of proteoforms and is fully compatible with subsequent mass spectrometric analyses. Testing included various fractionation and non-fractionation approaches. The data show that resolving 500 µg protein on 17 cm 3–10 non-linear immobilised pH gradient strips in the first dimension followed by second dimension resolution on 7–20% gradient gels with a combination of lithium dodecyl sulfate (LDS) and sodium dodecyl sulfate (SDS) detergents markedly improves the resolution and detection of proteoforms in serum. In addition, well established third dimension electrophoretic separations in combination with deep imaging further contributed to the best available resolution, detection, and thus quantitative top-down analysis of serum proteomes.

Mass Spectrometry-Based Analysis for the Discovery and Validation of Potential Colorectal Cancer Stool Biomarkers

Colorectal cancer (CRC) is the third leading cause of cancer mortality for both men and women, and the second leading cause of cancer death for men and women combined. If detected early, before metastasis has occurred, survival following surgical resection of the tumor is >90%. Early detection is therefore critical for effective disease surveillance. Unfortunately, current biomarker assays lack the necessary sensitivity and specificity for reliable early disease detection. Development of new robust, non- or minimally invasive specific and sensitive biomarkers or panels with improved compliance and performance is therefore urgently required. The use of fecal samples offers several advantages over other clinical biospecimens (e.g., plasma or serum) as a source of CRC biomarkers, including: collection is noninvasive, the test can be performed at home, one is not sample limited, and the stool effectively samples the entire length of the inner bowel wall contents (including tumor) as it passes down the gastrointestinal tract. Recent advances in mass spectrometry now facilitate both the targeted discovery and validation of potential CRC biomarkers. We describe, herein, detailed protocols that can be used to mine deeply into the fecal proteome to reveal candidate proteins, identify proteotypic/unitypic peptides (i.e., peptides found in only a single known human protein that serve to identify that protein) suitable for sensitive and specific quantitative multiplexed analysis, and undertake high-throughput analysis of clinical samples. Finally, we discuss future directions that may further position this technology to support the current switch in translation research toward personalized medicine.

Macroporous reversed-phase separation of proteins combined with reversed-phase separation of phosphopeptides and tandem mass spectrometry for profiling the phosphoproteome of MDA-MB-231 cells

A new method of combining macroporous RP (mRP) protein fractionation with RPLC peptide separation MS/MS is reported for profiling the phosphoproteome of a complex sample. In this method, an mRP-C18 column was used to fractionate the proteins from a whole cell lysate of a breast cancer cell line, MDA-MB-231, into 38 fractions. Each fraction was subjected to tryptic digestion, sequential phosphopeptide enrichment by immobilized metal ion affinity chromatography and titanium dioxide (TiO2 ), followed by capillary RPLC-MS/MS analysis. For comparison, the conventional method of using strong cation exchange RPLC separation of peptides combined with MS/MS was also used for analyzing the phosphoproteome. Replicate experiments by the mRP-RPLC method identified 1585 distinct phosphoproteins with 4519 phosphopeptides, compared to 1585 phosphoproteins with 4297 phosphopeptides by strong cation exchange RPLC, with a total of 1947 phosphoproteins and 6278 phosphopeptides identified from the combined results. While the two methods have similar ability in the identification of the phosphoproteome, they produce complementary information. The phosphoproteins identified in this study, including 67 novel phosphorylation sites from 56 breast cancer related proteins, can serve as the entry point for future validation with biological implications in breast cancer. The MS proteomics data have been deposited to the ProteomeXchange Consortium via the PRIDE partner repository with the dataset identifier PXD000948 and DOI 10.6019/PXD000948.

Strategies to reduce aspecific adsorption of peptides and proteins in liquid chromatography-mass spectrometry based bioanalyses: an overview

