One-Step Fractionation of Complex Proteomes Enables Detection of Low Abundant Analytes Using Antibody-Based Microarrays

Multiplex profiling of serum proteins in solution using barcoded antibody fragments and next generation sequencing

The composition of serum proteins is reflecting the current health status and can, with the right tools, be used to detect early signs of disease, such as an emerging cancer. An earlier diagnosis of cancer would greatly increase the chance of an improved outcome for the patients. However, there is still an unmet need for proficient tools to decipher the information in the blood proteome, which calls for further technological development. Here, we present a proof-of-concept study that demonstrates an alternative approach for multiplexed protein profiling of serum samples in solution, using DNA barcoded scFv antibody fragments and next generation sequencing. The outcome shows high accuracy when discriminating samples derived from pancreatic cancer patients and healthy controls and represents a scalable alternative for serum analysis.

Brofelth, Ekstrand et al use DNA barcoded scFv antibody fragments and next generation sequencing for multiplex profiling of proteins in serum from pancreatic cancer patients with high accuracy. This approach can potentially be used in high throughput precision diagnosis.

Application of multiplex immunoassay technology to investigations of ocular disease

Eye-derived fluids, including tears, aqueous humour and vitreous humour often contain molecular signatures of ocular disease states. These signatures can be composed of cytokines, chemokines, growth factors, proteases and soluble receptors. However, the small quantities (<10 µl) of these fluids severely limit the detection of these proteins by traditional enzyme-linked immunosorbent assay or Western blot. To maximise the amount of information generated from the analysis of these specimens, many researchers have employed multiplex immunoassay technologies for profiling the expression or modification of multiple proteins from minute sample volumes.

Exploiting fluorescence for multiplex immunoassays on protein microarrays

Protein microarray technology is becoming the method of choice for identifying protein interaction partners, detecting specific proteins, carbohydrates and lipids, or for characterizing protein interactions and serum antibodies in a massively parallel manner. Availability of the well-established instrumentation of DNA arrays and development of new fluorescent detection instruments promoted the spread of this technique. Fluorescent detection has the advantage of high sensitivity, specificity, simplicity and wide dynamic range required by most measurements. Fluorescence through specifically designed probes and an increasing variety of detection modes offers an excellent tool for such microarray platforms. Measuring for example the level of antibodies, their isotypes and/or antigen specificity simultaneously can offer more complex and comprehensive information about the investigated biological phenomenon, especially if we take into consideration that hundreds of samples can be measured in a single assay. Not only body fluids, but also cell lysates, extracted cellular components, and intact living cells can be analyzed on protein arrays for monitoring functional responses to printed samples on the surface. As a rapidly evolving area, protein microarray technology offers a great bulk of information and new depth of knowledge. These are the features that endow protein arrays with wide applicability and robust sample analyzing capability. On the whole, protein arrays are emerging new tools not just in proteomics, but glycomics, lipidomics, and are also important for immunological research. In this review we attempt to summarize the technical aspects of planar fluorescent microarray technology along with the description of its main immunological applications.

High-throughput proteomics using antibody microarrays: an update

Antibody-based microarrays are a rapidly emerging technology that has advanced from the first proof-of-concept studies to demanding serum protein profiling applications during recent years, displaying great promise within disease proteomics. Miniaturized micro- and nanoarrays can be fabricated with an almost infinite number of antibodies carrying the desired specificities. While consuming only minute amounts of reagents, multiplexed and ultrasensitive assays can be performed targeting high- as well as low-abundance analytes in complex nonfractionated proteomes. The microarray images generated can then be converted into protein expression profiles or protein atlases, revealing a detailed composition of the sample. The technology will provide unique opportunities for fields such as disease diagnostics, biomarker discovery, patient stratification, predicting disease recurrence and drug target discovery. This review describes an update of high-throughput proteomics, using antibody-based microarrays, focusing on key technological advances and novel applications that have emerged over the last 3 years.

Advances in assay of complement function and activation

The main function of the complement system is pattern recognition of danger. Typical exogenous danger signals are pathogen associated molecular patterns inducing a protective inflammatory response. Other examples are exposure to foreign surfaces of biomedical materials including nanoparticles, which principally induce the same inflammatory response. If a surface is "foreign" to the host, it induces complement activation. Development of monoclonal antibodies to neoepitopes on complement activation products introduced an entirely new set of methods for assay of complement activation. Activation of complement by a surface occurs by impairment of the fine balance of the control system, e.g. by preferred binding of factor B at the expense of factor H. Sensitive methods to detect complement activation on surfaces and in the fluid phase are a prerequisite for investigation of the biocompatibility of artificial materials. This information can be used to develop new materials with enhanced biocompatibility. Here we review available methods to study human and animal complement function and activation in vitro and in vivo.

