Rapid Covalent-Probe Discovery by Electrophile-Fragment Screening.

Covalent probes can display unmatched potency, selectivity, and duration of action; however, their discovery is challenging. In principle, fragments that can irreversibly bind their target can overcome the low affinity that limits reversible fragment screening, but such electrophilic fragments were considered nonselective and were rarely screened. We hypothesized that mild electrophiles might overcome the selectivity challenge and constructed a library of 993 mildly electrophilic fragments. We characterized this library by a new high-throughput thiol-reactivity assay and screened them against 10 cysteine-containing proteins. Highly reactive and promiscuous fragments were rare and could be easily eliminated. In contrast, we found hits for most targets. Combining our approach with high-throughput crystallography allowed rapid progression to potent and selective probes for two enzymes, the deubiquitinase OTUB2 and the pyrophosphatase NUDT7. No inhibitors were previously known for either. This study highlights the potential of electrophile-fragment screening as a practical and efficient tool for covalent-ligand discovery.

Native Mass Spectrometry of Recombinant Proteins from Crude Cell Lysates.

Determining the properties of proteins prior to purification saves time and labor. Here, we demonstrate a native mass spectrometry approach for rapid characterization of overexpressed proteins directly in crude cell lysates. The method provides immediate information on the identity, solubility, oligomeric state, overall structure, and stability, as well as ligand binding, without the need for purification.

Multifunctional Magnetic Particles for Combined Circulating Tumor Cells Isolation and Cellular Metabolism Detection.

We for the first time demonstrate multi-functional magnetic particles based rare cell isolation combined with the downstream laser desorption/ionization mass spectrometry (LDI-MS) to measure the metabolism of enriched circulating tumor cells (CTCs). The characterization of CTCs metabolism plays a significant role in understanding the tumor microenvironment, through exploring the diverse cellular process. However, characterizing cell metabolism is still challenging due to the low detection sensitivity, high sample complexity, and tedious preparation procedures, particularly for rare cells analysis in clinical study. Here we conjugate ferric oxide magnetic particles with anti-EpCAM on the surface for specific, efficient enrichment of CTCs from PBS and whole blood with cells concentration of 6-100 cells per mL. Moreover, these hydrophilic particles as matrix enable sensitive and selective LDI-MS detection of small metabolites (MW<500 Da) in complex bio-mixtures and can be further coupled with isotopic quantification to monitor selected molecules metabolism of ~50 CTCs. Our unique approach couples the immunomagnetic separation of CTCs and LDI-MS based metabolic analysis, which represents a key step forward for downstream metabolites analysis of rare cells to investigate the biological features of CTCs and their cellular responses in both pathological and physiological phenomena.

Ultrasensitive Detection of Low-Abundance Protein Biomarkers by Mass Spectrometry Signal Amplification Assay.

A mass spectrometry signal amplification method is developed for the ultrasensitive and selective detection of low-abundance protein biomarkers by utilizing tag molecules on gold nanoparticles (AuNPs). EpCAM and thrombin as model targets are captured by specific aptamers immobilized on the AuNPs. With laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF MS), the mass tag molecules are detected to represent the protein biomarkers. Benefiting from the MS signal amplification, the assay can achieve a limit of detection of 100 aM. The method is further applied to detect thrombin in fetal bovine serum and EpCAM in cell lysates to demonstrate its selectivity and feasibility in complex biological samples. With the high sensitivity and specificity, the protocol shows great promise for providing a new route to single-cell analysis and early disease diagnosis.

Direct imaging of elemental distributions in tissue sections by laser ablation mass spectrometry.

We present a strategy for imaging of elements in biological tissues using laser ablation (LA) mass spectrometry (MS), which was compared to laser ablation inductively coupled plasma (LA-ICP) MS. Both methods were adopted for quantitative imaging of elements in mouse kidney, as well as traumatic brain injury model tissue sections. MS imaging (MSI) employing LA provides quantitative data by comparing signal abundances of sodium from tissues to those obtained by imaging quantitation calibration standards of the target element applied to adjacent control tissue sections. LA-ICP MSI provided quantitative data for several essential elements in both brain and kidney tissue sections using a dried-droplet approach. Both methods were used to image a rat model of traumatic brain injury, revealing accumulations of sodium and calcium in the impact area and its peripheral regions. LA MSI is shown to be a viable option for quantitative imaging of specific elements in biological tissue sections.

Porous silica enhanced proteolysis during Off-Gel separation for efficient protein identification.