In the drug-discovery setting, the development of new peptide and protein-based biopharmaceuticals attracts increased attention from the pharmaceutical industry and consequently demands the development of high-throughput LC-MS methods. Regulatory guidelines require bioanalytical methods to be validated not only in terms of linearity, sensitivity, accuracy, precision, selectivity and stability, but also in terms of carryover. Carryover results from the aspecific adsorption of analyte(s) to parts of the analytical system and thus introduces bias in both identification and quantification assays. Moreover, nonspecific binding occurs at the surface of materials used during sample preparation, such as pipette tips, sample tubes and LC-vials. Hence, linearity, sensitivity and repeatability of the analyses are negatively affected. Due to the great diversity in physicochemical properties of biomolecules, there is no general approach available to minimize adsorption phenomena. Therefore, we aim to present different strategies which can be generically applied to reduce nonspecific binding of peptides and proteins. In the first part of this review, a systematic approach is proposed to guide the reader through the different solvents which can be used to dissolve the analyte of interest. Indeed, proper solubilization is one of the most important factors for a successful analysis. In addition, alternative approaches are described to improve analyte recovery from the sample vial. The second part focuses on strategies to efficiently reduce adsorption at components of the autosampler, column and mass spectrometer. Thereby carryover is reduced while maintaining a sufficiently wide dynamic range of the assay.

HPLC enrichment/isolation of proteins for post-translational modification studies from complex mixtures

The paper describes a macroporous RP-HPLC method for separation and isolation/enrichment of proteins from complex mixtures. The method is robust and efficient; using 2.1 or 4.6mm diameter columns provides sufficient material for subsequent proteomic analysis. The main advantage of the method is that most protein variants are isolated in the same fraction, as separation is not based on differences in isoelectric point. This is highly advantageous for studying complex mixtures and post-translational modifications. Examples related to glycosylation analysis are discussed in detail.

Automated sample preparation platform for mass spectrometry-based plasma proteomics and biomarker discovery

The identification of novel biomarkers from human plasma remains a critical need in order to develop and monitor drug therapies for nearly all disease areas. The discovery of novel plasma biomarkers is, however, significantly hampered by the complexity and dynamic range of proteins within plasma, as well as the inherent variability in composition from patient to patient. In addition, it is widely accepted that most soluble plasma biomarkers for diseases such as cancer will be represented by tissue leakage products, circulating in plasma at low levels. It is therefore necessary to find approaches with the prerequisite level of sensitivity in such a complex biological matrix. Strategies for fractionating the plasma proteome have been suggested, but improvements in sensitivity are often negated by the resultant process variability. Here we describe an approach using multidimensional chromatography and on-line protein derivatization, which allows for higher sensitivity, whilst minimizing the process variability. In order to evaluate this automated process fully, we demonstrate three levels of processing and compare sensitivity, throughput and reproducibility. We demonstrate that high sensitivity analysis of the human plasma proteome is possible down to the low ng/mL or even high pg/mL level with a high degree of technical reproducibility.

Coupling immunoaffinity techniques with MS for quantitative analysis of low-abundance protein biomarkers

The field of proteomics is rapidly turning towards targeted mass spectrometry (MS) methods to quantify putative markers or known proteins of biological interest. Historically, the enzyme-linked immunosorbent assay (ELISA) has been used for targeted protein analysis, but, unfortunately, it is limited by the excessive time required for antibody preparation, as well as concerns over selectivity. Despite the ability of proteomics to deliver increasingly quantitative measurements, owing to limited sensitivity, the leads generated are in the microgram per milliliter range. This stands in stark contrast to ELISA, which is capable of quantifying proteins at low picogram per milliliter levels. To bridge this gap, targeted liquid chromatography (LC) tandem MS (MS/MS) analysis of tryptic peptide surrogates using selected reaction monitoring detection has emerged as a viable option for rapid quantification of target proteins. The precision of this approach has been enhanced by the use of stable isotope-labeled peptide internal standards to compensate for variation in recovery and the influence of differential matrix effects. Unfortunately, the complexity of proteinaceous matrices, such as plasma, limits the usefulness of this approach to quantification in the mid-nanogram per milliliter range (medium-abundance proteins). This article reviews the current status of LC/MS/MS using selected reaction monitoring for protein quantification, and specifically considers the use of a single antibody to achieve superior enrichment of either the protein target or the released tryptic peptide. Examples of immunoaffinity-assisted LC/MS/MS are reviewed that demonstrate quantitative analysis of low-abundance proteins (subnanogram per milliliter range). A strategy based on this technology is proposed for the expedited evaluation of novel protein biomarkers, which relies on the synergy created from the complementary nature of MS and ELISA.

