Alexandre Masselot

neXtProt: a knowledge platform for human proteins

neXtProt (http://www.nextprot.org/) is a new human protein-centric knowledge platform. Developed at the Swiss Institute of Bioinformatics (SIB), it aims to help researchers answer questions relevant to human proteins. To achieve this goal, neXtProt is built on a corpus containing both curated knowledge originating from the UniProtKB/Swiss-Prot knowledgebase and carefully selected and filtered high-throughput data pertinent to human proteins. This article presents an overview of the database and the data integration process. We also lay out the key future directions of neXtProt that we consider the necessary steps to make neXtProt the one-stop-shop for all research projects focusing on human proteins.

Exploring glycopeptide-resistance in Staphylococcus aureus: a combined proteomics and transcriptomics approach for the identification of resistance-related markers

BackgroundTo unravel molecular targets involved in glycopeptide resistance, three isogenic strains of Staphylococcus aureus with different susceptibility levels to vancomycin or teicoplanin were subjected to whole-genome microarray-based transcription and quantitative proteomic profiling. Quantitative proteomics performed on membrane extracts showed exquisite inter-experimental reproducibility permitting the identification and relative quantification of >30% of the predicted S. aureus proteome.ResultsIn the absence of antibiotic selection pressure, comparison of stable resistant and susceptible strains revealed 94 differentially expressed genes and 178 proteins. As expected, only partial correlation was obtained between transcriptomic and proteomic results during stationary-phase. Application of massively parallel methods identified one third of the complete proteome, a majority of which was only predicted based on genome sequencing, but never identified to date. Several over-expressed genes represent previously reported targets, while series of genes and proteins possibly involved in the glycopeptide resistance mechanism were discovered here, including regulators, global regulator attenuator, hyper-mutability factor or hypothetical proteins. Gene expression of these markers was confirmed in a collection of genetically unrelated strains showing altered susceptibility to glycopeptides.ConclusionOur proteome and transcriptome analyses have been performed during stationary-phase of growth on isogenic strains showing susceptibility or intermediate level of resistance against glycopeptides. Altered susceptibility had emerged spontaneously after infection with a sensitive parental strain, thus not selected in vitro. This combined analysis allows the identification of hundreds of proteins considered, so far as hypothetical protein. In addition, this study provides not only a global picture of transcription and expression adaptations during a complex antibiotic resistance mechanism but also unravels potential drug targets or markers that are constitutively expressed by resistant strains regardless of their genetic background, amenable to be used as diagnostic targets.

X-Rank: A Robust Algorithm for Small Molecule Identification Using Tandem Mass Spectrometry

The diversity of experimental workflows involving LC-MS/MS and the extended range of mass spectrometers tend to produce extremely variable spectra. Variability reduces the accuracy of compound identification produced by commonly available software for a spectral library search. We introduce here a new algorithm that successfully matches MS/MS spectra generated by a range of instruments, acquired under different conditions. Our algorithm called X-Rank first sorts peak intensities of a spectrum and second establishes a correlation between two sorted spectra. X-Rank then computes the probability that a rank from an experimental spectrum matches a rank from a reference library spectrum. In a training step, characteristic parameter values are generated for a given data set. We compared the efficiency of the X-Rank algorithm with the dot-product algorithm implemented by MS Search from the National Institute of Standards and Technology (NIST) on two test sets produced with different instruments. Overall the X-Rank algorithm accurately discriminates correct from wrong matches and detects more correct substances than the MS Search. Furthermore, X-Rank could correctly identify and top rank eight chemical compounds in a commercially available test mix. This confirms the ability of the algorithm to perform both a straight single-platform identification and a cross-platform library search in comparison to other tools. It also opens the possibility for efficient general unknown screening (GUS) against large compound libraries.

A simple workflow to increase MS2 identification rate by subsequent spectral library search.

Searching a spectral library for the identification of protein MS/MS data has proven to be a fast and accurate method, while yielding a high identification rate. We investigated the potential to increase peptide discovery rate, with little increase in computational time, by constructing a workflow based on a sequence search with Phenyx followed by a library search with SpectraST. Searching a consensus library compiled from the search results of the prior Phenyx search increased the number of confidently matched spectra by up to 156%. Additionally matched spectra by SpectraST included noisy spectra, spectra representing missed cleaved peptides as well as spectra from post‐translationally modified peptides.

SwissPIT: An workflow-based platform for analyzing tandem-MS spectra using the Grid

The identification and characterization of peptides from MS/MS data represents a critical aspect of proteomics. It has been the subject of extensive research in bioinformatics resulting in the generation of a fair number of identification software tools. Most often, only one program with a specific and unvarying set of parameters is selected for identifying proteins. Hence, a significant proportion of the experimental spectra do not match the peptide sequences in the screened database due to inappropriate parameters or scoring schemes. The Swiss protein identification toolbox (swissPIT) project provides the scientific community with an expandable multitool platform for automated in‐depth analysis of MS data also able to handle data from high‐throughput experiments. With swissPIT many problems have been solved: The missing standards for input and output formats (A), creation of analysis workflows (B), unified result visualization (C), and simplicity of the user interface (D). Currently, swissPIT supports four different programs implementing two different search strategies to identify MS/MS spectra. Conceived to handle the calculation‐intensive needs of each of the programs, swissPIT uses the distributed resources of a Swiss‐wide computer Grid (http://www.swing‐grid.ch).