Antibody array analysis of labelled proteomes: how should we control specificity?

Researchers who use protein binders in multiplexed assays can be divided into two camps. One believes that arrays with proteome-wide coverage will become a reality once we have developed binders for all proteins. The sceptics claim that detection with immobilized protein binders and sample labelling will not provide the required specificity. In this article, we review the evidence showing that antibody array analysis of labelled samples can provide meaningful data and discuss the issues raised by the sceptics. We argue that direct the evidence for monospecificity has yet to be published. This will require assays designed to resolve the proteins captured by each binder. One option is to combine array measurement with protein separation. We have developed an assay where labelled sample proteins are separated by size exclusion chromatography (SEC) before contact with microsphere-based arrays (Size-MAP; size exclusion chromatography-resolved microsphere-based affinity proteomics). The effect is an 'antibody array Western blot' where reactivity of immobilized binders is resolved against the size of the proteins in the sample. We show that Size-MAP is useful to discriminate monospecific- and polyreactive antibodies and for automatic detection of reacting with the same target. The possibility to test specificity directly in array-based measurement should be useful to select the best binders and to determine whether the DNA microarray for the proteome is a realistic goal or not.

Dual-color proteomic profiling of complex samples with a microarray of 810 cancer-related antibodies

Antibody microarrays have the potential to enable comprehensive proteomic analysis of small amounts of sample material. Here, protocols are presented for the production, quality assessment, and reproducible application of antibody microarrays in a two-color mode with an array of 1,800 features, representing 810 antibodies that were directed at 741 cancer-related proteins. In addition to measures of array quality, we implemented indicators for the accuracy and significance of dual-color detection. Dual-color measurements outperform a single-color approach concerning assay reproducibility and discriminative power. In the analysis of serum samples, depletion of high-abundance proteins did not improve technical assay quality. On the contrary, depletion introduced a strong bias in protein representation. In an initial study, we demonstrated the applicability of the protocols to proteins derived from urine samples. We identified differences between urine samples from pancreatic cancer patients and healthy subjects and between sexes. This study demonstrates that biomedically relevant data can be produced. As demonstrated by the thorough quality analysis, the dual-color antibody array approach proved to be competitive with other proteomic techniques and comparable in performance to transcriptional microarray analyses.

Protein-protein interaction studies on protein arrays: effect of detection strategies on signal-to-background ratios

Protein arrays hold great promise for proteome-scale analysis of protein-protein interaction networks, but the technical challenges have hindered their adoption by proteomics researchers. The crucial issue of design and fabrication of protein arrays have been addressed in several studies, but the detection strategies used for identifying protein-protein interactions have received little attention. In this study, we evaluated six different detection strategies to identify four different protein-protein interaction pairs. We discuss each detection approach in terms of signal-to-background (S/B) ratio, ease of use, and adaptability to high-throughput format. Protein arrays for this study were made by expressing both the bait proteins (proteins captured at the surface) and prey proteins (probes) in cell-free rabbit reticulocyte lysate (RRL) systems. Bait proteins were expressed as HaloTag fusions that allow covalent capture on a HaloTag ligand-coated glass without any prior protein purification step. Prey proteins were expressed and modified with either tags (protein or peptides) or labels (fluorescent or radiometric) for detection. This simple method for creating protein arrays in combination with our analyses of several detection strategies should increase the usefulness of protein array technologies.

Sample preparation and fractionation for proteome analysis and cancer biomarker discovery by mass spectrometry

Sample preparation and fractionation technologies are one of the most crucial processes in proteomic analysis and biomarker discovery in solubilized samples. Chromatographic or electrophoretic proteomic technologies are also available for separation of cellular protein components. There are, however, considerable limitations in currently available proteomic technologies as none of them allows for the analysis of the entire proteome in a simple step because of the large number of peptides, and because of the wide concentration dynamic range of the proteome in clinical blood samples. The results of any undertaken experiment depend on the condition of the starting material. Therefore, proper experimental design and pertinent sample preparation is essential to obtain meaningful results, particularly in comparative clinical proteomics in which one is looking for minor differences between experimental (diseased) and control (nondiseased) samples. This review discusses problems associated with general and specialized strategies of sample preparation and fractionation, dealing with samples that are solution or suspension, in a frozen tissue state, or formalin-preserved tissue archival samples, and illustrates how sample processing might influence detection with mass spectrometric techniques. Strategies that dramatically improve the potential for cancer biomarker discovery in minimally invasive, blood-collected human samples are also presented.

