Analysis of the Cerebrospinal Fluid Proteome in Alzheimer's Disease.

Alzheimer's disease is a neurodegenerative disorder accounting for more than 50% of cases of dementia. Diagnosis of Alzheimer's disease relies on cognitive tests and analysis of amyloid beta, protein tau, and hyperphosphorylated tau in cerebrospinal fluid. Although these markers provide relatively high sensitivity and specificity for early disease detection, they are not suitable for monitor of disease progression. In the present study, we used label-free shotgun mass spectrometry to analyse the cerebrospinal fluid proteome of Alzheimer's disease patients and non-demented controls to identify potential biomarkers for Alzheimer's disease. We processed the data using five programs (DecyderMS, Maxquant, OpenMS, PEAKS, and Sieve) and compared their results by means of reproducibility and peptide identification, including three different normalization methods. After depletion of high abundant proteins we found that Alzheimer's disease patients had lower fraction of low-abundance proteins in cerebrospinal fluid compared to healthy controls (p<0.05). Consequently, global normalization was found to be less accurate compared to using spiked-in chicken ovalbumin for normalization. In addition, we determined that Sieve and OpenMS resulted in the highest reproducibility and PEAKS was the programs with the highest identification performance. Finally, we successfully verified significantly lower levels (p<0.05) of eight proteins (A2GL, APOM, C1QB, C1QC, C1S, FBLN3, PTPRZ, and SEZ6) in Alzheimer's disease compared to controls using an antibody-based detection method. These proteins are involved in different biological roles spanning from cell adhesion and migration, to regulation of the synapse and the immune system.

Immunocapture strategies in translational proteomics.

Aiming at clinical studies of human diseases, antibody-assisted assays have been applied to biomarker discovery and toward a streamlined translation from patient profiling to assays supporting personalized treatments. In recent years, integrated strategies to couple and combine antibodies with mass spectrometry-based proteomic efforts have emerged, allowing for novel possibilities in basic and clinical research. Described in this review are some of the field's current and emerging immunocapture approaches from an affinity proteomics perspective. Discussed are some of their advantages, pitfalls and opportunities for the next phase in clinical and translational proteomics.

Neuroproteomic profiling of human body fluids.

Analysis of protein expression and abundance provides a possibility to extend the current knowledge on disease-associated processes and pathways. The human brain is a complex organ and dysfunction or damage can give rise to a variety of neurological diseases. Although many proteins potentially reflecting disease progress are originating from brain, the scarce availability of human tissue material has lead to utilization of body fluids such as cerebrospinal fluid and blood in disease-related research. Within the most common neurological disorders, much effort has been spent on studying the role of a few hallmark proteins in disease pathogenesis but despite extensive investigation, the signatures they provide seem insufficient to fully understand and predict disease progress. In order to expand the view the field of neuroproteomics has lately emerged alongside developing technologies, such as affinity proteomics and mass spectrometry, for multiplexed and high-throughput protein profiling. Here, we provide an overview of how such technologies have been applied to study neurological disease and we also discuss some important considerations concerning discovery of disease-associated profiles.

Defining the Human Brain Proteome Using Transcriptomics and Antibody-Based Profiling with a Focus on the Cerebral Cortex.

The mammalian brain is a complex organ composed of many specialized cells, harboring sets of both common, widely distributed, as well as specialized and discretely localized proteins. Here we focus on the human brain, utilizing transcriptomics and public available Human Protein Atlas (HPA) data to analyze brain-enriched (frontal cortex) polyadenylated messenger RNA and long non-coding RNA and generate a genome-wide draft of global and cellular expression patterns of the brain. Based on transcriptomics analysis of altogether 27 tissues, we have estimated that approximately 3% (n=571) of all protein coding genes and 13% (n=87) of the long non-coding genes expressed in the human brain are enriched, having at least five times higher expression levels in brain as compared to any of the other analyzed peripheral tissues. Based on gene ontology analysis and detailed annotation using antibody-based tissue micro array analysis of the corresponding proteins, we found the majority of brain-enriched protein coding genes to be expressed in astrocytes, oligodendrocytes or in neurons with molecular properties linked to synaptic transmission and brain development. Detailed analysis of the transcripts and the genetic landscape of brain-enriched coding and non-coding genes revealed brain-enriched splice variants. Several clusters of neighboring brain-enriched genes were also identified, suggesting regulation of gene expression on the chromatin level. This multi-angle approach uncovered the brain-enriched transcriptome and linked genes to cell types and functions, providing novel insights into the molecular foundation of this highly specialized organ.