Proteomic Workflows for Biomarker Identification Using Mass Spectrometry - Technical and Statistical Considerations during Initial Discovery

Identification of biomarkers capable of differentiating between pathophysiological states of an individual is a laudable goal in the field of proteomics. Protein biomarker discovery generally employs high throughput sample characterization by mass spectrometry (MS), being capable of identifying and quantifying thousands of proteins per sample. While MS-based technologies have rapidly matured, the identification of truly informative biomarkers remains elusive, with only a handful of clinically applicable tests stemming from proteomic workflows. This underlying lack of progress is attributed in large part to erroneous experimental design, biased sample handling, as well as improper statistical analysis of the resulting data. This review will discuss in detail the importance of experimental design and provide some insight into the overall workflow required for biomarker identification experiments. Proper balance between the degree of biological vs. technical replication is required for confident biomarker identification.

A universal, high recovery assay for protein quantitation through temperature programmed liquid chromatography (TPLC)

As an alternative to direct UV absorbance measurements, estimation of total protein concentration is typically conducted through colorimetric reagent assays. However, for protein-limited applications, the proportion of the sample sacrificed to the assay becomes increasingly significant. This work demonstrates a method for quantitation of protein samples with high recovery. Temperature programmed liquid chromatography (TPLC) with absorbance detection at 214nm permits accurate estimation of total protein concentration from samples containing as little as 0.75μg. The method incorporates a temperature gradient from 25 to 80°C to facilitate elution of total protein into a single fraction. Analyte recovery, as measured from 1 and 10μg protein extracts of Escherichia coli, is shown to exceed 93%. Extinction coefficients at 214nm were calculated across the human proteome, providing a relative standard deviation of 21% (versus 42% at 280nm), suggesting absorbance values at 214nm provide a more consistent measure of protein concentration. These results translate to a universal protein detection strategy exhibiting a coefficient of variation below 10%. Together with the sensitivity and tolerance to contaminants, TPLC with UV detection is a favorable alternative to colorimetric assay for total protein quantitation, particularly in sample-limited applications.

Human blood plasma proteome mapping for search of potential markers of the lung squamous cell carcinoma

Blood plasma proteomes obtained from 77 lung squamous cell carcinoma (LSCC) patients (Stages I-III) and 67 healthy controls (all males) were analyzed by using the label-free liquid chromatography tandem mass spectrometry (LC-MS/MS) method for the search of potential cancer biomarkers. All plasma samples were depleted of 14 highly-abundant plasma proteins by immune-affinity column chromatography before LC-MS/MS. We identified and quantified 809 differential proteins with molecular weights from 6.4 kDa to 3900 kDa using a label-free method. Three hundred and sixty four proteins were identified in all three groups. Changes in levels of an expression of blood plasma proteins associated with LSCC were discovered. Among them, 43 proteins were overexpressed and 39 proteins were down-regulated by more than two-fold between the plasmas of lung cancer patients and healthy men. We focused our attention on proteins whose expression levels increased from control to early stage and then to advanced stage tumor. Each of the 43 unique overexpressed proteins was classified according to its cellular localization, biological processes, molecular function and classes. Many of these proteins are involved in biological pathways pertinent to tumor progression and metastasis and some of these deregulated proteins may be useful clinical markers.

Affinity depletion of plasma and serum for mass spectrometry-based proteome analysis

Protein biomarker discovery in blood plasma and serum is severely hampered by the vast dynamic range of the proteome. With protein concentrations spanning 12 orders of magnitude, conventional mass spectrometric analysis allows for detection of only a few low-abundance proteins. Prior depletion of high-abundant proteins from the sample can increase analytical depth considerably and has become a widely used practice. We describe in detail an affinity depletion method that selectively removes 14 of the most abundant proteins in plasma and serum.

Identificationof potential lung cancer biomarkers by liquid chromatography tandem mass spectrometry-based proteomics analysis of secretomes of two lung cancer cell lines

A label-free nano-liquid chromatography tandem mass spectrometry proteomics analysis on the conditioned media (CM) of two lung cancer cell lines of different histological backgrounds to identify secreted or membrane-bound proteins as novel lung cancer biomarkers was performed. Five hundred and seventy seven proteins were identified and 38% of them were classified as extracellular or membrane-bound. For the search of potential biomarkers of lung cancer a series of selection criteria were proposed. We detected known or putative lung cancer markers. In addition, 40 novel proteins were identified, whose role as biomarkers of lung cancer should be explored further.