Decomplexing biofluids using microchip based acoustophoresis

Highly efficient washing and extraction of microbeads to decomplex analytes ranging from small peptides to large viruses was realised in a microscaled continuous flow format. The bead washing principle reported herein is based on acoustophoresis, i.e. the primary acoustic radiation force in an ultrasonic standing wave and laminar flow properties are utilised to translate bioanalytes trapped on functionalised microbeads from one carrier fluid to another. The carry-over of non-specific material ranges from 1 to 50 ppm relative to input levels depending on application, making acoustophoresis suitable for extraction of rare species from complex environments. Selective extraction of a phosphopeptide relative to its unphosphorylated counterpart is demonstrated using metal oxide affinity capture (MOAC) beads and MALDI-TOF MS readout. Acoustophoresis of microbeads activated with specific binders could be used to capture phage viral particles. The efficiency of the acoustophoretic washing principle was demonstrated by an unspecific phage cross contamination level of only 10(-6) of that in the input bead/phage mixture. The continuous flow format makes acoustophoretic washing flexible regarding sample volume and also allows for easy integration into a sequence of particle handling and analytical unit operations.

Design of high-density antibody microarrays for disease proteomics: key technological issues

Antibody-based microarray is a novel proteomic technology setting a new standard for molecular profiling of non-fractionated complex proteomes. The first generation of antibody microarrays has already demonstrated its potential for generating detailed protein expression profiles, or protein atlases, of human body fluids in health and disease, paving the way for new discoveries within the field of disease proteomics. The process of designing highly miniaturized, high-density and high-performing antibody microarray set-ups have, however, proven to be challenging. In this mini-review we discuss key technological issues that must be addressed in a cross-disciplinary manner before true global proteome analysis can be performed using antibody microarrays.

Antibody-based microarrays

Antibody-based microarrays are a new powerful proteomic technology that can be used to generate rapid and detailed expression profiles of defined sets of protein analytes in complex samples as well as high-resolution portraits of entire proteomes. Miniaturized micro- and nanoarrays can be printed with numerous antibodies carrying the desired specificities. Multiplexed and ultra-sensitive assays, specifically targeting several analytes in a single experiment, can be performed, while consuming only minute amounts of the sample. The array images generated can then be converted into protein expression profiles, or maps, revealing the detailed composition of the sample. This promising proteomic research tool will thus provide unique opportunities for e.g. disease proteomics, biomarker discovery, disease diagnostics, and patient stratification. This review describes the antibody-based microarray technology and applications thereof.

Electrochemical biochips for protein analysis

Proteins bear important functions for most life processes. It is estimated that the human proteome comprises more than 250,000 proteins. Over the last years, highly sophisticated and powerful instruments have been developed that allow their detection and characterization with great precision and sensitivity. However, these instruments need well-equipped laboratories and a well-trained staff. For the determination of proteins in a hospital, in a doctor's office, or at home, low-budget protein analysis methods are needed that are easy to perform. In addition, for a proteomic approach, highly parallel measurements with small sample sizes are required. Biochips are considered as promising tools for such applications. The following chapter describes electrochemical biochips for protein analysis that use antibodies or aptamers as recognition elements.

Antibody microarray analysis of directly labelled complex proteomes

In recent years, the antibody microarray technology has made significant progress, going from proof-of-concept designs to established high-performing technology platforms capable of targeting non-fractionated complex proteomes. In these cross-disciplinary efforts, a particular focus has lately been placed on two key technological issues: the sample and data handling. To this end, robust protocols have been designed for direct labelling of whole proteomes compatible with a sensitive fluorescent-based sensing. Tagging of the proteins with biotin in a single-colour approach has, in many cases, proven to be the preferred approach. Furthermore, based on modified approaches, adopted from the DNA microarray field, the first bioinformatic standards for performing the antibody microarray data analysis have emerged, though general standard operating procedure(s) remains to be implemented.