Proteomic profiling reveals autoimmune targets in sarcoidosis.

RATIONALE: There is a need to further characterize the antibody repertoire in relation to sarcoidosis and potentially related autoantigens. OBJECTIVES: We investigated bronchoalveolar lavage (BAL) and serum samples from patients with sarcoidosis and healthy and diseased control subjects to discover sarcoidosis-associated autoantigens. METHODS: Antigen microarrays built on 3,072 protein fragments were used to screen for IgG reactivity in 73 BAL samples from subjects with sarcoidosis, subjects with asthma, and healthy subjects. A set of 131 targets were selected for subsequent verification on suspension bead arrays using 272 additional BAL samples and 141 paired sera. Reactivity to four antigens was furthermore analyzed in 22 unprocessed BAL samples from patients with fibrosis and 269 plasma samples from patients diagnosed with myositis. MEASUREMENTS AND MAIN RESULTS: Reactivity toward zinc finger protein 688 and mitochondrial ribosomal protein L43 were discovered with higher frequencies in patients with sarcoidosis, for mitochondrial ribosomal protein L43 especially in patients with non-Löfgren syndrome. Increased reactivity toward nuclear receptor coactivator 2 was also observed in patients with non-Löfgren syndrome as compared with patients with Löfgren syndrome. The antigen representing adenosine diphosphate-ribosylation factor GTPase activating protein 1 revealed high reactivity frequency in all sample groups but with significantly higher level of IgG reactivities in patients with sarcoidosis. CONCLUSIONS: Autoantigen reactivity was present in most BAL and serum samples analyzed, and the results revealed high interindividual heterogeneity, with most of the reactivities observed in single individuals only. Four proteins are here proposed as sarcoidosis-associated autoimmune targets and of interest for further validation in independent cohorts.

Plasma profiling reveals three proteins associated to amyotrophic lateral sclerosis.

OBJECTIVE: Amyotrophic lateral sclerosis (ALS) is the most common adult motor neuron disease leading to muscular paralysis and death within 3-5 years from onset. Currently, there are no reliable and sensitive markers able to substantially shorten the diagnosis delay. The objective of the study was to analyze a large number of proteins in plasma from patients with various clinical phenotypes of ALS in search for novel proteins or protein profiles that could serve as potential indicators of disease. METHODS: Affinity proteomics in the form of antibody suspension bead arrays were applied to profile plasma samples from 367 ALS patients and 101 controls. The plasma protein content was directly labeled and protein profiles obtained using 352 antibodies from the Human Protein Atlas targeting 278 proteins. A focused bead array was then built to further profile eight selected protein targets in all available samples. RESULTS: Disease-associated significant differences were observed and replicated for profiles from antibodies targeting the proteins: neurofilament medium polypeptide (NEFM), solute carrier family 25 (SLC25A20), and regulator of G-protein signaling 18 (RGS18). INTERPRETATION: Upon further validation in several independent cohorts with inclusion of a broad range of other neurological disorders as controls, the alterations of these three protein profiles in plasma could potentially provide new molecular markers of disease that contribute to the quest of understanding ALS pathology.

Affinity proteomic profiling of plasma, cerebrospinal fluid, and brain tissue within multiple sclerosis.

The brain is a vital organ and because it is well shielded from the outside environment, possibilities for noninvasive analysis are often limited. Instead, fluids taken from the spinal cord or circulatory system are preferred sources for the discovery of candidate markers within neurological diseases. In the context of multiple sclerosis (MS), we applied an affinity proteomic strategy and screened 22 plasma samples with 4595 antibodies (3450 genes) on bead arrays, then defined 375 antibodies (334 genes) for targeted analysis in a set of 172 samples and finally used 101 antibodies (43 genes) on 443 plasma as well as 573 cerebrospinal spinal fluid (CSF) samples. This revealed alteration of protein profiles in relation to MS subtypes for IRF8, IL7, METTL14, SLC30A7, and GAP43. Respective antibodies were subsequently used for immunofluorescence on human post-mortem brain tissue with MS pathology for expression and association analysis. There, antibodies for IRF8, IL7, and METTL14 stained neurons in proximity of lesions, which highlighted these candidate protein targets for further studies within MS and brain tissue. The affinity proteomic translation of profiles discovered by profiling human body fluids and tissue provides a powerful strategy to suggest additional candidates to studies of neurological disorders.

Developing new methods to answer old and new questions in neurodegenerative diseases: 21(st) Workshop of the HUPO Brain Proteome Project (HBPP) 23-24 January 2014, Honolulu, Hawaii.

