New Quantitative Mass Spectrometry Approaches Reveal Different ADP-ribosylation Phases Dependent On the Levels of Oxidative Stress

SERBP1 interacts with PARP1 and is present in PARylation-dependent protein complexes regulating splicing, cell division, and ribosome biogenesis

RNA binding proteins (RBPs) containing intrinsically disordered regions (IDRs) are present in diverse molecular complexes where they function as dynamic regulators. Their characteristics promote liquid-liquid phase separation (LLPS) and the formation of membraneless organelles such as stress granules and nucleoli. IDR-RBPs are particularly relevant in the nervous system and their dysfunction is associated with neurodegenerative diseases and brain tumor development. Serpine1 mRNA-binding protein 1 (SERBP1) is a unique member of this group, being mostly disordered and lacking canonical RNA-binding domains. We defined SERBP1's interactome, uncovered novel roles in splicing, cell division and ribosomal biogenesis, and showed its participation in pathological stress granules and Tau aggregates in Alzheimer's brains. SERBP1 preferentially interacts with other G-quadruplex (G4) binders, implicated in different stages of gene expression, suggesting that G4 binding is a critical component of SERBP1 function in different settings. Similarly, we identified important associations between SERBP1 and PARP1/polyADP-ribosylation (PARylation). SERBP1 interacts with PARP1 and its associated factors and influences PARylation. Moreover, protein complexes in which SERBP1 participates contain mostly PARylated proteins and PAR binders. Based on these results, we propose a feedback regulatory model in which SERBP1 influences PARP1 function and PARylation, while PARylation modulates SERBP1 functions and participation in regulatory complexes.

SERBP1 interacts with PARP1 and is present in PARylation-dependent protein complexes regulating splicing, cell division, and ribosome biogenesis

RNA binding proteins (RBPs) containing intrinsically disordered regions (IDRs) are present in diverse molecular complexes where they function as dynamic regulators. Their characteristics promote liquid-liquid phase separation (LLPS) and the formation of membraneless organelles such as stress granules and nucleoli. IDR-RBPs are particularly relevant in the nervous system and their dysfunction is associated with neurodegenerative diseases and brain tumor development. Serpine1 mRNA-binding protein 1 (SERBP1) is a unique member of this group, being mostly disordered and lacking canonical RNA-binding domains. We defined SERBP1's interactome, uncovered novel roles in splicing, cell division and ribosomal biogenesis, and showed its participation in pathological stress granules and Tau aggregates in Alzheimer's brains. SERBP1 preferentially interacts with other G-quadruplex (G4) binders, implicated in different stages of gene expression, suggesting that G4 binding is a critical component of SERBP1 function in different settings. Similarly, we identified important associations between SERBP1 and PARP1/polyADP-ribosylation (PARylation). SERBP1 interacts with PARP1 and its associated factors and influences PARylation. Moreover, protein complexes in which SERBP1 participates contain mostly PARylated proteins and PAR binders. Based on these results, we propose a feedback regulatory model in which SERBP1 influences PARP1 function and PARylation, while PARylation modulates SERBP1 functions and participation in regulatory complexes.

ADP-ribosylation: An emerging direction for disease treatment

ADP-ribosylation (ADPr) is a dynamically reversible post-translational modification (PTM) driven primarily by ADP-ribosyltransferases (ADPRTs or ARTs), which have ADP-ribosyl transfer activity. ADPr modification is involved in signaling pathways, DNA damage repair, metabolism, immunity, and inflammation. In recent years, several studies have revealed that new targets or treatments for tumors, cardiovascular diseases, neuromuscular diseases and infectious diseases can be explored by regulating ADPr. Here, we review the recent research progress on ART-mediated ADP-ribosylation and the latest findings in the diagnosis and treatment of related diseases.

A Combined Gas-Phase Separation Strategy for ADP-ribosylated Peptides

ADP-ribosylation (ADPr) is a post-translational modification that is best studied using mass spectrometry. Method developments that are permissive with low inputs or baseline levels of protein ribosylation represent the next frontier in the field. High-field asymmetric waveform ion mobility spectrometry (FAIMS) reduces peptide complexity in the gas phase, providing a means to achieve maximal ADPr peptide sequencing depth. We therefore investigated the extent to which FAIMS with or without traditional gas-phase fractionation-separation (GPS) can increase the number of ADPr peptides. We examined ADPr peptides enriched from mouse spleens. We gleaned additional insight by also reporting findings from the corresponding non-ADPr peptide contaminants and the peptide inputs for ADPr peptide enrichment. At increasingly higher negative compensation voltages, ADPr peptides were more stable, whereas the non-ADPr peptides were filtered out. A combination of 3 GPS survey scans, each with 8 compensation voltages, resulted in 790 high-confidence ADPr peptides, compared to 90 with GPS alone. A simplified acquisition strategy requiring only two injections corresponding to two MS1 scan ranges coupled to optimized compensation voltage settings provided 402 ADPr peptides corresponding to 234 ADPr proteins. We conclude that our combined GPS strategy is a valuable addition to any ADP-ribosylome workflow. The data are available via ProteomeXchange with identifier PXD040898.