Early activation of the fatty acid metabolism pathway by chronic high glucose exposure in rat insulin secretory β‐cells

Pancreatic β‐cells are responsible for insulin secretion that regulates blood glucose homeostasis. In the development of type II diabetes, a progressive impairment of insulin secretion by the pancreatic β‐cells occurs called β‐cell dysfunction or β‐cell failure. Chronic hyperglycemia has been shown being involved in β‐cell dysfunction, a phenomenon known as glucotoxicity. The molecular mechanisms underlying the impairment of insulin secretion by β‐cells induced by glucotoxicity are still not fully understood. In this work, quantitative proteomics was employed to identify early key players involved in β‐cell dysfunction induced by glucotoxicity. For this, the stable isotope labeling by amino acids in cell culture strategy was used on the slowly‐growing rat β‐cell line INS‐1E. We showed that the stable isotope labeling by amino acids in cell culture approach did not induce any detectable biological effects on these β‐cells, as measured at both the transcriptomic and proteomic levels. Proteins differentially expressed between control cells and cells submitted to chronic high glucose concentrations were identified and verified. The results obtained reinforce the link between glucotoxicity and lipogenesis and suggest that the fatty acid metabolism pathway may rapidly be stimulated in β‐cells submitted to chronic high glucose concentrations.

A Systematic Statistical Analysis of Ion Trap Tandem Mass Spectra in View of Peptide Scoring

Tandem mass spectrometry has become central in proteomics projects. In particular, it is of prime importance to design sensitive and selective score functions to reliably identify peptides in databases. By using a huge collection of 140 000+ peptide MS/MS spectra, we systematically study the importance of many characteristics of a match (peptide sequence/spectrum) to include in a score function. Besides classical match characteristics, we investigate the value of new characteristics such as amino acid dependence and consecutive fragment matches. We finally select a combination of promising characteristics and show that the corresponding score function achieves very low false positive rates while being very sensitive, thereby enabling highly automated peptide identification in large proteomics projects. We compare our results to widely used protein identification systems and show a significant reduction in false positives.

swissPIT: A novel approach for pipelined analysis of mass spectrometry data

The identification and characterization of peptides from tandem mass spectrometry (MS/MS) data represents a critical aspect of proteomics. Today, tandem MS analysis is often performed by only using a single identification program achieving identification rates between 10-50% (Elias and Gygi, 2007). Beside the development of new analysis tools, recent publications describe also the pipelining of different search programs to increase the identification rate (Hartler et al., 2007; Keller et al., 2005). The Swiss Protein Identification Toolbox (swissPIT) follows this approach, but goes a step further by providing the user an expandable multi-tool platform capable of executing workflows to analyze tandem MS-based data. One of the major problems in proteomics is the absent of standardized workflows to analyze the produced data. This includes the pre-processing part as well as the final identification of peptides and proteins. The main idea of swissPIT is not only the usage of different identification tool in parallel, but also the meaningful concatenation of different identification strategies at the same time. The swissPIT is open source software but we also provide a user-friendly web platform, which demonstrates the capabilities of our software and which is available at http://swisspit.cscs.ch upon request for account.

Mass spectrometry has married statistics: uncle is functionality, children are selectivity and sensitivity

Abstract Techniques for analyzing tandem mass spectrometry (MS/MS) data are moving from empirically determined heuristics to standard statistical methods. OLAV, an advanced database search engine, applies statistical models to capture essential structural properties of correct peptide identifications. Moreover, as exemplified by OLAV, it is important to have extended functionalities in MS/MS search engines both to exploit database annotations and to carry out advanced search strategies.

Varapp: A reactive web-application for variants filtering

Summary: Varapp is an open-source web application to filter variants from large sets of exome data stored in a relational database. Varapp offers a reactive graphical user interface, very fast data pro-cessing, security and facility to save, reproduce and shareresults. Typically, a few seconds suffice to apply non-trivial filters to a set of half a million variants and extract a handful of potential clinically relevant targets. Varapp implements different scenarios for Mendelian diseases (dominant, recessive, de novo, X-linked, andcompound heterozygous), and allows searching for variants in genes or chro-mosomal regions of interest. Availability: The application is made of a Javascript front-end and a Python back-end. Its source code is hosted at https://github.com/varapp. A demo version isavailable at https://varapp-demo.vital-it.ch. The full documentation can be found at https://varapp-demo.vital-it.ch/docs. Contact: sylvain.pradervand@chuv.ch