The HUPO Brain Proteome Project (HUPO BPP) held its 21(st) workshop in Honolulu, Hawaii. During the 23-24 January 2014 the island became the center of the open workshop of the scientific community.

Autoantibody profiling in multiple sclerosis using arrays of human protein fragments.

Profiling the autoantibody repertoire with large antigen collections is emerging as a powerful tool for the identification of biomarkers for autoimmune diseases. Here, a systematic and undirected approach was taken to screen for profiles of IgG in human plasma from 90 individuals with multiple sclerosis related diagnoses. Reactivity pattern of 11,520 protein fragments (representing ∼38% of all human protein encoding genes) were generated on planar protein microarrays built within the Human Protein Atlas. For more than 2,000 antigens IgG reactivity was observed, among which 64% were found only in single individuals. We used reactivity distributions among multiple sclerosis subgroups to select 384 antigens, which were then re-evaluated on planar microarrays, corroborated with suspension bead arrays in a larger cohort (n = 376) and confirmed for specificity in inhibition assays. Among the heterogeneous pattern within and across multiple sclerosis subtypes, differences in recognition frequencies were found for 51 antigens, which were enriched for proteins of transcriptional regulation. In conclusion, using protein fragments and complementary high-throughput protein array platforms facilitated an alternative route to discovery and verification of potentially disease-associated autoimmunity signatures, that are now proposed as additional antigens for large-scale validation studies across multiple sclerosis biobanks.

Antibody-based profiling of cerebrospinal fluid within multiple sclerosis.

Antibody suspension bead arrays have proven to enable multiplexed and high-throughput protein profiling in unfractionated plasma and serum samples through a direct labeling approach. We here describe the development and application of an assay for protein profiling of cerebrospinal fluid (CSF). While setting up the assay, systematic intensity differences between sample groups were observed that reflected inherent sample specific total protein amounts. Supplementing the labeling reaction with BSA and IgG diminished these differences without impairing the apparent sensitivity of the assay. We also assessed the effects of heat treatment on the analysis of CSF proteins and applied the assay to profile 43 selected proteins by 101 antibodies in 339 CSF samples from a multiple sclerosis (MS) cohort. Two proteins, GAP43 and SERPINA3 were found to have a discriminating potential with altered intensity levels between sample groups. GAP43 was detected at significantly lower levels in secondary progressive MS compared to early stages of MS and the control group of other neurological diseases. SERPINA3 instead was detected at higher levels in all MS patients compared to controls. The developed assay procedure now offers new possibilities for broad-scale protein profiling of CSF within neurological disorders.

Classification of protein profiles from antibody microarrays using heat and detergent treatment.

Antibody microarrays offer new opportunities for exploring the proteome and to identify biomarker candidates in human serum and plasma. Here, we have investigated the effect of heat and detergents on an antibody-based suspension bead array (SBA) assay using polyclonal antibodies and biotinylated plasma samples. With protein profiles from more than 2300 antibodies generated in 384-plex antibody SBAs, three major classes of heat and detergent susceptibility could be described. The results show that washing of the beads with SDS (rather than Tween) after target binding lowered intensity levels of basically all profiles and that about 50% of the profiles appeared to be lowered to a similar extent by heating of the sample. About 33% of the profiles appeared to be insensitive to heat treatment while another 17% showed a positive influence of heat to yield elevated profiles. The results suggest that the classification of antibodies is driven by the molecular properties of the antibody-antigen interaction and can generally not be predicted based on protein class or Western blot data. The experimental scheme presented here can be used to systematically categorize antibodies and thereby combine antibodies with similar properties into targeted arrays for analysis of plasma and serum.

Systematic antibody and antigen-based proteomic profiling with microarrays.

Current approaches within affinity-based proteomics are driven both by the accessibility and availability of antigens and capture reagents, and by suitable multiplexed technologies onto which these are implemented. By combining planar microarrays and other multiparallel systems with sets of reagents, possibilities to discover new and unpredicted protein-disease associations, either via directed hypothesis-driven or via undirected hypothesis-generating target selection, can be created. In the following stages, the discoveries made during these screening phases have to be verified for potential clinical relevance based on both technical and biological aspects. The use of affinity tools throughout discovery and verification has the potential to streamline the introduction of new markers, as transition into clinically required assay formats appears straightforward. In this article, we summarize some of the current building blocks within array- and affinity-based proteomic profiling with a focus on body fluids, by giving a perspective on how current and upcoming developments in this bioscience could enable a path of pursuit for biomarker discovery.