Proteome resolution by two-dimensional gel electrophoresis varies with the commercial source of IPG strips

Proteomics-The State of the Field: The Definition and Analysis of Proteomes Should Be Based in Reality, Not Convenience

With growing recognition and acknowledgement of the genuine complexity of proteomes, we are finally entering the post-proteogenomic era. Routine assessment of proteomes as inferred correlates of gene sequences (i.e., canonical 'proteins') cannot provide the necessary critical analysis of systems-level biology that is needed to understand underlying molecular mechanisms and pathways or identify the most selective biomarkers and therapeutic targets. These critical requirements demand the analysis of proteomes at the level of proteoforms/protein species, the actual active molecular players. Currently, only highly refined integrated or integrative top-down proteomics (iTDP) enables the analytical depth necessary to provide routine, comprehensive, and quantitative proteome assessments across the widest range of proteoforms inherent to native systems. Here we provide a broad perspective of the field, taking in historical and current realities, to establish a more balanced understanding of where the field has come from (in particular during the ten years since Proteomes was launched), current issues, and how things likely need to proceed if necessary deep proteome analyses are to succeed. We base this in our firm belief that the best proteomic analyses reflect, as closely as possible, the native sample at the moment of sampling. We also seek to emphasise that this and future analytical approaches are likely best based on the broad recognition and exploitation of the complementarity of currently successful approaches. This also emphasises the need to continuously evaluate and further optimize established approaches, to avoid complacency in thinking and expectations but also to promote the critical and careful development and introduction of new approaches, most notably those that address proteoforms. Above all, we wish to emphasise that a rigorous focus on analytical quality must override current thinking that largely values analytical speed; the latter would certainly be nice, if only proteoforms could thus be effectively, routinely, and quantitatively assessed. Alas, proteomes are composed of proteoforms, not molecular species that can be amplified or that directly mirror genes (i.e., 'canonical'). The problem is hard, and we must accept and address it as such, but the payoff in playing this longer game of rigorous deep proteome analyses is the promise of far more selective biomarkers, drug targets, and truly personalised or even individualised medicine.

2DE: the phoenix of proteomics

Given the rapid developments in mass spectrometry (MS) in terms of sensitivity, mass accuracy, and throughput, some have suggested that two-dimensional gel electrophoresis (2DE) may no longer be a method of choice for proteomic analyses. However, as recognition of issues with these newer shotgun-MS approaches grows, there is a fresh and growing regard for the maturity of 2DE-MS as a genuine top-down analytical approach, particularly as it resolves thousands of intact protein species in a single run, enabling the simultaneous analysis of total protein complement, including isoforms and post-translational modifications. Given the strengths of both, it is most appropriate to view these as complementary or at least parallel approaches: as proteins encompass a myriad of physico-chemical properties, and the real aim is to explore proteomes as deeply as possible, all available resolving strategies must be considered in terms of the complexity encountered. It is time to critically and constructively focus on the optimization and integration of existing techniques rather than simplistically suggesting that one should replace the other. Our intention here is thus to present an overview of protein resolving techniques, focusing on milestones associated with 2DE, including pros, cons, advances and variations, in particular relative to shotgun proteomic approaches.

BIOLOGICAL SIGNIFICANCE

Proteomic researchers recognize the importance of 2DE in the history of proteomics. But the latest developments in mass spectrometry-based techniques have led some researchers to retire 2DE in their labs. However, we argue here that 2DE-MS is a genuine top-down analytical approach. The significance of this discussion is to make proteomic researchers aware of the importance of this technique in a proteomic pipeline. This article is part of a Special Issue entitled: Environmental and structural proteomics.

A prospective proteomic-based study for identifying potential biomarkers for the diagnosis of cholangiocarcinoma

BACKGROUND

Cholangiocarcinoma (CCA) is becoming a common fatal hepatic tumor. Early detection of CCA is hampered by the absence of a sufficiently accurate and noninvasive diagnostic test. Proteomic analysis would be a powerful tool to identify potential biomarkers of this cancer.

AIMS

This study aims to identify new protein markers that are specific for CCA using proteomic approaches and to evaluate the performance of S100 calcium-binding protein A9 (S100A9) and chaperonin-containing TCR1, subunit 3 (CCTγ) as diagnostic markers for screening test of CCA.

METHODS

Two-dimensional differential gel electrophoresis (2-D DIGE) coupled with matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry were used to analyze and screen biomarker candidates in the proteomes of five human CCA samples and five healthy control samples. Subsequently, two potential biomarkers, S100A9 and CCTγ, were chosen for validation and analysis by immunohistochemical methods using CCA tissue microarrays.

RESULTS

Twenty protein spots were significantly elevated and five protein spots were downregulated in all patients (p < 0.05). The positive rate was significantly higher in patients with CCA (48 ± 35%) compared with the normal liver control group (5 ± 10%, p < 0.001), the hepatocellular carcinoma group (15 ± 20%, p < 0.001), and the cirrhosis group (12 ± 16%, p < 0.001). A greater proportion of patients with CCA were positive for CCTγ (72 ± 18%) compared with the normal liver control group (43 ± 22%, p < 0.001), the hepatocellular carcinoma group (45 ± 20%, p < 0.001), and the cirrhosis group (39 ± 25%, p < 0.001).