The dynamic process of covalent and non-covalent PARylation in the maintenance of genome integrity: a focus on PARP inhibitors

Poly(ADP-ribosylation) (PARylation) by poly(ADP-ribose) polymerases (PARPs) is a highly regulated process that consists of the covalent addition of polymers of ADP-ribose (PAR) through post-translational modifications of substrate proteins or non-covalent interactions with PAR via PAR binding domains and motifs, thereby reprogramming their functions. This modification is particularly known for its central role in the maintenance of genomic stability. However, how genomic integrity is controlled by an intricate interplay of covalent PARylation and non-covalent PAR binding remains largely unknown. Of importance, PARylation has caught recent attention for providing a mechanistic basis of synthetic lethality involving PARP inhibitors (PARPi), most notably in homologous recombination (HR)-deficient breast and ovarian tumors. The molecular mechanisms responsible for the anti-cancer effect of PARPi are thought to implicate both catalytic inhibition and trapping of PARP enzymes on DNA. However, the relative contribution of each on tumor-specific cytotoxicity is still unclear. It is paramount to understand these PAR-dependent mechanisms, given that resistance to PARPi is a challenge in the clinic. Deciphering the complex interplay between covalent PARylation and non-covalent PAR binding and defining how PARP trapping and non-trapping events contribute to PARPi anti-tumour activity is essential for developing improved therapeutic strategies. With this perspective, we review the current understanding of PARylation biology in the context of the DNA damage response (DDR) and the mechanisms underlying PARPi activity and resistance.

Profiling of the ADP-Ribosylome in Living Cells

Post-translational modification (PTM) with ADP-ribose and poly(ADP-ribose) using nicotinamide adenine dinucleotide (NAD+ ) as substrate is involved in the regulation of numerous cellular pathways in eukaryotes, notably the response to DNA damage caused by cellular stress. Nevertheless, due to intrinsic properties of NAD+ e.g., high polarity and associated poor cell passage, these PTMs are difficult to characterize in cells. Here, two new NAD+ derivatives are presented, which carry either a fluorophore or an affinity tag and, in combination with developed methods for mild cell delivery, allow studies in living human cells. We show that this approach allows not only the imaging of ADP-ribosylation in living cells but also the proteome-wide analysis of cellular adaptation by protein ADP-ribosylation as a consequence of environmental changes such as H2 O2 -induced oxidative stress or the effect of the approved anti-cancer drug olaparib. Our results therefore pave the way for further functional and clinical studies of the ADP-ribosylated proteome in living cells in health and disease.

The RiboMaP Spectral Annotation Method Applied to Various ADP-Ribosylome Studies Including INF-γ-Stimulated Human Cells and Mouse Tissues

ADP-ribosylation is a post-translational modification that is catalyzed by the ADP-ribosyltransferase enzyme family. Major emphasis to date has been ADP-ribosylation's role in cancer; however, there is growing interest in its role in inflammation and cardiovascular disease. Despite a recent boom in ADP-ribosylation mass spectrometry-based proteomics, there are limited computational resources to evaluate the quality of reported ADP-ribosylated (ADPr) proteins. We recently developed a novel mass spectral annotation strategy (RiboMaP) that facilitates identification and reporting of ADPr peptides and proteins. This strategy can monitor the fragmentation properties of ADPr peptide-unique fragment ions, termed m-ions and p-ions, that in turn provide spectral quality scores for candidate ADP-ribosyl peptides. In this study, we leveraged the availability of publicly available ADP-ribosylome data, acquired on various mass spectrometers, to evaluate the broader applicability of RiboMaP. We observed that fragmentation spectra of ADPr peptides vary considerably across datasets; nonetheless, RiboMaP improves ADPr peptide spectral annotation across all studies. We then reanalyzed our own previously published in vitro ADP-ribosylome data to determine common responses to the pro-inflammatory cytokine, IFN-γ. We conclude that despite these recent advances in the field of ADPr proteomics, studies in the context of inflammation and cardiovascular disease still require further bench-to-informatics workflow development in order to capture ADPr signaling events related to inflammatory pathways.

