Proteomics Using Protease Alternatives to Trypsin Benefits from Sequential Digestion with Trypsin.

Trypsin is the most used enzyme in proteomics. Nevertheless, proteases with complementary cleavage specificity have been applied in special circumstances. In this work, we analyzed the characteristics of five protease alternatives to trypsin for protein identification and sequence coverage when applied to S. pombe whole cell lysates. The specificity of the protease heavily impacted the number of proteins identified. Proteases with higher specificity led to the identification of more proteins than proteases with lower specificity. However, AspN, GluC, chymotrypsin, and proteinase K largely benefited from being paired with trypsin in sequential digestion, as had been shown by us for elastase before. In the most extreme case, predigesting with trypsin improves the number of identified proteins for proteinase K by 731%. Trypsin predigestion also improved the protein identifications of other proteases, AspN (+62%), GluC (+80%), and chymotrypsin (+21%). Interestingly, the sequential digest with trypsin and AspN yielded even a higher number of protein identifications than digesting with trypsin alone.

An integrated workflow for crosslinking mass spectrometry.

We present a concise workflow to enhance the mass spectrometric detection of crosslinked peptides by introducing sequential digestion and the crosslink identification software xiSEARCH. Sequential digestion enhances peptide detection by selective shortening of long tryptic peptides. We demonstrate our simple 12-fraction protocol for crosslinked multi-protein complexes and cell lysates, quantitative analysis, and high-density crosslinking, without requiring specific crosslinker features. This overall approach reveals dynamic protein-protein interaction sites, which are accessible, have fundamental functional relevance and are therefore ideally suited for the development of small molecule inhibitors.

Common coding variant in SERPINA1 increases the risk for large artery stroke.

Large artery atherosclerotic stroke (LAS) shows substantial heritability not explained by previous genome-wide association studies. Here, we explore the role of coding variation in LAS by analyzing variants on the HumanExome BeadChip in a total of 3,127 cases and 9,778 controls from Europe, Australia, and South Asia. We report on a nonsynonymous single-nucleotide variant in serpin family A member 1 (SERPINA1) encoding alpha-1 antitrypsin [AAT; p.V213A; P = 5.99E-9, odds ratio (OR) = 1.22] and confirm histone deacetylase 9 (HDAC9) as a major risk gene for LAS with an association in the 3'-UTR (rs2023938; P = 7.76E-7, OR = 1.28). Using quantitative microscale thermophoresis, we show that M1 (A213) exhibits an almost twofold lower dissociation constant with its primary target human neutrophil elastase (NE) in lipoprotein-containing plasma, but not in lipid-free plasma. Hydrogen/deuterium exchange combined with mass spectrometry further revealed a significant difference in the global flexibility of the two variants. The observed stronger interaction with lipoproteins in plasma and reduced global flexibility of the Val-213 variant most likely improve its local availability and reduce the extent of proteolytic inactivation by other proteases in atherosclerotic plaques. Our results indicate that the interplay between AAT, NE, and lipoprotein particles is modulated by the gate region around position 213 in AAT, far away from the unaltered reactive center loop (357-360). Collectively, our findings point to a functionally relevant balance between lipoproteins, proteases, and AAT in atherosclerosis.

Sequential Digestion with Trypsin and Elastase in Cross-Linking Mass Spectrometry.

Cross-linking mass spectrometry has become an important approach for studying protein structures and protein-protein interactions. The amino acid compositions of some protein regions impede the detection of cross-linked residues, although it would yield invaluable information for protein modeling. Here, we report on a sequential-digestion strategy with trypsin and elastase to penetrate regions with a low density of trypsin-cleavage sites. We exploited intrinsic substrate-recognition properties of elastase to specifically target larger tryptic peptides. Our application of this protocol to the TAF4-12 complex allowed us to identify cross-links in previously inaccessible regions.

Impact of inflammatory preconditioning on murine microglial proteome response induced by focal ischemic brain injury.