CONCLUSIONS

Combined comparative proteomic analysis using 2-D DIGE and MALDI-TOF is an effective method for identifying differentially expressed proteins in CCA tissues. The expression of S100A9 and CCTγ showed promise as novel diagnostic markers for CCA.

Comparison of first dimension IPG and NEPHGE techniques in two-dimensional gel electrophoresis experiment with cytosolic unfolded protein response in Saccharomyces cerevisiae

Background

Two-dimensional gel electrophoresis (2DE) is one of the most popular methods in proteomics. Currently, most 2DE experiments are performed using immobilized pH gradient (IPG) in the first dimension; however, some laboratories still use carrier ampholytes-based isoelectric focusing technique. The aim of this study was to directly compare IPG-based and non-equilibrium pH gradient electrophoresis (NEPHGE)-based 2DE techniques by using the same samples and identical second dimension procedures. We have used commercially available Invitrogen ZOOM IPGRunner and WITAvision systems for IPG and NEPHGE, respectively. The effectiveness of IPG-based and NEPHGE-based 2DE methods was compared by analysing differential protein expression during cytosolic unfolded protein response (UPR-Cyto) in Saccharomyces cerevisiae.

Results

Protein loss during 2DE procedure was higher in IPG-based method, especially for basic (pI > 7) proteins. Overall reproducibility of spots was slightly better in NEPHGE-based method; however, there was a marked difference when evaluating basic and acidic protein spots. Using Coomassie staining, about half of detected basic protein spots were not reproducible by IPG-based 2DE, whereas NEPHGE-based method showed excellent reproducibility in the basic gel zone. The reproducibility of acidic proteins was similar in both methods. Absolute and relative volume variability of separate protein spots was comparable in both 2DE techniques. Regarding proteomic analysis of UPR-Cyto, the results exemplified parameters of general comparison of the methods. New highly basic protein Sis1p, overexpressed during UPR-Cyto stress, was identified by NEPHGE-based 2DE method, whereas IPG-based method showed unreliable results in the basic pI range and did not provide any new information on basic UPR-Cyto proteins. In the acidic range, the main UPR-Cyto proteins were detected and quantified by both methods. The drawback of NEPHGE-based 2DE method is its failure to detect some highly acidic proteins. The advantage of NEPHGE is higher protein capacity with good reproducibility and quality of spots at high protein load.

Conclusions

Comparison of broad range (pH 3–10) gradient-based 2DE methods suggests that NEPHGE-based method is preferable over IPG (Invitrogen) 2DE method for the analysis of basic proteins. Nevertheless, the narrow range (pH 4–7) IPG technique is a method of choice for the analysis of acidic proteins.

2D-ToGo workflow: increasing feasibility and reproducibility of 2-dimensional gel electrophoresis

Two-dimensional gel electrophoresis (2-DE) is one of the most powerful methods for studying global protein profiles. However, due to the multiple manual steps involved in gel based processing it is challenging to achieve the necessary overall reproducibility for a reliable comparative analysis, especially between different laboratories. To improve the 2-DE technique for quantitative analyses we have set up a robust 2-DE workflow, called 2D-ToGo, which utilizes latest innovations concerning instrumentation, consumables and protocols. Quantitative data analyses indicate the high reproducibility between replicate gels processed at a single site (intra-laboratory variation: CV 20%). The data-sets of the inter-laboratory comparison revealed similar results displaying a variation of CV 23%. The technical improvements given by our 2-DE workflow have a positive impact on process robustness and most importantly, reproducibility. Accordingly, many of the well-known challenges for resolving and quantitating up to thousands of different protein components in a given biological sample are minimized.

Proteomic analysis of interstitial aortic valve cells acquiring a pro-calcific profile

Cell-driven processes are now considered of relevance for the pathogenesis of aortic stenosis. In particular, during calcific valve degeneration, interstitial valve cells (VIC) resident in the leaflet can acquire an osteogenic/pro-calcific profile and actively contribute to matrix mineralization. The proteomic study described in this chapter is undertaken to investigate modifications in the proteome of bovine aortic VIC acquiring a calcifying phenotype. This approach can be useful to clarify cellular pathways involved in VIC pro-calcific differentiation and identify innovative therapeutic targets.