Serum and plasma proteomics and its possible use as detector and predictor of radiation diseases

All tissues can be damaged by ionizing radiation. Early biomarkers of radiation injury are critical for triage, treatment and follow-up of large numbers of people exposed to ionizing radiation after terrorist attacks or radiological accident, and for prediction of normal tissue toxicity before, during and after a treatment by radiotherapy. The comparative proteomic approach is a promising and powerful tool for the discovery of new radiation biomarkers. In association with multivariate statistics, proteomics enables measurement of the level of hundreds or thousands of proteins at the same time and identifies set of proteins that can discriminate between different groups of individuals. Human serum and plasma are the preferred samples for the study of normal and disease-associated proteins. Extreme complexity, extensive dynamic range, genetic and physiological variations, protein modifications and incompleteness of sampling by two-dimensional electrophoresis and mass spectrometry represent key challenges to reproducible, high-resolution, and high-throughput analyses of serum and plasma proteomes. The future of radiation research will possibly lie in molecular networks that link genome, transcriptome, proteome and metabolome variations to radiation pathophysiology and serve as sensors of radiation disease. This chapter reviews recent advances in proteome analysis of serum and plasma as well as its applications to radiation biology and radiation biomarker discovery for both radiation exposure and radiation tissue toxicity.

MRM-based multiplexed quantitation of 67 putative cardiovascular disease biomarkers in human plasma

A highly-multiplexed MRM-based assay for determination of cardiovascular disease (CVD) status and disease classification has been developed for clinical research. A high-flow system using ultra-high performance LC and an Agilent 6490 triple quadrupole mass spectrometer, equipped with an ion funnel, provided ease of use and increased the robustness of the assay. The assay uses 135 stable isotope-labeled peptide standards for the quantitation of 67 putative biomarkers of CVD in tryptic digests of whole plasma in a 30-min assay. Eighty-five analyses of the same sample showed no loss of sensitivity (<20% CV for 134/135 peptides) and no loss of retention time accuracy (<0.5% CV for all peptides). The maximum linear dynamic range of the MRM assays ranged from 10(3) -10(5) for 106 of the assays. Excellent linear responses (r >0.98) were obtained for 117 of the 135 peptide targets with attomole level limits of quantitation (<20% CV and accuracy 80-120%) for 81 of the 135 peptides. The assay presented in this study is easy to use, robust, sensitive, and has high-throughput capabilities through short analysis time and complete automated sample preparation. It is therefore well suited for CVD biomarker validation and discovery in plasma.

A method for isolation and identification of urinary biomarkers in patients with diabetic nephropathy

PURPOSE

The poor performance of current tests for predicting the onset, progression and treatment response of diabetic nephropathy has engendered a search for more sensitive and specific urinary biomarkers. Our goal was to develop a new method for protein biomarker discovery in urine from these patients.

EXPERIMENTAL DESIGN

We analyzed urine from normal subjects and patients with early and advanced nephropathy. Proteins were separated using a novel analysis process including immunodepletion of high-abundance proteins followed by two-stage LC fractionation of low-abundance proteins. The proteins in the fractions were sequenced using MS/MS.

RESULTS

Immunodepletion of selected high-abundance proteins followed by two-stage LC produced approximately 700 fractions, each less complex and more amenable to analysis than the mixture and requiring minimal processing for MS identification. Comparison of fractions between normal and diabetic nephropathy subjects revealed several low-abundance proteins that reproducibly distinguished low glomerular filtration rate (GFR) from both high GFR diabetic and normal subjects, including uteroglobin, a protein previously associated with renal scarring.

CONCLUSIONS AND CLINICAL RELEVANCE

We developed a novel method to identify low-abundance urinary proteins that enables the discovery of potential biomarkers to improve the diagnosis and management of patients with diabetic nephropathy.