Comparative proteomic profiling in Chinese shrimp Fenneropenaeus chinensis under low pH stress

Lower pH gives rise to a harmful stress to crustacean. Here, we analyzed the proteomic response of Fenneropenaeus chinensis from control pH (pH value 8.2) and low pH (pH value 6.5) - treated groups by employing absolute quantitation-based quantitative proteomic (iTRAQ) analysis. Among the identified proteins, a total of 76 proteins differed in their abundance levels, including 45 upregulated and 31 downregulated proteins. The up-regulation of proteins like citrate synthase, cytochrome c oxidase, V-type proton ATPase, glyceraldehyde-3-phosphate dehydrogenase and fructose 1,6-bisphosphate-aldolase as well as the enrichment of the DEPs in multiple metabolic processes and pathways illustrated that increased energy and substrates metabolism was essential for F. chinensis to counteract low pH stress. Ion transporting related proteins, such as Na⁺/K⁺/2Cl^- cotransporter and calmodulin, participated in the homeostatic maintenance of pH in F. chinensis. There were significant downregulation expressions of lectin, lipopolysaccharide- and beta-1,3-glucan binding protein, chitinase, cathepsin L and beta-glucuronidase, which indicating the immune dysfunction of F. chinensis when exposure to low pH condition. These findings can extend our understanding on the defensive mechanisms of the low pH stress and accelerate the breeding process of low pH tolerance in F. chinensis.

Temporal and Site-Specific ADP-Ribosylation Dynamics upon Different Genotoxic Stresses

The DNA damage response revolves around transmission of information via post-translational modifications, including reversible protein ADP-ribosylation. Here, we applied a mass-spectrometry-based Af1521 enrichment technology for the identification and quantification of ADP-ribosylation sites as a function of various DNA damage stimuli and time. In total, we detected 1681 ADP-ribosylation sites residing on 716 proteins in U2OS cells and determined their temporal dynamics after exposure to the genotoxins H₂O₂ and MMS. Intriguingly, we observed a widespread but low-abundance serine ADP-ribosylation response at the earliest time point, with later time points centered on increased modification of the same sites. This suggests that early serine ADP-ribosylation events may serve as a platform for an integrated signal response. While treatment with H₂O₂ and MMS induced homogenous ADP-ribosylation responses, we observed temporal differences in the ADP-ribosylation site abundances. Exposure to MMS-induced alkylating stress induced the strongest ADP-ribosylome response after 30 min, prominently modifying proteins involved in RNA processing, whereas in response to H₂O₂-induced oxidative stress ADP-ribosylation peaked after 60 min, mainly modifying proteins involved in DNA damage pathways. Collectively, the dynamic ADP-ribosylome presented here provides a valuable insight into the temporal cellular regulation of ADP-ribosylation in response to DNA damage.

Establishment of a Mass-Spectrometry-Based Method for the Identification of the In Vivo Whole Blood and Plasma ADP-Ribosylomes

Blood and plasma proteins are heavily investigated as biomarkers for different diseases. However, the post-translational modification states of these proteins are rarely analyzed since blood contains many enzymes that rapidly remove these modifications after sampling. In contrast to the well-described role of protein ADP-ribosylation in cells and organs, its role in blood remains mostly uncharacterized. Here, we discovered that plasma phosphodiesterases and/or ADP-ribosylhydrolases rapidly demodify in vitro ADP-ribosylated proteins. Thus, to identify the in vivo whole blood and plasma ADP-ribosylomes, we established a mass-spectrometry-based workflow that was applied to blood samples collected from LPS-treated pigs (Sus scrofa domesticus), which serves as a model for human systemic inflammatory response syndrome. These analyses identified 60 ADP-ribosylated proteins, 17 of which were ADP-ribosylated plasma proteins. This new protocol provides an important step forward for the rapidly developing field of ADP-ribosylation and defines the blood and plasma ADP-ribosylomes under both healthy and disease conditions.

Progress and outlook in studying the substrate specificities of PARPs and related enzymes

Despite decades of research on ADP-ribosyltransferases (ARTs) from the poly(ADP-ribose) polymerase (PARP) family, one key aspect of these enzymes - their substrate specificity - has remained unclear. Here, we briefly discuss the history of this area and, more extensively, the recent breakthroughs, including the identification of protein serine residues as a major substrate of PARP1 and PARP2 in human cells and of cysteine and tyrosine as potential targets of specific PARPs. On the molecular level, the modification of serine residues requires a composite active site formed by PARP1 or PARP2 together with a specificity-determining factor, HPF1; this represents a new paradigm not only for PARPs but generally for post-translational modification (PTM) catalysis. Additionally, we discuss the identification of DNA as a substrate of PARP1, PARP2 and PARP3, and some bacterial ARTs and the discovery of noncanonical RNA capping by several PARP family members. Together, these recent findings shed new light on PARP-mediated catalysis and caution to 'expect the unexpected' when it comes to further potential substrates.

