Parallel reaction monitoring on a Q Exactive mass spectrometer increases reproducibility of phosphopeptide detection in bacterial phosphoproteomics measurements

Targeted proteomics addresses selectivity and complexity of protein degradation by autophagy

Macroautophagy/autophagy is a constitutively active catabolic lysosomal degradation pathway, often found dysregulated in human diseases. It is often considered to act in a cytoprotective manner and is commonly upregulated in cells undergoing stress. Its initiation is regulated at the protein level and does not require de novo protein synthesis. Historically, autophagy has been regarded as nonselective; however, it is now clear that different stimuli can lead to the selective degradation of cellular components via selective autophagy receptors (SARs). Due to its selective nature and the existence of multiple degradation pathways potentially acting in concert, monitoring of autophagy flux, i.e. selective autophagy-dependent protein degradation, should address this complexity. Here, we introduce a targeted proteomics approach monitoring abundance changes of 37 autophagy-related proteins covering process-relevant proteins such as the initiation complex and the Atg8-family protein lipidation machinery, as well as most known SARs. We show that proteins involved in autophagosome biogenesis are upregulated and spared from degradation under autophagy-inducing conditions in contrast to SARs, in a cell-line dependent manner. Classical bulk stimuli such as nutrient starvation mainly induce degradation of ubiquitin-dependent soluble SARs and not of ubiquitin-independent, membrane-bound SARs. In contrast, treatment with the iron chelator deferiprone leads to the degradation of ubiquitin-dependent and -independent SARs linked to mitophagy and reticulophagy/ER-phagy. Our approach is automatable and supports large-scale screening assays paving the way to (pre)clinical applications and monitoring of specific autophagy flux.Abbreviation: AMBRA1: autophagy and beclin 1 regulator 1; ATG: autophagy related; BafA1: bafilomycin A1; BNIP1: BCL2 interacting protein 1; BNIP3: BCL2 interacting protein 3; BNIP3L/NIX: BCL2 interacting protein 3-like; CALCOCO2/NDP52: calcium binding and coiled-coil domain 2; CCPG1: cell cycle progression 1; CV: coefficients of variations; CCCP: carbonyl cyanide m-chlorophenyl hydrazone; DFP: deferiprone; ER: endoplasmic reticulum; FKBP8: FKBP prolyl isomerase 8; GABARAPL: GABA type A receptor associated protein like; LC: liquid chromatography; LOD: limit of detection; LOQ: limit of quantification; MAP1LC3: microtubule associated protein 1 light chain 3; MS: mass spectrometry; NCOA4: nuclear receptor coactivator 4; NBR1: NBR1 autophagy cargo receptor; NUFIP1: nuclear FMR1 interacting protein 1; OPTN: optineurin; PHB2: prohibitin 2; PNPLA2/ATGL: patatin like phospholipase domain containing 2; POI: protein of interest; PTM: posttranslational modification; PRM: parallel reaction monitoring; RB1CC1/FIP200: RB1 inducible coiled-coil 1; RETREG1/FAM134B: reticulophagy regulator 1; RPS6KB1: ribosomal protein S6 kinase B1; RTN3: reticulon 3; SARs: selective autophagy receptors; SQSTM1/p62: sequestosome 1; STBD1: starch binding domain 1; TAX1BP1: Tax1 binding protein 1; TFEB: transcription factor EB; TNIP1: TNFAIP3 interacting protein 1; TOLLIP: toll interacting protein; ULK1: unc-51 like autophagy activating kinase 1; WBP2: WW domain binding protein 2; WDFY3/Alfy: WD repeat and FYVE domain containing 3; WIPI2: WD repeat domain, phosphoinositide interacting 2.

The ABL-MYC axis controls WIPI1-enhanced autophagy in lifespan extension

Human WIPI β-propellers function as PI3P effectors in autophagy, with WIPI4 and WIPI3 being able to link autophagy control by AMPK and TORC1 to the formation of autophagosomes. WIPI1, instead, assists WIPI2 in efficiently recruiting the ATG16L1 complex at the nascent autophagosome, which in turn promotes lipidation of LC3/GABARAP and autophagosome maturation. However, the specific role of WIPI1 and its regulation are unknown. Here, we discovered the ABL-ERK-MYC signalling axis controlling WIPI1. As a result of this signalling, MYC binds to the WIPI1 promoter and represses WIPI1 gene expression. When ABL-ERK-MYC signalling is counteracted, increased WIPI1 gene expression enhances the formation of autophagic membranes capable of migrating through tunnelling nanotubes to neighbouring cells with low autophagic activity. ABL-regulated WIPI1 function is relevant to lifespan control, as ABL deficiency in C. elegans increased gene expression of the WIPI1 orthologue ATG-18 and prolonged lifespan in a manner dependent on ATG-18. We propose that WIPI1 acts as an enhancer of autophagy that is physiologically relevant for regulating the level of autophagic activity over the lifespan.