Comparative (meta)genomic analysis and ecological profiling of human gut-specific bacteriophage φB124-14

Bacteriophage associated with the human gut microbiome are likely to have an important impact on community structure and function, and provide a wealth of biotechnological opportunities. Despite this, knowledge of the ecology and composition of bacteriophage in the gut bacterial community remains poor, with few well characterized gut-associated phage genomes currently available. Here we describe the identification and in-depth (meta)genomic, proteomic, and ecological analysis of a human gut-specific bacteriophage (designated φB124-14). In doing so we illuminate a fraction of the biological dark matter extant in this ecosystem and its surrounding eco-genomic landscape, identifying a novel and uncharted bacteriophage gene-space in this community. φB124-14 infects only a subset of closely related gut-associated Bacteroides fragilis strains, and the circular genome encodes functions previously found to be rare in viral genomes and human gut viral metagenome sequences, including those which potentially confer advantages upon phage and/or host bacteria. Comparative genomic analyses revealed φB124-14 is most closely related to φB40-8, the only other publically available Bacteroides sp. phage genome, whilst comparative metagenomic analysis of both phage failed to identify any homologous sequences in 136 non-human gut metagenomic datasets searched, supporting the human gut-specific nature of this phage. Moreover, a potential geographic variation in the carriage of these and related phage was revealed by analysis of their distribution and prevalence within 151 human gut microbiomes and viromes from Europe, America and Japan. Finally, ecological profiling of φB124-14 and φB40-8, using both gene-centric alignment-driven phylogenetic analyses, as well as alignment-free gene-independent approaches was undertaken. This not only verified the human gut-specific nature of both phage, but also indicated that these phage populate a distinct and unexplored ecological landscape within the human gut microbiome.

Advancing the sensitivity of selected reaction monitoring-based targeted quantitative proteomics

Selected reaction monitoring (SRM) - also known as multiple reaction monitoring (MRM) - has emerged as a promising high-throughput targeted protein quantification technology for candidate biomarker verification and systems biology applications. A major bottleneck for current SRM technology, however, is insufficient sensitivity for, e.g. detecting low-abundance biomarkers likely present at the low ng/mL to pg/mL range in human blood plasma or serum, or extremely low-abundance signaling proteins in cells or tissues. Herein, we review recent advances in methods and technologies, including front-end immunoaffinity depletion, fractionation, selective enrichment of target proteins/peptides including posttranslational modifications, as well as advances in MS instrumentation which have significantly enhanced the overall sensitivity of SRM assays and enabled the detection of low-abundance proteins at low- to sub-ng/mL level in human blood plasma or serum. General perspectives on the potential of achieving sufficient sensitivity for detection of pg/mL level proteins in plasma are also discussed.

Hexapeptide library as a universal tool for sample preparation in protein glycosylation analysis

Recent analytical advancements allow for large-scale glycomics and glycan-biomarker research with N-glycans released from complex protein mixtures of e.g. plasma with a wide range of protein concentrations. Protein enrichment techniques to obtain samples with a better representation of low-abundance proteins are hardy applied. In this study, hexapeptide ligands previously described for enrichment of low-abundance proteins in proteomics are evaluated for glycan analysis. A repeatable on-bead glycan release strategy was developed, and glycans were analyzed using capillary sieving electrophoresis on a DNA analyzer. Binding of proteins to the hexapeptide library occurred via the protein backbone. At neutral pH no discrimination between protein glycoforms was observed. Interestingly, glycan profiles of plasma with and without hexapeptide library enrichment revealed very similar patterns, despite the vast changes in protein concentrations in the samples. The most significant differences in glycosylation profiles were ascribed to a reduction in immunoglobulin-derived glycans. These results suggest that specific and sensitive biomarkers will be hard to access on the full plasma level using protein enrichment in combination with glycan analysis. Instead, fractionation techniques or profiling strategies on the glycopeptide level after enrichment are proposed for in-depth glycoproteomics research.

Improvement of 2D-PAGE resolution of human, porcine and canine follicular fluid: comparison of two immunodepletion columns

Follicular fluid provides the microenvironment within which somatic cells proliferate and differentiate, and the oocyte matures. It contains a number of soluble factors implicated in various stages of follicular development, most of them being functionally unknown. The presence of several high-abundance proteins, mainly originating from the blood circulation, is a major challenge of follicular fluid proteomic analysis, as these proteins can mask or decrease the visualization of follicle-specific proteins. In this study, we evaluated the efficiency of two immunodepletion columns (ProteomeLab™ IgY-HSA and MARS-6) on follicular fluids of human, porcine and canine prior to 2D-PAGE. Our results showed that both columns were suitable to remove some of the high-abundance proteins present in human and canine follicular fluid. In conclusion, we demonstrated that the immunodepletion strategy enables the detection of new protein spots, increases resolution and highly improves the intensity of low-abundance proteins by 2D-PAGE.