Preconditioning with lipopolysaccharide (LPS) induces neuroprotection against subsequent cerebral ischemic injury, mainly involving innate immune pathways. Microglia are resident immune cells of the central nervous system (CNS) that respond early to danger signals through memory-like differential reprogramming. However, the cell-specific molecular mechanisms underlying preconditioning are not fully understood. To elucidate the distinct molecular mechanisms of preconditioning on microglia, we compared these cell-specific proteomic profiles in response to LPS preconditioning and without preconditioning and subsequent transient focal brain ischemia and reperfusion, - using an established mouse model of transient focal brain ischemia and reperfusion. A proteomic workflow, based on isolated microglia obtained from mouse brains by cell sorting and coupled to mass spectrometry for identification and quantification, was applied. Our data confirm that LPS preconditioning induces marked neuroprotection, as indicated by a significant reduction in brain infarct volume. The established brain cell separation method was suitable for obtaining an enriched microglial cell fraction for valid proteomic analysis. The results show a significant impact of LPS preconditioning on microglial proteome patterns by type I interferons, presumably driven by the interferon cluster regulator proteins signal transducer and activator of transcription1/2 (STAT1/2).

The NLRP3/ASC/Caspase-1 axis regulates IL-1ß processing in neutrophils.

Neutrophils play a pivotal role in the defense against bacterial, viral and fungal infections and are important mediators in the acute inflammatory response. At the same time, neutrophils are also in volved in sterile inflammatory responses that are triggered by endogenous ligands. A series of immediate effector functions and the expression of proinflammatory genes enable neutrophils to initiate the immune response against the injurious agent. Among these, interleukin-1ß (IL-1ß) plays a key role in the orchestration of the inflammatory response. Induction of IL-1ß expression leads to production of cytosolic pro-IL-1ß, which requires further processing by a proteolytic cleavage event. Caspase-1 was initially identified as the main IL-1ß-converting enzyme, and the upstream events leading to caspase-1 activation were identified as so-called inflammasome complexes. Up to now, the inflammasome system has mainly been studied in macrophages, whereas the inflammasome was thought to play a redundant or no role in the cell intrinsic processing of pro-IL-1ß in neutrophils. Here, we identify the expression of the components of the NLRP3 inflammasome complex in neutrophils and show that the NLRP3 inflammasome pathway is indeed operational in neutrophils. Our findings establish the NLRP3 inflammasome as a key step in the secretion of matured IL-1ß by neutrophils.

Protein structure prediction with in-cell photo-crosslinking mass spectrometry and deep learning.

While AlphaFold2 can predict accurate protein structures from the primary sequence, challenges remain for proteins that undergo conformational changes or for which few homologous sequences are known. Here we introduce AlphaLink, a modified version of the AlphaFold2 algorithm that incorporates experimental distance restraint information into its network architecture. By employing sparse experimental contacts as anchor points, AlphaLink improves on the performance of AlphaFold2 in predicting challenging targets. We confirm this experimentally by using the noncanonical amino acid photo-leucine to obtain information on residue-residue contacts inside cells by crosslinking mass spectrometry. The program can predict distinct conformations of proteins on the basis of the distance restraints provided, demonstrating the value of experimental data in driving protein structure prediction. The noise-tolerant framework for integrating data in protein structure prediction presented here opens a path to accurate characterization of protein structures from in-cell data.

Osteocytes Serve as a Reservoir for Intracellular Persisting Staphylococcus aureus Due to the Lack of Defense Mechanisms.

Chronic staphylococcal osteomyelitis can persist for long time periods causing bone destruction. The ability of Staphylococcus aureus to develop chronic infections is linked to its capacity to invade and replicate within osteoblasts and osteocytes and to switch to a dormant phenotype called small colony variants. Recently, osteocytes were described as a main reservoir for this pathogen in bone tissue. However, the mechanisms involved in the persistence of S. aureus within these cells are still unknown. Here, we investigated the interaction between S. aureus and osteoblasts or osteocytes during infection. While osteoblasts are able to induce a strong antimicrobial response and eliminate intracellular S. aureus, osteocytes trigger signals to recruit immune cells and enhance inflammation but fail an efficient antimicrobial activity to clear the bacterial infection. Moreover, we found that extracellular signals from osteocytes enhance intracellular bacterial clearance by osteoblasts. Even though both cell types express Toll-like receptor (TLR) 2, the main TLR responsible for S. aureus detection, only osteoblasts were able to increase TLR2 expression after infection. Additionally, proteomic analysis indicates that reduced intracellular bacterial killing activity in osteocytes is related to low antimicrobial peptide expression. Nevertheless, high levels of lipid mediators and cytokines were secreted by osteocytes, suggesting that they can contribute to inflammation. Taken together, our results demonstrate that osteocytes contribute to severe inflammation observed in osteomyelitis and represent the main niche for S. aureus persistence due to their poor capacity for intracellular antimicrobial response.