Serum proteome profiling of metastatic breast cancer using recombinant antibody microarrays

The driving force behind oncoproteomics is to identify biomarker signatures associated with a particular malignancy. Here, we have for the first time used large-scale recombinant scFv antibody microarrays in an attempt to classify metastatic breast cancer versus healthy controls, based on differential protein expression profiling of whole serum samples. Using this multiplexed and miniaturised assay set-up providing pM range sensitivities, breast cancer could be classified with a specificity and sensitivity of 85% based on 129 serum analytes. However, by adopting a condensed 11 analyte biomarker signature, composed of nine non-redundant serum proteins, we were able to distinguish cancer versus healthy serum proteomes with a 95% sensitivity and specificity, respectively. When a subgroup of patients, not receiving anti-inflammatory drugs, was analysed, a novel eight analyte biomarker signature with a further improved predictive power was indicated. In a longer perspective, antibody microarray analysis could provide a tool for the development of improved diagnostics and intensified biomarker discovery for breast cancer patients.

Design of recombinant antibody microarrays for cell surface membrane proteomics

Generating proteomic maps of membrane proteins, common targets for therapeutic interventions and disease diagnostics, has turned out to be a major challenge. Antibody-based microarrays are among the novel rapidly evolving proteomic technologies that may enable global proteome analysis to be performed. Here, we have designed the first generation of a scaleable human recombinant scFv antibody microarray technology platform for cell surface membrane proteomics as well as glycomics targeting intact cells. The results showed that rapid and multiplexed profiling of the cell surface proteome (and glycome) could be performed in a highly specific and sensitive manner and that differential expression patterns due to external stimuli could be monitored.

Design of recombinant antibody microarrays for complex proteome analysis: choice of sample labeling-tag and solid support

Antibody-based microarray is a novel technology with great potential within high-throughput proteomics. The process of designing high-performing antibody (protein) microarrays has, however, turned out to be a challenging process. In this study, we have developed further our human recombinant single-chain variable-fragment (scFv) antibody microarray methodology by addressing two crucial technological issues, choice of sample labeling-tag and solid support. We examined the performance of a range of dyes in a one- or two-color approach on a selection of solid supports providing different surface and coupling chemistries, and surface structures. The set-ups were evaluated in terms of sensitivity, specificity, and selectivity. The results showed that a one-color approach, based on NHS-biotin (or ULS-biotin) labeling, on black polymer Maxisorb slides (or Nexterion slide H) was the superior approach for targeting low-abundant (pg/mL) analytes in nonfractionated, complex proteomes, such as human serum or crude cell supernatants. Notably, microarrays displaying adequate spot morphologies, high S/Ns, minimized nonspecific binding, and most importantly a high selectivity, specificity, and sensitivity (>or=fM range) were obtained. Taken together, we have designed the first generation of a high-performing recombinant scFv antibody microarray technology platform on black polymer Maxisorb slides for sensitive profiling of low-abundant analytes in nonfractionated biotinylated complex proteomes.

[Protein arrays and perspectives of medical applications]

Protein microarrays make it possible to detect molecular interactions with various partners (proteins, peptides, nucleic acids, sugars, etc.). Their advantages are crucial for high-throughput analysis of proteomes of different organisms. Moreover, the recent data reveal the performance of microarrays over current immunological methods. Therefore, the antigen and antibody microarrays become indispensable for medical applications, in particular, for diagnosis and prognosis of microbial infections, autoimmune and allergic diseases. The further technological progress might provide the extension of the miniaturized assays for multiparametric monitoring of human pathologies in practical medicine.

Sample preparation

A panorama of sample preparation methods has been composed from 481 references, with a highlight of some promising methods fast developed during recent years and a somewhat brief introduction on most of the well-developed methods. All the samples were commonly referred to molecular composition, being extendable to particles including cells but not to organs, tissues and larger bodies. Some criteria to evaluate or validate a sample preparation method were proposed for reference. Strategy for integration of several methods to prepare complicated protein samples for proteomic studies was illustrated and discussed.

How will haematologists use proteomics?