Absolute quantitation of protein therapeutics in biological matrices by enzymatic digestion and LC–MS

The advancement of biotechnology has led to an increase in biotherapeutic drugs, especially recombinant proteins and monoclonal antibodies. Ligand-binding assays or immunoassays are the standard methods of choice in pharmacokinetic studies in support of drug discovery and development for protein therapeutics. LC–MS-based methodologies are increasingly used as alternatives to immunoassays for absolute protein quantitation in biological samples. We review recent advancements in absolute quantitation of protein therapeutics in biological matrices by enzymatic digestion and LC–MS.

Proteomic analysis of mare follicular fluid during late follicle development

Background

Follicular fluid accumulates into the antrum of follicle from the early stage of follicle development. Studies on its components may contribute to a better understanding of the mechanisms underlying follicular development and oocyte quality. With this objective, we performed a proteomic analysis of mare follicular fluid. First, we hypothesized that proteins in follicular fluid may differ from those in the serum, and also may change during follicle development. Second, we used four different approaches of Immunodepletion and one enrichment method, in order to overcome the masking effect of high-abundance proteins present in the follicular fluid, and to identify those present in lower abundance. Finally, we compared our results with previous studies performed in mono-ovulant (human) and poly-ovulant (porcine and canine) species in an attempt to identify common and/or species-specific proteins.

Methods

Follicular fluid samples were collected from ovaries at three different stages of follicle development (early dominant, late dominant and preovulatory). Blood samples were also collected at each time. The proteomic analysis was carried out on crude, depleted and enriched follicular fluid by 2D-PAGE, 1D-PAGE and mass spectrometry.

Results

Total of 459 protein spots were visualized by 2D-PAGE of crude mare follicular fluid, with no difference among the three physiological stages. Thirty proteins were observed as differentially expressed between serum and follicular fluid. Enrichment method was found to be the most powerful method for detection and identification of low-abundance proteins from follicular fluid. Actually, we were able to identify 18 proteins in the crude follicular fluid, and as many as 113 in the enriched follicular fluid. Inhibins and a few other proteins involved in reproduction could only be identified after enrichment of follicular fluid, demonstrating the power of the method used. The comparison of proteins found in mare follicular fluid with proteins previously identified in human, porcine and canine follicular fluids, led to the identification of 12 common proteins and of several species-specific proteins.

Conclusions

This study provides the first description of mare follicular fluid proteome during the late follicle development stages. We identified several proteins from crude, depleted and enriched follicular fluid. Our results demonstrate that the enrichment method, combined with 2D-PAGE and mass spectrometry, can be successfully used to visualize and further identify the low-abundance proteins in the follicular fluid.

Identification of vitronectin as a novel serum marker for early breast cancer detection using a new proteomic approach

PURPOSE

Breast cancer is the most frequent malignancy in women. However, no useful serum markers with high sensitivity and specificity for the detection of early breast cancer have been identified. The search for biological markers of early breast cancer is of continual interest in experimental and clinical breast cancer research. We recently described a simple and highly reproducible three-step proteome analysis for identifying potential disease-marker candidates among the low-abundance serum proteins.

METHODS

Serum samples from breast ductal carcinoma in situ (DCIS) patients and normal controls were subjected to a three-step serum proteome analysis. The steps were the following: first, immunodepletion of most abundant proteins; second, fractionation using reverse-phase high-performance liquid chromatography; and third, separation using two-dimensional electrophoresis (2-DE). Differences revealed by protein staining were further confirmed by Western blotting, immunohistochemical staining, and enzyme-linked immunosorbent assays (ELISA).

RESULTS

Twenty-two upregulated and 26 downregulated spots were detected on the 2-DE gels, and a total of 33 proteins were identified by liquid chromatography and tandem mass spectrometry. Western blotting confirmed that the level of vitronectin was significantly increased in DCIS patients compared with that of normal controls. Immunohistochemical staining of vitronectin in breast cancer tissue revealed high expression in small vessel walls surrounding cancer cells and the extracellular matrix of stroma. Moreover, vitronectin serum concentrations, as measured by ELISA, were significantly increased in patients with DCIS or more advanced breast cancer compared with those of normal controls.