ADP-ribosylation of histone variant H2AX promotes base excision repair

Optimal DNA damage response is associated with ADP-ribosylation of histones. However, the underlying molecular mechanism of DNA damage-induced histone ADP-ribosylation remains elusive. Herein, using unbiased mass spectrometry, we identify that glutamate residue 141 (E141) of variant histone H2AX is ADP-ribosylated following oxidative DNA damage. In-depth studies performed with wild-type H2AX and the ADP-ribosylation-deficient E141A mutant suggest that H2AX ADP-ribosylation plays a critical role in base excision repair (BER). Mechanistically, ADP-ribosylation on E141 mediates the recruitment of Neil3 glycosylase to the sites of DNA damage for BER. Moreover, loss of this ADP-ribosylation enhances serine-139 phosphorylation of H2AX (γH2AX) upon oxidative DNA damage and erroneously causes the accumulation of DNA double-strand break (DSB) response factors. Taken together, these results reveal that H2AX ADP-ribosylation not only facilitates BER repair, but also suppresses the γH2AX-mediated DSB response.

Mitochondria are devoid of poly(ADP-ribose)polymerase-1, but harbor its product oligo(ADP-ribose)

There are conflicting data about localization of poly(ADP-ribose)polymerase-1 and its product poly(ADP-ribose) in mitochondria. To finally clarify the discussion, we investigated with biochemical and cell biological methods the potential presence of poly(ADP-ribose) polymerase-1 in these organelles. Our data show that endogenous and overexpressed poly(ADP-ribose)polymerase 1 is only localized to the nucleus with a clear exclusion of cytosolic compartments. In addition, highly purified mitochondria devoid of nuclear contaminations do not contain poly(ADP-ribose)polymerase-1. Although no poly(ADP-ribose)polymerase-1 enzyme is detectable in mitochondria, a shorter variant of its product poly(ADP-ribose) is present, associated specifically with a small subset of mitochondrial proteins as revealed by immunoprecipitation and protein fingerprint analysis. These proteins are located at key-points of the Krebs-cycle, are chaperones involved in mitochondrial functionality and quality-control, and are RNA-binding proteins important for transcript stability, respectively. Of note, despite the fact that especially poly(ADP-ribose)polymerase-1 is its own major target for modification, we could not detect this enzyme by mass spectrometry in these organelles. These data suggests a new way of targeted nuclear-mitochondrial signaling, mediated by nuclear poly(ADP-ribosyl)ation dependent on poly(ADP-ribose)polymerase-1.

WTAP and BIRC3 are involved in the posttranscriptional mechanisms that impact on the expression and activity of the human lactonase PON2

The activity of human paraoxonase 2 (PON2) is rapidly reduced in cells incubated with the bacterial quorormone 3-Oxo-dodecanoyl Homoserine Lactone (3OC12HSL), an observation that led to hypothesize a fast PON2 post-translational modification (PTM). Recently, we detected a 3OC12HSL-induced PTM in a cell-free system in which a crude extract from 3OC12HSL-treated HeLa cells was able to inactivate and ubiquitinate at position 144 a recombinant PON2. Here we show the occurrence of this and new PTMs on PON2 in HeLa cells. PTMs were found to gather nearby the two SNPs, A148G, and S311C, that are related to type-2 diabetes and its complications. Furthermore, we detected a PTM nearby a 12 amino acids region that is deleted in PON2 Isoform 2. An in vitro mutation analysis showed that the SNPs and the deletion are involved in PON2 activity and suggested a role of PTMs on its modulation, while a SAXS analysis pointed to Isoform 2 as being largely unstructured, compared to the wild type. Besides, we discovered a control of PON2 expression via a putative mRNA operon involving the Wilms tumor 1 associated protein (WTAP) and the E3 ubiquitin ligase (E3UbL) baculoviral IAP repeat-containing 3 (BIRC3).

Poly(ADP-ribose): A Dynamic Trigger for Biomolecular Condensate Formation

Poly(ADP-ribose) (PAR) is a nucleic acid-like protein modification that can seed the formation of microscopically visible cellular compartments that lack enveloping membranes, recently termed biomolecular condensates. These PAR-mediated condensates are linked to cancer, viral infection, and neurodegeneration. Recent data have shown the therapeutic potential of modulating PAR conjugation (PARylation): PAR polymerase (PARP) inhibitors can modulate the formation and dynamics of these condensates as well as the trafficking of their components - many of which are key disease factors. However, the way in which PARylation facilitates these functions remains unclear, partly because of our lack of understanding of the fundamental parameters of intracellular PARylation, including the sites that are conjugated, PAR chain length and structure, and the physicochemical properties of the conjugates. This review first introduces the role of PARylation in regulating biomolecular condensates, followed by discussion of current knowledge gaps, potential solutions, and therapeutic applications.

