Enhancing Cysteine Chemoproteomic Coverage Through Systematic Assessment of Click Chemistry Product Fragmentation

Mass spectrometry-based chemoproteomics has enabled functional analysis and small molecule screening at thousands of cysteine residues in parallel. Widely adopted chemoproteomic sample preparation workflows rely on the use of pan-cysteine reactive probes such as iodoacetamide alkyne combined with biotinylation via copper-catalyzed azide–alkyne cycloaddition (CuAAC) or ‘click chemistry’ for cysteine capture. Despite considerable advances in both sample preparation and analytical platforms, current techniques only sample a small fraction of all cysteines encoded in the human proteome. Extending the recently introduced labile mode of the MSFragger search engine, here we report an in-depth analysis of cysteine biotinylation via click chemistry (CBCC) reagent gas-phase fragmentation during MS/MS analysis. We find that CBCC conjugates produce both known and novel diagnostic fragments and peptide remainder ions. Among these species, we identified a candidate signature ion for CBCC peptides, the oxonium-biotin fragment ion that is generated upon fragmentation of the N(triazole)–C(alkyl) bond together with cyclization. Guided by our empirical comparison of the fragmentation patterns of five CBCC reagent combinations, we achieved enhanced coverage of cysteine labeled peptides. For larger, fragmentation-prone biotinylation reagents, implementation of labile search afforded unique PSMs and provides a roadmap for the utility of such searches in enhancing chemoproteomic peptide coverage.

Small-molecule suppression of calpastatin degradation reduces neuropathology in models of Huntington’s disease

Mitochondrial dysfunction is a common hallmark of neurological disorders, and reducing mitochondrial damage is considered a promising neuroprotective therapeutic strategy. Here, we used high-throughput small molecule screening to identify CHIR99021 as a potent enhancer of mitochondrial function. CHIR99021 improved mitochondrial phenotypes and enhanced cell viability in several models of Huntington’s disease (HD), a fatal inherited neurodegenerative disorder. Notably, CHIR99201 treatment reduced HD-associated neuropathology and behavioral defects in HD mice and improved mitochondrial function and cell survival in HD patient-derived neurons. Independent of its known inhibitory activity against glycogen synthase kinase 3 (GSK3), CHIR99021 treatment in HD models suppressed the proteasomal degradation of calpastatin (CAST), and subsequently inhibited calpain activation, a well-established effector of neural death, and Drp1, a driver of mitochondrial fragmentation. Our results established CAST-Drp1 as a druggable signaling axis in HD pathogenesis and highlighted CHIR99021 as a mitochondrial function enhancer and a potential lead for developing HD therapies.

Identification of first-in-class plasmodium OTU inhibitors with potent anti-malarial activity

OTU proteases antagonize the cellular defense in the host cells and involve in pathogenesis. Intriguingly, P. falciparum, P. vivax, and P. yoelii have an uncharacterized and highly conserved viral OTU-like proteins. However, their structure, function or inhibitors have not been previously reported. To this end, we have performed structural modeling, small molecule screening, deconjugation assays to characterize and develop first-in-class inhibitors of P. falciparum, P. vivax, and P. yoelii OTU-like proteins. These Plasmodium OTU-like proteins have highly conserved residues in the catalytic and inhibition pockets similar to viral OTU proteins. Plasmodium OTU proteins demonstrated Ubiquitin and ISG15 deconjugation activities as evident by intracellular ubiquitinated protein content analyzed by western blot and flow cytometry. We screened a library of small molecules to determine plasmodium OTU inhibitors with potent anti-malarial activity. Enrichment and correlation studies identified structurally similar molecules. We have identified two small molecules that inhibit P. falciparum, P. vivax, and P. yoelii OTU proteins (IC50 values as low as 30nM) with potent anti-malarial activity (IC50 of 4.1-6.5 μM). We also established enzyme kinetics, druglikeness, ADME, and QSAR model. MD simulations allowed us to resolve how inhibitors interacted with plasmodium OTU proteins. These findings suggest that targeting malarial OTU-like proteases is a plausible strategy to develop new anti-malarial therapies.