DeepFLR facilitates false localization rate control in phosphoproteomics

Protein phosphorylation is a post-translational modification crucial for many cellular processes and protein functions. Accurate identification and quantification of protein phosphosites at the proteome-wide level are challenging, not least because efficient tools for protein phosphosite false localization rate (FLR) control are lacking. Here, we propose DeepFLR, a deep learning-based framework for controlling the FLR in phosphoproteomics. DeepFLR includes a phosphopeptide tandem mass spectrum (MS/MS) prediction module based on deep learning and an FLR assessment module based on a target-decoy approach. DeepFLR improves the accuracy of phosphopeptide MS/MS prediction compared to existing tools. Furthermore, DeepFLR estimates FLR accurately for both synthetic and biological datasets, and localizes more phosphosites than probability-based methods. DeepFLR is compatible with data from different organisms, instruments types, and both data-dependent and data-independent acquisition approaches, thus enabling FLR estimation for a broad range of phosphoproteomics experiments.

Progress in Targeted Mass Spectrometry (Parallel Accumulation-Serial Fragmentation) and Its Application in Plasma/Serum Proteomics

Targeted mass spectrometry using multiple reaction monitoring (MRM) or parallel reaction monitoring (PRM) has been commonly used for protein biomarker validation in plasma, serum, or other clinically relevant specimens due to its high specificity, selectivity, and multiplexing capability compared with immunoassays. As the emerging mode termed parallel accumulation-serial fragmentation (prmPASEF) significantly improved analyte throughput (100-1000), sensitivity (attomole level), and acquisition speed, it promises to broaden the application of targeted mass spectrometry to simultaneous biomarker discovery and validation with high accuracy. Here, we summarize the general approach of the MRM and PRM techniques used for serum/plasma proteomics and describe a detailed step-by-step procedure for the development of MRM/PRM assays for secreted proteins.

Identification of peptides of cinobufacini by gel filter chromatography and peptidomics

Aqueous extract of toad skin (named as Cinobufacini or Huachansu) provides plentiful sources of bioactive peptides that remain undetected and unidentified. High-resolution mass spectrometry-based peptidomics platforms have developed into a major approach to the discovery of natural peptides, with data-dependent acquisition modes providing a wealth of peptide profiling information. In this study, we used a gel- and HLB (a solid phase extraction cartridge)-based two-dimensional separation and purification system and nano-liquid chromatography-tandem mass spectrometry-based peptidomic studies with homology matching for the identification of peptides from Cinobufacini. We evaluated 232 multi-charged peptides and found several specific peptides, some of which were validated by target parallel reaction monitoring mode. These peptides are the first to be identified in Cinobufacini and are completely different from ones identified in toad venom. So, this mapping provides key peptide information for the quality control of Bufo bufo gargarizans skin and its preparation.

A Strategy for Rapid Discovery of Marker Peptides Associated with Fibrinolytic Efficacy of Pheretima aspergillum Based on Bioinformatics Combined with Parallel Reaction Monitoring

Quality control of animal-derived traditional Chinese medicines has improved dramatically as proteomics research advanced in the past few decades. However, it remains challenging to identify quality attributes with routine proteomics approaches since protein with fibrinolytic activity is rarely reported in pheretima, a typical animal-derived traditional medicine. A novel strategy based on bioinformatics combined with parallel reaction monitoring (PRM) was developed here to rapidly discover the marker peptides associated with a fibrinolytic effect. Potential marker peptides were found by lumbrokinase sequences’ alignment and in silico digestion. The fibrinogen zymography was used to visually identify fibrinolytic proteins in pheretima. As a result, it was found that the fibrinolytic activity varied among different portions of pheretima. Fibrinolytic proteins were distributed regionally in the anterior and anterior-mid portion and there was no significant fibrinogenolytic activity observed in the mid-posterior and posterior portion. Finally, PRM experiments were deployed to validate and quantify selected marker peptides and a total of 11 peptides were identified as marker peptides, which could be potentially used in quality control of pheretima. This strategy provides a robust workflow to benefit the quality control of other animal-derived traditional medicines.

Importance of protein Ser/Thr/Tyr phosphorylation for bacterial pathogenesis

Protein phosphorylation regulates a large variety of biological processes in all living cells. In pathogenic bacteria, the study of serine, threonine, and tyrosine (Ser/Thr/Tyr) phosphorylation has shed light on the course of infectious diseases, from adherence to host cells to pathogen virulence, replication, and persistence. Mass spectrometry (MS)-based phosphoproteomics has provided global maps of Ser/Thr/Tyr phosphosites in bacterial pathogens. Despite recent developments, a quantitative and dynamic view of phosphorylation events that occur during bacterial pathogenesis is currently lacking. Temporal, spatial, and subpopulation resolution of phosphorylation data is required to identify key regulatory nodes underlying bacterial pathogenesis. Herein, we discuss how technological improvements in sample handling, MS instrumentation, data processing, and machine learning should improve bacterial phosphoproteomic datasets and the information extracted from them. Such information is expected to significantly extend the current knowledge of Ser/Thr/Tyr phosphorylation in pathogenic bacteria and should ultimately contribute to the design of novel strategies to combat bacterial infections.