Chemical ADP-ribosylation: mono-ADPr-peptides and oligo-ADP-ribose

ADP-ribosylation is an important post-translational modification that plays a pivotal role in many cellular processes, including cell signaling, DNA repair, gene regulation and apoptosis. Although chemical synthesis of mono- or poly-ADP-ribosylated biomolecules is extremely difficult due to the challenges in regio- and stereoselective glycosylation, suitable protective group manipulations and pyrophosphate construction, synthetic procedures towards these bio-related targets have been reported in recent years. Chemically synthesized well-defined ADP-ribose derivatives serve as useful tools in biological experiments aimed to further elucidate native ADP-ribosylation. In this review, we will discuss the synthetic studies on mono-ADP-ribosylated proteins and oligo-ADP-ribose chains. Future possible synthetic targets and upcoming new methods for the synthesis of these molecules are also included.

Ion-Pairing with Triethylammonium Acetate Improves Solid-Phase Extraction of ADP-Ribosylated Peptides

ADP-ribosylation refers to the post-translational modification of protein substrates with monomers or polymers of the small molecule ADP-ribose. ADP-ribosylation is enzymatically regulated and plays roles in cellular processes including DNA repair, nucleic acid metabolism, cell death, cellular stress responses, and antiviral immunity. Recent advances in the field of ADP-ribosylation have led to the development of proteomics approaches to enrich and identify endogenous ADP-ribosylated peptides by liquid chromatography tandem mass spectrometry (LC-MS/MS). A number of these methods rely on reverse-phase solid-phase extraction as a critical step in preparing cellular peptides for further enrichment steps in proteomics workflows. The anionic ion-pairing reagent trifluoroacetic acid (TFA) is typically used during reverse-phase solid-phase extraction to promote retention of tryptic peptides. Here we report that TFA and other carboxylate ion-pairing reagents are inefficient for reverse-phase solid-phase extraction of ADP-ribosylated peptides. Substitution of TFA with cationic ion-pairing reagents, such as triethylammonium acetate (TEAA), improves recovery of ADP-ribosylated peptides. We further demonstrate that substitution of TFA with TEAA in a proteomics workflow specific for identifying ADP-ribosylated peptides increases identification rates of ADP-ribosylated peptides by LC-MS/MS.

Proteomic Characterization of the Heart and Skeletal Muscle Reveals Widespread Arginine ADP-Ribosylation by the ARTC1 Ectoenzyme

The clostridium-like ecto-ADP-ribosyltransferase ARTC1 is expressed in a highly restricted manner in skeletal muscle and heart tissue. Although ARTC1 is well studied, the identification of ARTC1 targets in vivo and subsequent characterization of ARTC1-regulated cellular processes on the proteome level have been challenging and only a few ARTC1-ADP-ribosylated targets are known. Applying our recently developed mass spectrometry-based workflow to C2C12 myotubes and to skeletal muscle and heart tissues from wild-type mice, we identify hundreds of ARTC1-ADP-ribosylated proteins whose modifications are absent in the ADP-ribosylome of ARTC1-deficient mice. These proteins are ADP-ribosylated on arginine residues and mainly located on the cell surface or in the extracellular space. They are associated with signal transduction, transmembrane transport, and muscle function. Validation of hemopexin (HPX) as a ARTC1-target protein confirmed the functional importance of ARTC1-mediated extracellular arginine ADP-ribosylation at the systems level.

ADPredict: ADP-ribosylation site prediction based on physicochemical and structural descriptors

Motivation

ADP-ribosylation is a post-translational modification (PTM) implicated in several crucial cellular processes, ranging from regulation of DNA repair and chromatin structure to cell metabolism and stress responses. To date, a complete understanding of ADP-ribosylation targets and their modification sites in different tissues and disease states is still lacking. Identification of ADP-ribosylation sites is required to discern the molecular mechanisms regulated by this modification. This motivated us to develop a computational tool for the prediction of ADP-ribosylated sites.

