Dynamic Range Of Protein Concentrations Research Articles

Recent Progress in Selected Reaction Monitoring MS-Driven Plasma Protein Biomarker Analysis

The plasma proteome offers a wealth of opportunity to develop protein-based assays for diagnostic, prognostic and predictive biomarkers. Nonetheless, the unique properties of plasma, with its high dynamic range of protein concentrations and the vast complexity of protein species, present considerable analytical challenges. The continuing maturation of proteomic technologies, in particular biological MS instrumentation and immunoaffinity sample preparation strategies, is driving progress in the field. Selected reaction monitoring (SRM)-MS of peptides derived from plasma proteins is one such development that facilitates high-fidelity selection and quantitation with only minimal prior sample enrichment, while coupling targeted immunoaffinity enrichment prior to SRM-MS further enhances detection of less-abundant plasma proteins present in the subnanogram per millilitre concentration. This article reviews recent progress in the bioanalysis of plasma proteins driven by SRM-MS.

Glycosylation as means of reducing sample complexity to enable quantitative proteomics

Quantitative proteomics is a rapidly expanding field, in particular, the application to clinical biomarker studies for diagnosis or prognosis of diseases, and the systematic analysis of protein functions in biological systems. Isolation of a class of peptides or a subproteome enables reduction of sample complexity, which is essential to perform sensitive, quantitative analyses over a wider dynamic range of protein concentrations. Glycosylation is one of the most frequent PTMs, and glycans have unique chemical properties that can be leveraged to selectively enrich for a subset of peptides, and thus facilitate the downstream analysis. The isolation of glycopeptides and its benefits for mass spectrometric measurements is discussed.

Microvolume Spectrophotometric and Fluorometric Determination of Protein Concentration

Methods for determining protein concentration that use progressively smaller amounts of material are continually being developed. A new way of minimizing the amount of sample used for spectroscopic analysis is providing more opportunities for greater quality control. Traditional spectrophotometric and fluorometric methods for determination of protein concentrations have long required placing samples into containment devices such as cuvettes or capillaries. A microsample retention system is changing that paradigm by using natural surface tension properties to capture and hold microvolume samples in place during measurement without traditional containment devices. The advantage of such a system is to dramatically reduce the amount of sample required (1 to 2 microl) while greatly increasing the dynamic range of protein concentrations that can be measured. Modifications to classic protein concentration determination protocols are presented to provide a microvolume alternative to traditional cuvette-based methods.

Approaching clinical proteomics: current state and future fields of application in fluid proteomics

The field of clinical proteomics offers opportunities to identify new disease biomarkers in body fluids, cells and tissues. These biomarkers can be used in clinical applications for diagnosis, stratification of patients for specific treatment, or therapy monitoring. New protein array formats and improved spectrometry technologies have brought these analyses to a level with potential for use in clinical diagnostics. The nature of the human body fluid proteome with its large dynamic range of protein concentrations presents problems with quantitation. The extreme complexity of the proteome in body fluids presents enormous challenges and requires the establishment of standard operating procedures for handling of specimens, increasing sensitivity for detection and bioinformatical tools for distribution of proteomic data into the public domain. From studies of in vitro diagnostics, especially in clinical chemistry, it is evident that most errors occur in the preanalytical phase and during implementation of the diagnostic strategy. This is also true for clinical proteomics, and especially for fluid proteomics because of the multiple pretreatment processes. These processes include depletion of high-abundance proteins from plasma or enrichment processes for urine where biological variation or differences in proteolytic activities in the sample along with preanalytical variables such as inter- and intra-assay variability will likely influence the results of proteomics studies. However, before proteomic analysis can be introduced at a broader level into the clinical setting, standardization of the preanalytical phase including patient preparation, sample collection, sample preparation, sample storage, measurement and data analysis needs to be improved. In this review, we discuss the recent technological advances and applications that fulfil the criteria for clinical proteomics, with the focus on fluid proteomics. These advances relate to preanalytical factors, analytical standardization and quality-control measures required for effective implementation into routine laboratory testing in order to generate clinically useful information. With new disease biomarker candidates, it will be crucial to design and perform clinical studies that can identify novel diagnostic strategies based on these techniques, and to validate their impact on clinical decision-making.

DNA Aptamer for Molecular Recognition and Protein Expression Profiling

Aptamer has attracted much attention in both scientific community and pharmaceutical industry in the past few years. Its high affinity and specificity to target molecules have render aptamer with wide applications in both diagnostic and therapeutic fields. We demonstrate the molecular recognition capability of aptamers by conjugating them to resin beads and packing them in an HPLC column for affinity purification of Taq DNA polymerase. The concept of screening aptamers for a multitude of proteins is demonstrated by using protein extract of E. coli in a multiplex SELEX selection process. To reduce the wide dynamic range of protein concentration, the bacterial cell lysate was fractionated by two-dimensional chromatography. An aptamer microarray was constructed by using randomly selected 384 aptamer probes to illustrate the capability of aptamer microarray in profiling protein expression and in identifying differentially expressed genes. The aptamer microarray successfully discriminated two different bacterial species and identified a protein which was over-expressed when the E. coli was treated with ampicillin.