CONCLUSIONS

Vitronectin could serve as a promising serum marker for the detection of primary breast cancer.

Protein identification using nano-HPLC-MS: ESI-MS and MALDI-MS interfaces

Body fluids and body tissues have a myriad of peptides and proteins that, very often, the traditional methodologies of proteomics, such as conventional gel electrophoresis or mass spectrometry, are unable to characterize. We describe two protocols to characterize high molecular weight peptides (>3 kDa) and intact proteins involving on-line trypsin digestion, separation of the digests by nano-HPLC, and analysis by mass spectrometry using two different ionization sources (matrix-assisted laser desorption and electrospray ionization). These protocols have the advantage of promoting protein denaturation in an aqueous-organic solvent, which reduces the derivatization of the sample and facilitates an in-depth analysis for detection and identification of proteins. Additional advantages include the following: (1) integration of these protocols into standard proteomic workflows after the preprocessing of samples and separation; (2) use of high-resolution monolithic columns; and (3) the ability to acquire information from minimal amounts of sample.

Quantitative liquid chromatography tandem mass spectrometry analysis of macromolecules using signature peptides in biological fluids

Targeted protein quantification using peptide surrogates has increasingly become important to the validation of biomarker candidates and development of protein therapeutics. These approaches have been proposed and employed as alternatives to immunoassays in biological fluids. Technological advances over the last 20 years in biochemistry and mass spectrometry have prompted the use of peptides as surrogates to quantify enzyme digested proteins using triple quadrupole mass spectrometers. Multiple sample preparation processes are often incorporated to achieve quantification of target proteins using these signature peptides. This review article focuses on these processes or hyphenated techniques for quantification of proteins with peptide surrogates. The most recent advances and strategies involved with hyphenated techniques are discussed.

Evaluation of different two-dimensional chromatographic techniques for proteomic analysis of mouse cardiac tissue

In proteomics experiments the first critical step after sampling is certainly sample preparation. Multidimensional chromatography techniques have emerged as a powerful tool for the large-scale analysis of such complex samples as biological samples. In order to evaluate these separation techniques, microgram quantities of protein extracted from mouse heart tissue were fractionated by four different chromatographic methods. Regarding peptide-level fractionation, the first dimension of separation was performed with high-pH reversed-phase chromatography (pH-RP) and strong cation exchange chromatography (SCX). Regarding protein-level fractionation, C(8) protein reversed-phase (C(8) -RP Prot) and high-recovery protein reversed-phase (hr-RP Prot) were used instead. The second dimension consisted of a reversed-phase nano-HPLC on-Chip coupled to an electrospray ionization quadrupole time-of-flight mass spectrometer for tandem mass spectrometric analysis. The performance and relative fractionation efficiencies of each technique were assessed by comparing the total number of proteins identified by each method. The peptide-level pH-RP and the hr-RP Prot protein-level separations were the best methods, identifying 1338 and 1303 proteins, respectively. The peptide-level SCX, with 509 proteins identified, was the worst method.

High-abundance proteins depletion for serum proteomic analysis: concomitant removal of non-targeted proteins

In clinical and pharmaceutical proteomics, serum and plasma are frequently used for detection of early diagnostic biomarkers for therapeutic targets. Although obtaining these body fluid samples is non-invasive and easy, they contain some abundant proteins that mask other protein components present at low concentrations. The challenge in identifying serum biomarkers is to remove the abundant proteins, uncovering and enriching at the same time the low-abundance ones. The depletion strategies, however, could lead to the concomitant removal of some non-targeted proteins that may be of potential interest. In this study, we compared three different methods aimed to deplete high-abundance proteins from human serum, focusing on the identification of non-specifically bound proteins which might be eventually removed. A Cibacron blue-dye-based method for albumin removal, an albumin and IgG immunodepletion method and an immunoaffinity column (Multiple Affinity Removal System) that simultaneously removes a total of six high-abundance proteins, were investigated. The bound proteins were eluted, separated by two-dimensional gel electrophoresis and identified by Nano LC-CHIP-MS system. Flow-through fractions and bound fractions were also analysed with the ProteinChip technology SELDI-TOF-MS. Our results showed that the methods tested removed not only the targeted proteins with high efficiency, but also some non-targeted proteins. We found that the Multiple Affinity Removal Column improved the intensity of low-abundance proteins, displayed new protein spots and increased resolution. Notably, the column showed the lowest removal of untargeted proteins, proved to be the most promising depletion approach and a reliable method for serum preparation prior to proteomic studies.