Results

Here, we present ADPredict, the first dedicated computational tool for the prediction of ADP-ribosylated aspartic and glutamic acids. This predictive algorithm is based on (i) physicochemical properties, (ii) in-house designed secondary structure-related descriptors and (iii) three-dimensional features of a set of human ADP-ribosylated proteins that have been reported in the literature. ADPredict was developed using principal component analysis and machine learning techniques; its performance was evaluated both internally via intensive bootstrapping and in predicting two external experimental datasets. It outperformed the only other available ADP-ribosylation prediction tool, ModPred. Moreover, a novel secondary structure descriptor, HM-ratio, was introduced and successfully contributed to the model development, thus representing a promising tool for bioinformatics studies, such as PTM prediction.

Availability and implementation

ADPredict is freely available at www.ADPredict.net.

Supplementary information

Supplementary data are available at Bioinformatics online.

Targeting ADP-ribosylation as an antimicrobial strategy

ADP-ribosylation (ADPr) is an ancient reversible modification of cellular macromolecules controlling major biological processes as diverse as DNA damage repair, transcriptional regulation, intracellular transport, immune and stress responses, cell survival and proliferation. Furthermore, enzymatic reactions of ADPr are central in the pathogenesis of many human diseases, including infectious conditions. By providing a review of ADPr signalling in bacterial systems, we highlight the relevance of this chemical modification in the pathogenesis of human diseases depending on host-pathogen interactions. The post-antibiotic era has raised the need to find alternative approaches to antibiotic administration, as major pathogens becoming resistant to antibiotics. An in-depth understanding of ADPr reactions provides the rationale for designing novel antimicrobial strategies for treatment of infectious diseases. In addition, the understanding of mechanisms of ADPr by bacterial virulence factors offers important hints to improve our knowledge on cellular processes regulated by eukaryotic homologous enzymes, which are often involved in the pathogenesis of human diseases.

PARPs and PAR as novel pharmacological targets for the treatment of stress granule-associated disorders

Among the post-translational modifications, ADP-ribosylation has been for long time the least integrated in the scheme of the structural protein modifications affecting physiological functions. In spite of the original findings on bacterial-dependent ADP-ribosylation catalysed by toxins such as cholera and pertussis toxin, only with the discovery of the poly-ADP-ribosyl polymerase (PARP) family the field has finally expanded and the role of ADP-ribosylation has been recognised in both physiological and pathological processes, including cancer, infectious and neurodegenerative diseases. This is now a rapidly expanding field of investigation, centred on the role of the different PARPs and their substrates in various diseases, and on the potential of PARP inhibitors as novel pharmacological tools to be employed in relevant pathological context. In this review we analyse the role that members of the PARP family and poly-ADP-ribose (PAR; the product of PARP1 and PARP5a activity) play in the processes following the exposure of cells to different stresses. The cell response that arises following conditions such as heat, osmotic, oxidative stresses or viral infection relies on the formation of stress granules, which are transient cytoplasmic membrane-less structures, that include untranslated mRNA, specific proteins and PAR, this last one serving as the "collector" of all components (that bind to it in a non-covalent manner). The resulting phenotypes are cells in which translation, intracellular transport or pro-apoptotic pathways are reversibly inhibited, for the time the given stress holds. Interestingly, the formation of defective stress granules has been detected in diverse pathological conditions including neurological disorders and cancer. Analysing the molecular details of stress granule formation under these conditions offers a novel view on the pathogenesis of these diseases and, as a consequence, the possibility of identifying novel drug targets for their treatment.

A Study into the ADP-Ribosylome of IFN-γ-Stimulated THP-1 Human Macrophage-like Cells Identifies ARTD8/PARP14 and ARTD9/PARP9 ADP-Ribosylation

ADP-ribosylation is a post-translational modification that, until recently, has remained elusive to study at the cellular level. Previously dependent on radioactive tracers to identify ADP-ribosylation targets, several advances in mass spectrometric workflows now permit global identification of ADP-ribosylated substrates. In this study, we capitalized on two ADP-ribosylation enrichment strategies, and multiple activation methods performed on the Orbitrap Fusion Lumos, to identify IFN-γ-induced ADP-ribosylation substrates in macrophages. The ADP-ribosyl binding protein, Af1521, was used to enrich ADP-ribosylated peptides, and the antipoly-ADP-ribosyl antibody, 10H, was used to enrich ADP-ribosylated proteins. ADP-ribosyl-specific mass spectra were further enriched by an ADP-ribose product ion triggered EThcD and HCD activation strategy, in combination with multiple acquisitions that segmented the survey scan into smaller ranges. HCD and EThcD resulted in overlapping and unique ADP-ribosyl peptide identifications, with HCD providing more peptide identifications but EThcD providing more reliable ADP-ribosyl acceptor sites. Our acquisition strategies also resulted in the first ever characterization of ADP-ribosyl on three poly-ADP-ribose polymerases, ARTD9/PARP9, ARTD10/PARP10, and ARTD8/PARP14. IFN-γ increased the ADP-ribosylation status of ARTD9/PARP9, ARTD8/PARP14, and proteins involved in RNA processes. This study therefore summarizes specific molecular pathways at the intersection of IFN-γ and ADP-ribosylation signaling pathways.