Proteomic analysis of Aspergillus niger 3.316 under heat stress

β-Glucosidase production by Aspergillus niger is accompanied by an inevitable temperature increase in the industrial fermentation environment. Hence, the synthetic process of β-glucosidase is negatively affected. However, our understanding of the heat stress response (HSR) mechanism in A. niger is still incomplete. The current study explored the intracellular proteome profile of A. niger 3.316 in group T (50°C stress) and group C (30°C control) using two proteomic approaches (isobaric tags for relative and absolute quantitation [iTRAQ] and label-free) and examined the expression of four proteins using a parallel reaction monitoring (PRM) approach. Based on the result of the iTRAQ proteomic analysis, 1,025 proteins were differentially expressed in group T compared to group C. Using the label-free approach, we only focused on 77 proteins with significant changes in their protein expression levels. In addition, we performed bioinformatics analysis on all these proteins and obtained detailed gene ontology (GO) enrichment and Kyoto encyclopedia of genes and genomes (KEGG) pathway results. Under heat stress conditions, the relative expression levels of proteins with protection and repair functions were upregulated in A. niger 3.316. These proteins were involved in metabolic pathways, oxidative phosphorylation, porphyrin and chlorophyll metabolism, pyruvate metabolism, and the citrate cycle (TCA cycle). The insights obtained from the presented proteomics and bioinformatics analyses can be used to further explore the HSR mechanism of A. niger.

Phosphoproteomic Approaches to Discover Novel Substrates of Mycobacterial Ser/Thr Protein Kinases

Mycobacterial Ser/Thr protein kinases (STPKs) play a critical role in signal transduction pathways that ultimately determine mycobacterial growth and metabolic adaptation. Identification of key physiological substrates of these protein kinases is, therefore, crucial to better understand how Ser/Thr phosphorylation contributes to mycobacterial environmental adaptation, including response to stress, cell division, and host-pathogen interactions. Various substrate detection methods have been employed with limited success, with direct targets of STPKs remaining elusive. Recently developed mass spectrometry (MS)-based phosphoproteomic approaches have expanded the list of potential STPK substrate identifications, yet further investigation is required to define the most functionally significant phosphosites and their physiological importance. Prior to the application of MS workflows, for instance, GarA was the only known and validated physiological substrate for protein kinase G (PknG) from pathogenic mycobacteria. A subsequent list of at least 28 candidate PknG substrates has since been reported with the use of MS-based analyses. Herein, we integrate and critically review MS-generated datasets available on novel STPK substrates and report new functional and subcellular localization enrichment analyses on novel candidate protein kinase A (PknA), protein kinase B (PknB) and PknG substrates to deduce the possible physiological roles of these kinases. In addition, we assess substrate specificity patterns across different mycobacterial STPKs by analyzing reported sets of phosphopeptides, in order to determine whether novel motifs or consensus regions exist for mycobacterial Ser/Thr phosphorylation sites. This review focuses on MS-based techniques employed for STPK substrate identification in mycobacteria, while highlighting the advantages and challenges of the various applications.

Advanced Proteomics as a Powerful Tool for Studying Toxins of Human Bacterial Pathogens

Exotoxins contribute to the infectious processes of many bacterial pathogens, mainly by causing host tissue damages. The production of exotoxins varies according to the bacterial species. Recent advances in proteomics revealed that pathogenic bacteria are capable of simultaneously producing more than a dozen exotoxins. Interestingly, these toxins may be subject to post-transcriptional modifications in response to environmental conditions. In this review, we give an outline of different bacterial exotoxins and their mechanism of action. We also report how proteomics contributed to immense progress in the study of toxinogenic potential of pathogenic bacteria over the last two decades.

Import of extracellular ATP in yeast and man modulates AMPK and TORC1 signalling

AMP-activated kinase (AMPK) and target of rapamycin (TOR) signalling coordinate cell growth, proliferation, metabolism and cell survival with the nutrient environment of cells. The poor vasculature and nutritional stress experienced by cells in solid tumours raises the question: how do they assimilate sufficient nutrients to survive? Here, we show that human and fission yeast cells import ATP and AMP from their external environment to regulate AMPK and TOR signalling. Exposure of fission yeast (Schizosaccharomyces pombe) and human cells to external AMP impeded cell growth; however, in yeast this restraining impact required AMPK. In contrast, external ATP rescued the growth defect of yeast mutants with reduced TORC1 signalling; furthermore, exogenous ATP transiently enhanced TORC1 signalling in both yeast and human cell lines. Addition of the PANX1 channel inhibitor probenecid blocked ATP import into human cell lines suggesting that this channel may be responsible for both ATP release and uptake in mammals. In light of these findings, it is possible that the higher extracellular ATP concentration reported in solid tumours is both scavenged and recognized as an additional energy source beneficial for cell growth.