An experimental protocol for the fractionation and 2DE separation of HeLa and A-253 cell lysates suitable for the identification of the individual antigenic proteome in Sjögren's syndrome

Sjögren's syndrome (SS) is an autoimmune disease affecting exocrine glands, especially the salivary and lacrimal glands. Although most of the SS patients' sera have autoantibodies that can target a variety of antigens, it is not clear what determines which proteins will become autoantigens. The muscarinic receptor M3, an integral plasma membrane protein, has been proposed as a possible autoantigen in SS, and is endogenous in HeLa cells. The aim of this study was to develop a method that is able to separate and identify antigens recognised by sera from SS patients using lysates of HeLa and A-253 cells in 2D Western Blot (2DWB). The HeLa and A-253 cell lysates were fractionated in soluble and membrane-bound proteins, and the membrane-bound proteins were enriched for integral proteins. The fractions were tested using WB, confirming the presence of the main cell compartments. The rehydration solution containing ASB-14 performed better than the others in all three steps (active rehydration, focus and transfer), and efficiently separated the muscarinic receptor M3. The M3 receptor was also detected in lysates from A-253 cells. The presence of this receptor in this cell line has not been proven earlier. This work develops a suitable protocol to perform a mapping of the autoantibodies present in the sera of single SS patients, using lysates from epithelial cell lines that represent the main cell compartments as an antigen source. It is our future aim to use this protocol to perform a mapping of the antibodies present in the sera of individual SS patients.

Operator- and software-related post-experimental variability and source of error in 2-DE analysis

In the field of proteomics, several approaches have been developed for separating proteins and analyzing their differential relative abundance. One of the oldest, yet still widely used, is 2-DE. Despite the continuous advance of new methods, which are less demanding from a technical standpoint, 2-DE is still compelling and has a lot of potential for improvement. The overall variability which affects 2-DE includes biological, experimental, and post-experimental (software-related) variance. It is important to highlight how much of the total variability of this technique is due to post-experimental variability, which, so far, has been largely neglected. In this short review, we have focused on this topic and explained that post-experimental variability and source of error can be further divided into those which are software-dependent and those which are operator-dependent. We discuss these issues in detail, offering suggestions for reducing errors that may affect the quality of results, summarizing the advantages and drawbacks of each approach.

Identification of Cysteine Proteases and Screening of Cysteine Protease Inhibitors in Biological Samples by a Two-Dimensional Gel System of Zymography and Reverse Zymography

We have developed a two-dimensional (2D-) gel system of zymography and reverse zymography for the detection and characterization of proteases and protease inhibitors. Isoelectric focusing (IEF) agarose gels with pH gradients were employed for separation in the first-dimension and sodium dodecyl sulfate (SDS)-polyacrylamide gel copolymerized with gelatin used for the second dimension. Proteases and protease inhibitors separated by IEF gel were applied on the second gel without trichloroacetic acid (TCA) fixation. Protease activity in the 2D-gel was visualized as transparent spots where gelatin substrate was digested after commassie brilliant blue (CBB) staining. Some of the transparent spots from the skin mucus extract of rainbow trout were determined to be a cysteine protease through use of E-64 or CA-074. In the reverse zymography technique, the gel was incubated with papain solution at 37°C for 18 h. Cysteine protease inhibitors from broad bean seeds were detected as clear blue spots after CBB staining. The amino (N-) terminal sequences of four papain inhibitor spots thus detected were demonstrated to be identical to that of favin β chain, a broad bean lectin. Taken together, our system can be considered to be an efficient technique for discovering and characterizing new proteases and protease inhibitors in biological samples. This is the first report describing a 2D-gel system of zymography and reverse zymography.

Plant proteome analysis: a 2006 update

This 2006 'Plant Proteomics Update' is a continuation of the two previously published in 'Proteomics' by 2004 (Canovas et al., Proteomics 2004, 4, 285-298) and 2006 (Rossignol et al., Proteomics 2006, 6, 5529-5548) and it aims to bring up-to-date the contribution of proteomics to plant biology on the basis of the original research papers published throughout 2006, with references to those appearing last year. According to the published papers and topics addressed, we can conclude that, as observed for the three previous years, there has been a quantitative, but not qualitative leap in plant proteomics. The full potential of proteomics is far from being exploited in plant biology research, especially if compared to other organisms, mainly yeast and humans, and a number of challenges, mainly technological, remain to be tackled. The original papers published last year numbered nearly 100 and deal with the proteome of at least 26 plant species, with a high percentage for Arabidopsis thaliana (28) and rice (11). Scientific objectives ranged from proteomic analysis of organs/tissues/cell suspensions (57) or subcellular fractions (29), to the study of plant development (12), the effect of hormones and signalling molecules (8) and response to symbionts (4) and stresses (27). A small number of contributions have covered PTMs (8) and protein interactions (4). 2-DE (specifically IEF-SDS-PAGE) coupled to MS still constitutes the almost unique platform utilized in plant proteome analysis. The application of gel-free protein separation methods and 'second generation' proteomic techniques such as multidimensional protein identification technology (MudPIT), and those for quantitative proteomics including DIGE, isotope-coded affinity tags (ICAT), iTRAQ and stable isotope labelling by amino acids in cell culture (SILAC) still remains anecdotal. This review is divided into seven sections: Introduction, Methodology, Subcellular proteomes, Development, Responses to biotic and abiotic stresses, PTMs and Protein interactions. Section 8 summarizes the major pitfalls and challenges of plant proteomics.