In Vitro Techniques for ADP-Ribosylated Substrate Identification

ADP-ribosylation is a post-translational modification of proteins that has required the development of specific technical approaches for the full definition of its physiological roles and regulation. The identification of the enzymes and specific substrates of this reaction is an instrumental step toward these aims. Here we describe a method for the separation of ADP-ribosylated proteins based on the use of the ADP-ribose-binding macro domain of the thermophilic protein Af1521, coupled to mass spectrometry analysis for protein identification. This method foresees the coupling of the macro domain to resin, an affinity-based pull-down assay, coupled to a specificity step resulting from the clearing of cell lysates with a mutated macro domain unable to bind ADP-ribose. By this method both mono- and poly-ADP-ribosylated proteins have been identified.

Proteomics and Metabolomics for AKI Diagnosis

Acute kidney injury (AKI) is a severe and frequent condition in hospitalized patients. Currently, no efficient therapy of AKI is available. Therefore, efforts focus on early prevention and potentially early initiation of renal replacement therapy to improve the outcome in AKI. The detection of AKI in hospitalized patients implies the need for early, accurate, robust, and easily accessible biomarkers of AKI evolution and outcome prediction because only a narrow window exists to implement the earlier-described measures. Even more challenging is the multifactorial origin of AKI and the fact that the changes of molecular expression induced by AKI are difficult to distinguish from those of the diseases associated or causing AKI as shock or sepsis. During the past decade, a considerable number of protein biomarkers for AKI have been described and we expect from recent advances in the field of omics technologies that this number will increase further in the future and be extended to other sorts of biomolecules, such as RNAs, lipids, and metabolites. However, most of these biomarkers are poorly defined by their AKI-associated molecular context. In this review, we describe the state-of-the-art tissue and biofluid proteomic and metabolomic technologies and new bioinformatics approaches for proteomic and metabolomic pathway and molecular interaction analysis. In the second part of the review, we focus on AKI-associated proteomic and metabolomic biomarkers and briefly outline their pathophysiological context in AKI.

ADP-ribosylation and intracellular traffic: an emerging role for PARP enzymes

ADP-ribosylation is an ancient and reversible post-translational modification (PTM) of proteins, in which the ADP-ribose moiety is transferred from NAD+ to target proteins by members of poly-ADP-ribosyl polymerase (PARP) family. The 17 members of this family have been involved in a variety of cellular functions, where their regulatory roles are exerted through the modification of specific substrates, whose identification is crucial to fully define the contribution of this PTM. Evidence of the role of the PARPs is now available both in the context of physiological processes and of cell responses to stress or starvation. An emerging role of the PARPs is their control of intracellular transport, as it is the case for tankyrases/PARP5 and PARP12. Here, we discuss the evidence pointing at this novel aspect of PARPs-dependent cell regulation.

Emerging roles of ADP-ribosyl-acceptor hydrolases (ARHs) in tumorigenesis and cell death pathways

Malignant transformation may occur in the background of post-translational modification, such as ADP-ribosylation, phosphorylation and acetylation. Recent genomic analysis of ADP-ribosylation led to the discovery of more than twenty ADP-ribosyltransferases (ARTs), which catalyze either mono- or poly-ADP-ribosylation. ARTs catalyze the attachment of ADP-ribose to acceptor molecules. The ADP-ribose-acceptor bond can then be cleaved by a family of hydrolases in a substrate-specific manner, which is dependent on the acceptor and its functional group, e.g., arginine (guanidino), serine (hydroxyl), aspartate (carboxyl). These hydrolases vary in structure and function, and include poly-ADP-ribose glycohydrolase (PARG), MacroD1, MacroD2, terminal ADP-ribose protein glycohydrolase 1 (TARG1) and ADP-ribosyl-acceptor hydrolases (ARHs). In murine models, PARG deficiency increased susceptibility to alkylating agents-induced carcinogenesis. Similarly, by cleaving mono-ADP-ribosylated arginine on target proteins, ARH1 appears to inhibit tumor formation, suggesting that ARH1 is a tumor-suppressor gene. Although ARH3 is similar to ARH1 in amino acid sequence and crystal structure, ARH3 does not cleave ADP-ribose-arginine, rather it degrades in an exocidic manner, the PAR polymer and cleaves O-acetyl-ADP-ribose (OAADPr) and the ADP-ribose-serine linkage in acceptor proteins. Under conditions of oxidative stress, ARH3-deficient cells showed increased cytosolic PAR accumulation and PARP-1 mediated cell death. These findings expand our understanding of ADP-ribosylation and provide new therapeutic targets for cancer treatment. In the present review, research on ARH1-regulated tumorigenesis and cell death pathways that are enhanced by ARH3 deficiency are discussed.