Mapping protein carboxymethylation sites provides insights into their role in proteostasis and cell proliferation.

Posttranslational mechanisms play a key role in modifying the abundance and function of cellular proteins. Among these, modification by advanced glycation end products has been shown to accumulate during aging and age-associated diseases but specific protein targets and functional consequences remain largely unexplored. Here, we devise a proteomic strategy to identify sites of carboxymethyllysine modification, one of the most abundant advanced glycation end products. We identify over 1000 sites of protein carboxymethylation in mouse and primary human cells treated with the glycating agent glyoxal. By using quantitative proteomics, we find that protein glycation triggers a proteotoxic response and indirectly affects the protein degradation machinery. In primary endothelial cells, we show that glyoxal induces cell cycle perturbation and that carboxymethyllysine modification reduces acetylation of tubulins and impairs microtubule dynamics. Our data demonstrate the relevance of carboxymethyllysine modification for cellular function and pinpoint specific protein networks that might become compromised during aging.

C/EBPß expression in ALK-positive anaplastic large cell lymphomas is required for cell proliferation and is induced by the STAT3 signaling pathway.

BACKGROUND: Anaplastic lymphoma kinase (ALK)-positive anaplastic large cell lymphoma is characterized by the t(2;5) chromosomal translocation, resulting in the expression of a fusion protein formed of nucleophosmin (NPM) and ALK. Recently, we reported the abnormal expression of the transcription factor CCAAT/enhancer binding protein-beta (C/EBPbeta) in ALK-positive anaplastic large cell lymphomas, and demonstrated its dependence on NPM-ALK activity. DESIGN AND METHODS: In this study, the role of C/EBPbeta in proliferation and survival of ALK-positive anaplastic large cell lymphomas was investigated, as well as the mechanism of its expression and activity. Highly effective short hairpin RNA sequences and/or pharmacological inhibitors were used to abrogate the expression or activity of C/EBPbeta, signal transducer and activator of transcription 3 (STAT3), AKT, extracellular signal-related kinase 1/2 (ERK1/2) and mammalian target of rapamycin (mTOR). RESULTS: Interference with C/EBPbeta expression resulted in a dramatic decrease in cell proliferation in ALK-positive anaplastic large cell lymphomas, with a mild induction of apoptosis after 6 days. Down-regulation of STAT3 resulted in a marked decrease in C/EBPbeta mRNA and protein levels with impairment in cell proliferation and viability, underscoring the important role of these two proteins in ALK-mediated oncogenesis. Additionally, we demonstrated that reduction of ERK1/2 activity led to C/EBPbeta Thr(235) dephosphorylation and moderate growth retardation. The AKT/mTOR signaling pathway did not have any influence on C/EBPbeta expression or C/EBPbeta phosphorylation. CONCLUSIONS: These findings reveal the convergence of STAT3 and ERK1/2 signaling pathways activated by NPM-ALK in mediating the regulation of C/EBPbeta expression, a transcription factor central to NPM-ALK transformation.

Tailor-made inflammation: how neutrophil serine proteases modulate the inflammatory response.

Neutrophil granulocytes are important mediators of innate immunity, but also participate in the pathogenesis of (auto)inflammatory diseases. Neutrophils express a specific set of proteolytic enzymes, the neutrophil serine proteases (NSPs), which are stored in cytoplasmic granules and can be secreted into the extra- and pericellular space upon cellular activation. These NSPs, namely cathepsin G (CG), neutrophil elastase (NE), and proteinase 3 (PR3), have early been implicated in bacterial defense. However, NSPs also regulate the inflammatory response by specifically altering the function of cytokines and chemokines. For instance, PR3 and NE both inactivate the anti-inflammatory mediator progranulin, which may play a role in chronic inflammation. Here, we provide a concise update on NSPs as modulators of inflammation and discuss the biological and pathological significance of this novel function of NSPs. Mounting evidence support an important proinflammatory function for PR3, which may have been underestimated in the past.