Proteomics technologies are emerging as a useful tool in the identification of disease biomarkers, and in defining and characterising both normal physiological and disease processes. Many cellular changes in protein expression in response to an external stimulus or mutation can only be characterised at the proteome level. In these cases protein expression is often controlled by altered rates of translation and/or degradation, making proteomics an important tool in the analysis of biological systems. In the leukaemias, post-translational modification of proteins (e.g. phosphorylation, acetylation) plays a key role in the molecular pathology of the disease: such modifications can now be detected with novel proteomic methods. In a clinical setting, serum remains a relatively un-mined source of information for prognosis and response to therapy. This protein rich fluid represents an opportunity for proteomics research to benefit hematologists and others. In this review, we discuss the technologies available for the study of the proteome that offer realistic opportunities in haematology.

Design of Recombinant Antibody Microarrays for Serum Protein Profiling: Targeting of Complement Proteins

Antibody-based microarrays is a novel technology with great promise for high-throughput proteomics. The process of designing high-performing arrays has, however, turned out to be challenging. Here, we have designed the next generation of a human recombinant scFv antibody microarray platform for protein expression profiling of nonfractionated biotinylated human plasma and serum proteomes. The setup, based on black polymer Maxisorb slides interfaced with a fluorescent-based read-out system, was found to provide specific, sensitive (subpicomolar (pM) range) and reproducible means for protein profiling. Further, a chip-to-chip normalization protocol critical for comparing data generated on different chips was devised. Finally, the microarray data were found to correlate well with clinical laboratory data obtained using conventional methods, as demonstrated for a set of medium abundant (micromolar (microM) to nanomolar (nM) range) protein analytes in serum and plasma samples derived from healthy and complement-deficient individuals.

Reverse-Phase versus Sandwich Antibody Microarray, Technical Comparison from a Clinical Perspective

Protein microarrays are powerful tools to quantify and characterize proteins in multiplex assays. They have great potential within clinical diagnostics and prognostics, as they minimize consumption of both analyte and biological sample. Assays that do not require labeling of the biological specimen, henceforth called label-free, are vital for ease of clinical sample processing. Here, we evaluate two label-free techniques, reverse-phase and sandwich antibody assays, using microarrays on high-performance porous silicon surfaces and fluorescence detection. In view of increasing interest in reverse microarrays, this paper focuses on analytical sensitivity of the reverse assays compared to the more complex but highly sensitive sandwich assay. Sensitivity, linear range, and reproducibility of the two assays were compared using prostate-specific antigen (PSA) in buffer. The sandwich assay displayed 5 orders of magnitude lower detection limit (0.7 ng/mL) compared to the reverse assay (70 microg/mL). PSA at 50 nM (1.5 microg/mL) in cell lysates was detected by the sandwich assay but not by the reverse assay, demonstrating again a far lower detection limit for sandwich microarrays. In independent assay runs of PSA spiked in female serum, the sandwich assay had good linearity (R2 > 0.99) and reproducibility (coefficient of variation < or =15%), and the detection limit could be improved to 0.14 ng/mL. Without further signal amplification, the sandwich assay would be our choice for PSA analysis of clinical samples using a microarray technology platform.

Trends in sample preparation for classical and second generation proteomics

Sample preparation is a fundamental step in the proteomics workflow. However, it is not easy to find compiled information updating this subject. In this paper, the strategies and protocols for protein extraction and identification, following either classical or second generation proteomics methodologies, are reviewed. Procedures for: tissue disruption, cell lysis, sample pre-fractionation, protein separation by 2-DE, protein digestion, mass spectrometry analysis, multidimensional peptide separations and quantification of protein expression level are described.

Approaches for systematic proteome exploration

With the completion of the human genome project (HUGO) during recent years, gene function, protein abundance and expression patterns in tissues and cell types have emerged as central areas for the scientific community. A mapped human proteome will extend the value of the genome sequence and large-scale efforts aiming at elucidating protein localization, abundance and function are invaluable for biomarker and drug discovery. This research area, termed proteomics, is more demanding than any genome sequencing effort and to perform this on a wide scale is a highly diverse task. Therefore, the proteomics field employs a range of methods to examine different aspects of proteomics including protein localization, protein-protein interactions, posttranslational modifications and alteration of protein composition (e.g. differential expression) in tissues and body fluids. Here, some of the most commonly used methods, including chromatographic separations together with mass spectrometry and a number of affinity proteomics concepts are discussed and exemplified.