Small-molecule metabolome identifies potential therapeutic targets against COVID-19

Background: Respiratory viruses are transmitted and acquired via the nasal mucosa, and thereby may influence the nasal metabolome composed of biochemical products produced by both host cells and microbes. Studies of the nasal metabolome demonstrate virus-specific changes that sometimes correlate with viral load and disease severity. Here, we evaluate the nasopharyngeal metabolome of COVID-19 infected individuals and report several small molecules that may be used as potential therapeutic targets. Specimens were tested by qRT-PCR with target primers for three viruses: Influenza A (INFA), respiratory syncytial virus (RSV), and SARS-CoV-2, along with asymptomatic controls. The nasopharyngeal metabolome was characterized using an LC-MS/MS-based small-molecule screening kit capable of quantifying 141 analytes. A machine learning model identified 28 discriminating analytes and correctly categorized patients with a viral infection with an accuracy of 96% (R2=0.771, Q2=0.72). A second model identified 5 analytes to differentiate COVID19-infected patients from those with INFA or RSV with an accuracy of 85% (R2=0.442, Q2=0.301). Specifically, LysoPCaC18:2 concentration was significantly increased in COVID19 patients (P< 0.0001), whereas beta-hydroxybutyric acid, Met SO, succinic acid, and carnosine concentrations were significantly decreased (P< 0.0001). This study demonstrates that COVID19 infection results in a unique NP metabolomic signature with carnosine and LysoPCaC18:2 as potential therapeutic targets.

Novel AT1R-Allosteric Ligands Mask the Preeclampsia Auto-antibody Epitope and Decrease Angiotensin-induced Vasoconstriction

SummaryMaternal blood pressure regulation by the hormone angiotensin II (AngII) sustains fetal growth through feto-placental circulation. AngII binding to orthosteric pocket in the angiotensin type 1 receptor (AT1R) induces G protein and β-arrestin signaling. AT1R blocking drugs and β-arrestin biased ligands also bind to the orthosteric pocket but evoke different inactive and active states1–6. AT1R-directed auto-antibodies observed in preeclampsia bound outside the orthosteric pocket to extracellular loop-2 (ECL2) of AT1R7–9. How auto-antibodies modulate AT1R activity causing preeclampsia pathogenesis is unknown. Here we report a druggable cryptic allosteric pocket encompassing the preeclampsia epitope on ECL2. Using structure based high-throughput small molecule screening we discovered 18 ligands specific for AT1R’s allosteric pocket. After procuring these ligands we validated inhibition of preeclampsia epitope-specific antibody binding. We characterize their inhibitory effect on antibody and AngII-signaling in cells and vasoconstriction ex vivo. These novel AT1R allosteric ligands, thus act as dual action negative modulators of auto-antibody action and vasoconstriction. Our study demonstrates that positive allosteric modulator action of auto-antibody causes a disease linked to AT1R. We anticipate our findings to kindle structure-based discovery of AT1R allosteric ligands for intervention in maladies such as preeclampsia7–10, rejection of organ transplants11, vasodilatory shock12, 13 and metabolic syndrome14.

Bioinformatic analysis linking genomic defects to chemosensitivity and mechanism of action

A joint analysis of the NCI60 small molecule screening data, their genetically defective genes, and mechanisms of action (MOA) of FDA approved cancer drugs screened in the NCI60 is proposed for identifying links between chemosensitivity, genomic defects and MOA. Self-Organizing-Maps (SOMs) are used to organize the chemosensitivity data. Student’s t-tests are used to identify SOM clusters with enhanced chemosensitivity for tumor cell lines with versus without genetically defective genes. Fisher’s exact and chi-square tests are used to reveal instances where defective gene to chemosensitivity associations have enriched MOAs. The results of this analysis find a relatively small set of defective genes, inclusive of ABL1, AXL, BRAF, CDC25A, CDKN2A, IGF1R, KRAS, MECOM, MMP1, MYC, NOTCH1, NRAS, PIK3CG, PTK2, RPTOR, SPTBN1, STAT2, TNKS and ZHX2, as possible candidates for roles in chemosensitivity for compound MOAs that target primarily, but not exclusively, kinases, nucleic acid synthesis, protein synthesis, apoptosis and tubulin. These results find exploitable instances of enhanced chemosensitivity of compound MOA’s for selected defective genes. Collectively these findings will advance the interpretation of pre-clinical screening data as well as contribute towards the goals of cancer drug discovery, development decision making, and explanation of drug mechanisms.

Brief Guide: Experimental Strategies for High-Quality Hit Selection from Small-Molecule Screening Campaigns

Small-molecule screening is a powerful approach to identify modulators of either specific biological targets or cellular pathways with phenotypic endpoints. A myriad of assay technologies are available to assess the activity of enzymes, monitor protein–protein interactions, measure transcription factor activity in reporter assays, or detect cellular features and activities using high-content imaging. A common challenge during small-molecule screening is, however, the presence of hit compounds generating assay interference, thereby producing false-positive hits. Thus, efforts are needed to uncover such interferences to prioritize high-quality hits for further analysis. This process encompasses (1) computational approaches to flag undesirable compounds, and (2) the use of experimental approaches like counter, orthogonal, and cellular fitness screens to identify and eliminate artifacts. In this brief guide, we provide an overview for first-time users, highlighting experimental screening strategies to prioritize high-quality bioactive hits from high-throughput screening/high-content screening (HTS/HCS) campaigns.