Improved proteome coverage by using iTRAQ labelling and peptide OFFGEL fractionation

BackgroundThe development of mass spectrometric techniques and fractionation methods now allows the investigation of very complex protein mixtures ranging from subcellular structures to tissues. Nevertheless, this work is particularly difficult due to the wide dynamic range of protein concentration in eukaryotic tissues. In this paper, we present a shotgun method whereby the peptides are fractionated using OFFGEL electrophoresis after iTRAQ labelling.ResultsWe demonstrated that iTRAQ peptide labelling enhances MALDI ionisation and that the OFFGEL fractionation of the labelled peptides introduces a supplementary criterion (pI) useful for validation and identification of proteins. We showed that iTRAQ samples allowed lower-concentrated proteins identification in comparison with free-labelled samples.ConclusionThe combined use of iTRAQ labelling and OFFGEL fractionation allows a considerable increase in proteome coverage of very complex samples prepared from total cell extracts and supports the low-concentrated protein identification.

A high confidence, manually validated human blood plasma protein reference set

BackgroundThe immense diagnostic potential of human plasma has prompted great interest and effort in cataloging its contents, exemplified by the Human Proteome Organization (HUPO) Plasma Proteome Project (PPP) pilot project. Due to challenges in obtaining a reliable blood plasma protein list, HUPO later re-analysed their own original dataset with a more stringent statistical treatment that resulted in a much reduced list of high confidence (at least 95%) proteins compared with their original findings. In order to facilitate the discovery of novel biomarkers in the future and to realize the full diagnostic potential of blood plasma, we feel that there is still a need for an ultra-high confidence reference list (at least 99% confidence) of blood plasma proteins.MethodsTo address the complexity and dynamic protein concentration range of the plasma proteome, we employed a linear ion-trap-Fourier transform (LTQ-FT) and a linear ion trap-Orbitrap (LTQ-Orbitrap) for mass spectrometry (MS) analysis. Both instruments allow the measurement of peptide masses in the low ppm range. Furthermore, we employed a statistical score that allows database peptide identification searching using the products of two consecutive stages of tandem mass spectrometry (MS3). The combination of MS3 with very high mass accuracy in the parent peptide allows peptide identification with orders of magnitude more confidence than that typically achieved.ResultsHerein we established a high confidence set of 697 blood plasma proteins and achieved a high 'average sequence coverage' of more than 14 peptides per protein and a median of 6 peptides per protein. All proteins annotated as belonging to the immunoglobulin family as well as all hypothetical proteins whose peptides completely matched immunoglobulin sequences were excluded from this protein list. We also compared the results of using two high-end MS instruments as well as the use of various peptide and protein separation approaches. Furthermore, we characterized the plasma proteins using cellular localization information, as well as comparing our list of proteins to data from other sources, including the HUPO PPP dataset.ConclusionSuperior instrumentation combined with rigorous validation criteria gave rise to a set of 697 plasma proteins in which we have very high confidence, demonstrated by an exceptionally low false peptide identification rate of 0.29%.

Reduction of dynamic protein concentration range of biological extracts for the discovery of low-abundance proteins by means of hexapeptide ligand library

Deciphering the protein composition of complex extracts or discovering biologically relevant polypeptides is frequently hindered by large dynamic concentration ranges. The presence of high-abundance proteins suppresses the signal of low-abundance ones, and the most rare proteins are frequently below the sensitivity of available analytical methods. This protocol addresses this problem by using highly diversified hexapeptide ligand libraries for capturing proteins. A protein extract is mixed with the library and because the library has equal amounts of each ligand, theoretically the maximum amount of each protein bound is the same. Under overloading conditions this has the effect of diluting those proteins present in excess of the ligand concentration and concentrating those of relatively lower abundance. Unbound components are washed out and captured species are finally desorbed. The entire sample treatment process takes about half a day and yields a protein solution that could be used as such for mass spectrometry investigations.

Use of Peptide Analogue Diversity Library Beads for Increased Depth of Proteomic Analysis: Application to Cerebrospinal Fluid

Biological samples can contain proteins with concentrations that span more than 10 orders of magnitude. Given the limited dynamic range of analysis methods, observation of proteins present at the lower concentrations requires depletion of high-abundance proteins, or other means of reducing the dynamic range of concentrations. Hexapeptide diversity library beads have been used to bind proteins in a complex sample up to a given saturation limit, effectively truncating the maximum concentration of proteins at a desired level. To avoid the potential problem of susceptibility of the hexapeptides to cleavage by proteases in the sample and/or bacterial degradation, peptide analogues that exhibit similar binding characteristics to peptides can be used in place of peptides. We report here the use of hexameric peptoid diversity library beads to reduce the dynamic range of protein concentrations in human cerebrospinal fluid (CSF). Using this method in conjunction with 2D LC/MS/MS analyses, we identified 200 unique proteins, about twice the number identified in untreated CSF.