On-Chip Identification and Interaction Analysis of Gel-Resolved Proteins Using a Diamond-like Carbon-Coated Plate

We developed a novel protein chip made of a diamond-like, carbon-coated stainless steel plate (DLC plate), the surface of which is chemically modified with N-hydroxysuccinimide ester. To produce a high-density protein chip using the DLC plate, proteins separated by SDS gel electrophoresis or two-dimensional electrophoresis were electroblotted onto the DLC plate and immobilized covalently. A high blotting efficiency (25-70%) for transferring proteins from the gels onto the DLC plates was achieved by improvement of the electrophoresis device and electroblotting techniques. With the use of the DLC plate, we developed novel techniques to identify proteins immobilized on the chip and to detect protein-protein interactions on the chip by mass spectrometric analysis. We also developed a technique to identify post-translationally modified proteins, such as glycoproteins, on the protein chip.

Understanding proteomics

The purpose of this article is to describe proteomics, to discuss the importance of proteomics, to review different methods for protein measurement, and to illustrate how knowledge of proteomics might improve patient care. Among common laboratory determinations are those involving enzymatic (protein) function. Although the presence or activity of proteins may be seen clinically as incidental, proteins represent the engines through which critical life processes ensue. A selected review of the literature is presented to define and explain proteomics and to review the various techniques to measure proteins. A case-study approach is used to illustrate how proteomics can be utilized for detecting and monitoring disease. The human genome has been completely sequenced and proteomics has emerged as a way to unravel the biochemical and physiological mechanisms of diseases at the functional level. This review includes the recent discoveries regarding proteomics and its importance in the detection and treatment of disease.

Methods for samples preparation in proteomic research

Sample preparation is one of the most crucial processes in proteomics research. The results of the experiment depend on the condition of the starting material. Therefore, the proper experimental model and careful sample preparation is vital to obtain significant and trustworthy results, particularly in comparative proteomics, where we are usually looking for minor differences between experimental-, and control samples. In this review we discuss problems associated with general strategies of samples preparation, and experimental demands for these processes.

Design of atto-vial based recombinant antibody arrays combined with a planar wave-guide detection system

Antibody microarray is a rapidly emerging, powerful approach with great promise within high-throughput proteomics. However, before a truly proteome-wide analysis can be performed, the antibody array format needs to be miniaturized even further in order to enable ultradense arrays to be fabricated. To this end, we have designed and generated proof-of-concept for the first generation of an atto-vial based recombinant antibody array platform. Briefly, we have designed a novel nanostructured substrate using electron beam lithography. Vials, ranging in volume/size from 6 (200 nm in diameter) to 4000 aL (5 microm in diameter), were fabricated. Human recombinant single-chain Fv antibody fragments, microarray adopted by design, were used as probes. The set-up was interfaced with planar wave-guide technology for evanescant field fluorescence detection. The results showed that protein analytes could be specifically detected in the subzeptomole range for pure systems, using vials down to 57 aL. Further, low-abundant (pg/mL) protein analytes could be detected in directly labeled complex proteomes, such as human whole serum, using 157 aL-vials. Taken together, these results outline the potential of the atto-vial array set-up for miniaturized affinity proteomics-based approaches.

Endogenous Plasma Peptide Detection and Identification in the Rat by a Combination of Fractionation Methods and Mass Spectrometry

Mass spectrometry-based analyses are essential tools in the field of biomarker research. However, detection and characterization of plasma low abundance and/or low molecular weight peptides is challenged by the presence of highly abundant proteins, salts and lipids. Numerous strategies have already been tested to reduce the complexity of plasma samples. The aim of this study was to enrich the low molecular weight fraction of rat plasma. To this end, we developed and compared simple protocols based on membrane filtration, solid phase extraction, and a combination of both. As assessed by UV absorbance, an albumin depletion >99% was obtained. The multistep fractionation strategy (including reverse phase HPLC) allowed detection, in a reproducible manner (CV < 30%-35%), of more than 450 peaks below 3000 Da by MALDI-TOF/MS. A MALDI-TOF/MS-determined LOD as low as 1 fmol/μL was obtained, thus allowing nanoLC-Chip/MS/MS identification of spiked peptides representing ~10–6% of total proteins, by weight. Signal peptide recovery ranged between 5%-100% according to the spiked peptide considered. Tens of peptide sequence tags from endogenous plasma peptides were also obtained and high confidence identifications of low abundance fibrinopeptide A and B are reported here to show the efficiency of the protocol. It is concluded that the fractionation protocol presented would be of particular interest for future differential (high throughput) analyses of the plasma low molecular weight fraction.