Multicolumn separation platform for simultaneous depletion and prefractionation prior to 2-DE for facilitating in-depth serum proteomics profiling

In this report, we describe an integrated fluidic platform composed of tandem affinity columns for the depletion of high-abundance proteins from human serum and on-line fractionation/concentration of medium- and low-abundance proteins by tandem immobilized metal-ion affinity chromatography (IMAC) columns and reversed phase (RP) column for in-depth proteomics analysis. The depletion columns were based on monolithic polymethacrylate with surface immobilized protein A, protein G', and antibodies for depleting the top 8 high-abundance proteins. The IMAC fractionation/concentration columns consisted of monolithic stationary phases with surface bound iminodiacetic acid (IDA) chelated with Zn2+, Ni2+ and Cu2+, while the RP column was packed with nonpolar polymer beads. The integrated multicolumn fluidic platform was very effective in reducing simultaneously both the dynamic range differences among the protein constituents of serum and the complexity of the proteomics samples, thus, facilitating the in-depth proteomics analysis by 2-DE followed by MALDI-TOF and LC-MS/MS. In fact, the number of detected spots was approximately 1450 using SYPRO fluorescent stain from which 384 spots were subsequently detected by Coomassie Blue. Since the investigation was simply a proof of concept, 295 proteins were readily identified in some selected spots by MALDI-TOF and LC-MS/MS.

Integrated platform for proteome analysis with combination of protein and peptide separation via online digestion

An integrated platform with the combination of protein and peptide separation was established via online protein digestion, by which proteins were first separated by a microcolumn packed with mixed weak anion and weak cation exchange (WAX/WCX) particles under a series of salt steps, online digested by a trypsin immobilized microenzymatic reactor (IMER), trapped and desalted by two parallel C8 precolumns, separated by microreversed-phase liquid chromatography (muRPLC) under a linear gradient of organic modifier concentration, and finally identified by electrospray ionization-MS/MS (ESI-MS/MS). To evaluate the performance of such a platform, a mixture of myoglobin, cytochrome c, bovine serum albumin (BSA), and alpha-casein, with mass ranging from 25 ng to 2 microg, was analyzed. Compared to the methods by off-line protein fractionation and shotgun based strategy, the analysis time, including sample preparation, digestion, desalting, separation, and detection, was shortened from ca. 30 to 5 h, and cytochrome c with abundance of 25 ng could be identified with improved sequence coverage. Furthermore, such an integrated platform was successfully applied into the analysis of proteins extracted from human lung cancer cells. Compared with the results obtained by the shotgun approach, the identified protein number was increased by 30%. All these results demonstrated that such an integrated approach would be an attractive alternative to commonly applied approaches for proteome research.

Genomic and proteomic biomarkers for cancer: a multitude of opportunities

Biomarkers are molecular indicators of a biological status, and as biochemical species can be assayed to evaluate the presence of cancer and therapeutic interventions. Through a variety of mechanisms cancer cells provide the biomarker material for their own detection. Biomarkers may be detectable in the blood, other body fluids, or tissues. The expectation is that the level of an informative biomarker is related to the specific type of disease present in the body. Biomarkers have potential both as diagnostic indicators and monitors of the effectiveness of clinical interventions. Biomarkers are also able to stratify cancer patients to the most appropriate treatment. Effective biomarkers for the early detection of cancer should provide a patient with a better outcome which in turn will translate into more efficient delivery of healthcare. Technologies for the early detection of cancer have resulted in reductions in disease-associated mortalities from cancers that are otherwise deadly if allowed to progress. Such screening technologies have proven that early detection will decrease the morbidity and mortality from cancer. An emerging theme in biomarker research is the expectation that panels of biomarker analytes rather than single markers will be needed to have sufficient sensitivity and specificity for the presymptomatic detection of cancer. Biomarkers may provide prognostic information of disease enabling interventions using targeted therapeutic agents as well as course-corrections in cancer treatment. Novel genomic, proteomic and metabolomic technologies are being used to discover and validate tumor biomarkers individually and in panels.