Combination of affinity depletion of abundant proteins and reversed-phase fractionation in proteomic analysis of human plasma/serum

The tremendous complexity of the serum and plasma proteome presents extreme analytical challenges in comprehensive analysis due to the wide dynamic range of protein concentrations. Therefore, robust sample preparation methods remain one of the important steps in the proteome characterization workflow. We present the results on a new column for the specific depletion of 14 high-abundant proteins from human serum and plasma and the subsequent reversed-phase fractionation of the flow-through proteins. Analysis of tryptic peptides was accomplished with microfluidic HPLC-Chip/MS system. Results indicate that high-abundant protein depletion combined with RP fractionation of plasma showed an improved dynamic range for proteomic analysis and enabled the identification of low-abundant plasma proteins.

A combined dataset of human cerebrospinal fluid proteins identified by multi‐dimensional chromatography and tandem mass spectrometry

Human cerebrospinal fluid (CSF) is an important source for studying protein biomarkers of age-related neurodegenerative diseases. Before characterizing biomarkers unique to each disease, it is necessary to categorize CSF proteins systematically and extensively. However, the enormous complexity, great dynamic range of protein concentrations, and tremendous protein heterogeneity due to post-translational modification of CSF create significant challenges to the existing proteomics technologies for an in-depth, nonbiased profiling of the human CSF proteome. To circumvent these difficulties, in the last few years, we have utilized several different separation methodologies and mass spectrometric platforms that greatly enhanced the identification coverage and the depth of protein profiling of CSF to characterize CSF proteome. In total, 2594 proteins were identified in well-characterized pooled human CSF samples using stringent proteomics criteria. This report summarizes our efforts to comprehensively characterize the human CSF proteome to date.

Unraveling the Complex Proteome for Biomarker Discovery in Gastrointestinal and Liver Diseases

Unraveling the Complex Proteome for Biomarker Discovery in Gastrointestinal and Liver Diseases

Peptide profiling of cerebrospinal fluid by mass spectrometry

The search for biomarkers is driven by the increasing clinical importance of early diagnosis. Reliable biomarkers can also assist in directing therapy, monitoring disease activity and the efficacy of treatment. In addition, the discovery of novel biomarkers might provide clues to the pathogenesis of a disease. The dynamic range of protein concentrations in body fluids exceeds 10 orders of magnitude. These huge differences in concentrations complicate the detection of proteins with low expression levels. Since all classical biomarkers have low expression levels (e.g., prostate-specific antigen: 2–4 µg/l; and CA125: 20–35 U/ml), new developments with respect to identification and validation techniques of the low-abundance proteins are required. This review will discuss the current status of profiling cerebrospinal fluid using mass spectrometry-based techniques, and new developments in this area.

Effect of Immunoaffinity Depletion of Human Serum during Proteomic Investigations

Controversy exists regarding the proper mining of the human serum proteome. Because of the analytical challenges of accurately measuring samples containing a very large dynamic range of protein concentrations, current practices have employed depletion of the highly abundant housekeeping serum proteins, such as albumin and immunoglobins. There is question as to the selectivity of depletion, namely, is there loss of other non abundant serum proteins along with albumin, haptoglobin and other commonly depleted proteins. In this study, human serum was analyzed with and without immunoaffinity depletion of the six most abundant proteins by multidimensional liquid chromatography tandem mass spectrometry. Two replicates of each experiment were conducted and compared against one another. In both depleted and nondepleted replicates there was a 73% and 72% overlap of identified peptides and a 64% and 78% overlap of identified proteins, respectively. Of 262 unique proteins identified in the four experiments, 82 were found in common to all four experiments, 142 unique to the depleted serum, and 38 unique to the nondepleted serum. Although serum depletion of highly abundant proteins significantly increased the number of proteins identified, both the degree of sample complexity and this depletion method resulted in a nonselective loss of other proteins.

Analysis of the human serum proteome

Changes in serum proteins that signal histopathological states, such as cancer, are useful diagnostic and prognostic biomarkers. Unfortunately, the large dynamic concentration range of proteins in serum makes it a challenging proteome to effectively characterize. Typically, methods to deplete highly abundant proteins to decrease this dynamic protein concentration range are employed, yet such depletion results in removal of important low abundant proteins. A multi-dimensional peptide separation strategy utilizing conventional separation techniques combined with tandem mass spectrometry (MS/MS) was employed for a proteome analysis of human serum. Serum proteins were digested with trypsin and resolved into 20 fractions by ampholyte-free liquid phase isoelectric focusing. These 20 peptide fractions were further fractionated by strong cation-exchange chromatography, each of which was analyzed by microcapillary reversed-phase liquid chromatography coupled online with MS/MS analysis. This investigation resulted in the identification of 1444 unique proteins in serum. Proteins from all functional classes, cellular localization, and abundance levels were identified. This study illustrates that a majority of lower abundance proteins identified in serum are present as secreted or shed species by cells as a result of signalling, necrosis, apoptosis, and hemolysis. These findings show that the protein content of serum is quite reflective of the overall profile of the human organism and a conventional multidimensional fractionation strategy combined with MS/MS is entirely capable of characterizing a significant fraction of the serum proteome. We have constructed a publicly available human serum proteomic database (http://bpp.nci.nih.gov) to provide a reference resource to facilitate future investigations of the vast archive of pathophysiological content in serum.