Antibody microarrays: current status and key technological advances

Antibody-based microarrays are among the novel classes of rapidly evolving proteomic technologies that holds great promise in biomedicine. Miniaturized microarrays (< 1 cm2) can be printed with thousands of individual antibodies carrying the desired specificities, and with biological sample (e.g., an entire proteome) added, virtually any specifically bound analytes can be detected. While consuming only minute amounts (< microL scale) of reagents, ultra- sensitive assays (zeptomol range) can readily be performed in a highly multiplexed manner. The microarray patterns generated can then be transformed into proteomic maps, or detailed molecular fingerprints, revealing the composition of the proteome. Thus, protein expression profiling and global proteome analysis using this tool will offer new opportunities for drug target and biomarker discovery, disease diagnostics, and insights into disease biology. Adopting the antibody microarray technology platform, several biomedical applications, ranging from focused assays to proteome-scale analysis will be rapidly emerging in the coming years. This review will discuss the current status of the antibody microarray technology focusing on recent technological advances and key issues in the process of evolving the methodology into a high-performing proteomic research tool.

Generation of High Density Protein Microarrays by Cell-free in Situ Expression of Unpurified PCR Products

Due to the success of DNA microarrays and the growing numbers of available protein expression clones, protein microarrays have become more and more popular for the high throughput screening of protein interactions. However, the widespread applicability of protein microarrays is currently hampered by the large effort associated with their production. Apart from the requirement for a protein expression library, expression and purification of the proteins themselves and the lacking stability of many proteins remain the bottleneck. Here we present an approach that allows the generation of high density protein microarrays from unbound DNA template molecules on the chip. It is based on the multiple spotting technique and comprises the deposition of a DNA template in a first spotting step and the transfer of a cell-free transcription and translation mixture on top of the same spot in a second spotting step. Using wild-type green fluorescent protein as a model protein, we demonstrated the time and template dependence of this coupled transcription and translation and showed that enough protein was produced to yield signals that were comparable to 300 microg/ml spotted protein. Plasmids as well as unpurified PCR products can be used as templates, and as little as 35 fg of PCR product ( approximately 22,500 molecules) were sufficient for the detectable expression of full-length wild-type green fluorescent protein in subnanoliter volumes. We showed that both aminopropyltrimethoxysilane and nickel chelate surfaces can be used for capture of the newly synthesized proteins. Surprisingly we observed that nickel chelate-coated slides were binding the newly synthesized proteins in an unspecific manner. Finally we adapted the system to the high throughput expression of libraries by designing a single primer pair for the introduction of the required T7 promoter and demonstrated the in situ expression using 384 randomly chosen clones.

Identification of protein expression signatures associated with Helicobacter pylori infection and gastric adenocarcinoma using recombinant antibody microarrays

Antibody microarray based technology is a powerful emerging tool in proteomics, target discovery, and differential analysis. Here, we report the first study where recombinant antibody fragments have been used to construct large scale antibody microarrays, composed of 127 different antibodies against mostly immunoregulatory antigens. The arrays were based on single framework recombinant antibody fragments (SinFabs) designed for high on-chip stability and functionality and were used for the analysis of malignant and normal stomach tissue samples from Helicobacter pylori-positive and -negative patients. Our results demonstrate that distinct tumor- as well as infection-associated protein expression signatures could be identified from these complex tissue proteomes, as well as biomarkers such as IL-9, IL-11, and MCP-4, previously not found in these diseases. In a longer perspective, this study may improve the understanding of H. pylori-induced stomach cancer and lead to development of improved diagnostics.

Relative quantification in proteomics: new approaches for biochemistry

Recent developments in mass spectrometry and protein arrays provide opportunities to derive systematically proteomic information from small samples of cellular material. Relative quantification among samples can be achieved with either gel-based or gel-free approaches. Furthermore, the adaptation of specific techniques facilitates absolute quantification. Here, relative quantification in two-dimensional gel electrophoresis is contrasted with that in non-gel-based approaches such as isobaric tagging of peptides, pre-labelling of living cells with isotopomeric forms of essential amino acids and protein array platforms. In addition, novel flow-cytometry-based approaches are considered. These technologies can all be used to determine accurately the levels of proteins or biomarkers in a wide range of samples.