An advanced approach is developed in this work for simultaneous on-line separation and digestion of proteins by combining the Off-Gel isoelectric focusing (IEF) and enzymatic nanoreactor enhanced proteolysis. The nanoreactor was prepared by preloading trypsin in amino-functionalized macroporous silica, and then directly added into Off-Gel wells. With the nanoreactor loaded Off-Gel device, effective digestion of proteins happened during IEF electrophoresis to generate directly fractionated tryptic peptides, which not only accelerated the experimental flow but also avoided sample loss, leading to a more comprehensive protein identification from complex biological samples. A successful identification of 3592 proteins was achieved from Hela cell line by using the approach followed with LC-MS/MS analysis, while only 1877 proteins were identified from the same sample when using standard in-solution proteolysis followed with Off-Gel electrophoresis and then LC-MS/MS analysis. Therefore, we have demonstrated that the approach can greatly simplify high-throughput proteomics and significantly improve protein identification.

Designer SiO2@Au nanoshells towards sensitive and selective detection of small molecules in laser desorption ionization mass spectrometry.

We report a novel platform using optimized SiO2@Au core-shell structures as matrices for highly efficient laser desorption/ionization mass spectrometry analysis of small biomolecules (MW<700 Da). Owing to the designer structure, SiO2@Au nanoshells can achieve low detection-of-limits (~pmol-fmol) in mass spectrometry and selective laser desorption/ionization in bio-mixtures towards diverse small molecules. By further surface modification with aptamers, Apt-SiO2@Au nanoshells allowed simultaneously targeted enrichment and detection of kanamycin with a detection limit at 200 pM. Our work not only starts new applications of SiO2@Au nanoshells in mass spectrometry, but also contributes to advanced analysis of either a group of small molecules or one target small molecule from complex bio-samples in a pre-designed manner for bio-diagnostics. FROM THE CLINICAL EDITOR: Existing methods for the detection of small molecules are often not sensitive enough. Here, the authors developed aptamer functionalized SiO2@Au nanoshells for use in mass spectrometry, with very low detection limits. The new platform appeared to be simple and efficient and should be applicable in detection of clinical samples.

Amino-functionalized macroporous silica for efficient tryptic digestion in acidic solutions.

Amino-functionalized macroporous silica foam (NH2 -MOSF) has been developed as a host reactor to realize highly efficient proteolysis in acidic solutions where normal tryptic reactions cannot occur. The digestion protocol consists simply of adding the functionalized NH2 -MOSF into the protein and trypsin solutions without altering the bulk pH or preloading the enzymes on the materials. With this protocol, digestion of sample fractions from LC can be efficiently realized in the acidic solutions directly. Digestion of a protein fraction extracted from rat liver tissue after LC separation was performed to illustrate this principle, where 103 proteins were successfully identified at pH 3 after 1.5 h of tryptic digestion.

Periodic mesoporous organosilica as a multifunctional nanodevice for large-scale characterization of membrane proteins.

A versatile protocol has been developed for large-scale characterization of hydrophobic membrane proteins based on the periodic mesoporous organosilica (PMO) acting as both an extractor for hydrophobic substrate capture and a nanoreactor for efficient in situ digestion. With introduction of organic groups in the pore frameworks and the presence of hydrophilic silanol groups on the surface, PMO can be well-dispersed into not only an organic solution to concentrate the dissolved membrane proteins but also an aqueous solution containing enzymes for sequential rapid proteolysis in the nanopores. The unique amphiphilic property of PMO ensures a facile switch in different solutions to realize the processes of substrate dissolution, enrichment, and digestion effectively. Furthermore, this novel PMO-assisted protocol has been successfully applied for identification of complex membrane proteins extracted from mouse liver as proof of general applicability.

Label-free fluorescent sensor for mercury(II) ion by using carbon nanotubes to reduce background signal.

A simple, selective and sensitive turn-on fluorescent sensor for the detection of mercury(II) ion was developed using Sybr Green I as the signal reporter and SWCNTs as the quencher. Due to the affinity of SWCNTs towards ssDNA and organic dye, Sybr Green I, thymine-rich ssDNA and SWCNTs could form a self-assembly of three components, resulting in fluorescence quenching. Upon addition of another thymine-rich ssDNA and mercury(II) ion, formation of dsDNA via T-Hg(2+)-T base pairs enabled Sybr Green I to intercalate into the dsDNA, resulting in the restoration of fluorescence. SWCNTs were found to reduce the background signal and improve the analytical sensitivity. A linear relationship between the fluorescence intensity and the concentration of mercury(II) ion was observed in the range of 20-1250 nM (R = 0.9985) with a detection limit of 7.9 nM. The proposed method was applied to detect mercury(II) ion in tap water samples with good results.