A Cell-Line-Specific Atlas of PARP-Mediated Protein Asp/Glu-ADP-Ribosylation in Breast Cancer

PARP1 plays a critical role in regulating many biological processes linked to cellular stress responses. Although DNA strand breaks are potent stimuli of PARP1 enzymatic activity, the context-dependent mechanism regulating PARP1 activation and signaling is poorly understood. We performed global characterization of the PARP1-dependent, Asp/Glu-ADP-ribosylated proteome in a panel of cell lines originating from benign breast epithelial cells, as well as common subtypes of breast cancer. From these analyses, we identified 503 specific ADP-ribosylation sites on 322 proteins. Despite similar expression levels, PARP1 is differentially activated in these cell lines under genotoxic conditions, which generates signaling outputs with substantial heterogeneity. By comparing protein abundances and ADP-ribosylation levels, we could dissect cell-specific PARP1 targets that are driven by unique expression patterns versus cell-specific regulatory mechanisms of PARylation. Intriguingly, PARP1 modifies many proteins in a cell-specific manner, including those involved in transcriptional regulation, mRNA metabolism, and protein translation.

Comprehensive ADP-ribosylome analysis identifies tyrosine as an ADP-ribose acceptor site

Despite recent mass spectrometry (MS)-based breakthroughs, comprehensive ADP-ribose (ADPr)-acceptor amino acid identification and ADPr-site localization remain challenging. Here, we report the establishment of an unbiased, multistep ADP-ribosylome data analysis workflow that led to the identification of tyrosine as a novel ARTD1/PARP1-dependent in vivo ADPr-acceptor amino acid. MS analyses of in vitro ADP-ribosylated proteins confirmed tyrosine as an ADPr-acceptor amino acid in RPS3A (Y155) and HPF1 (Y238) and demonstrated that trans-modification of RPS3A is dependent on HPF1. We provide an ADPr-site Localization Spectra Database (ADPr-LSD), which contains 288 high-quality ADPr-modified peptide spectra, to serve as ADPr spectral references for correct ADPr-site localizations.

A Targeted Proteomic Approach for Heat Shock Proteins Reveals DNAJB4 as a Suppressor for Melanoma Metastasis

Heat shock proteins are molecular chaperones that are involved in protein folding. In this study, we developed a targeted proteomic method, relying on LC-MS/MS in the parallel-reaction monitoring (PRM) mode, for assessing quantitatively the human heat shock proteome. The method facilitated the coverage of approximately 70% of the human heat shock proteome and displayed much better throughput and sensitivity than the shotgun proteomic approach. We also applied the PRM method for assessing the differential expression of heat shock proteins in three matched primary/metastatic pairs of melanoma cell lines. We were able to quantify ∼45 heat shock proteins in each pair of cell lines, and the quantification results revealed that DNAJB4 is down-regulated in the three lines of metastatic melanoma cells relative to the corresponding primary melanoma cells. Interrogation of The Cancer Genome Atlas data showed that lower levels of DNAJB4 expression conferred poorer prognosis in melanoma patients. Moreover, we found that DNAJB4 suppresses the invasion of cultured melanoma cells through diminished expression and activities of matrix metalloproteinases 2 and 9 (MMP-2 and MMP-9). Together, we established, for the first time, a high-throughput targeted proteomics method for profiling quantitatively the human heat shock proteome and discovered DNAJB4 as a suppressor for melanoma metastasis.

Specificity of reversible ADP-ribosylation and regulation of cellular processes

Proper and timely regulation of cellular processes is fundamental to the overall health and viability of organisms across all kingdoms of life. Thus, organisms have evolved multiple highly dynamic and complex biochemical signaling cascades in order to adapt and survive diverse challenges. One such method of conferring rapid adaptation is the addition or removal of reversible modifications of different chemical groups onto macromolecules which in turn induce the appropriate downstream outcome. ADP-ribosylation, the addition of ADP-ribose (ADPr) groups, represents one of these highly conserved signaling chemicals. Herein we outline the writers, erasers and readers of ADP-ribosylation and dip into the multitude of cellular processes they have been implicated in. We also review what we currently know on how specificity of activity is ensured for this important modification.

Proteomic Analysis of the Downstream Signaling Network of PARP1

Poly-ADP-ribosylation (PARylation) is a protein posttranslational modification (PTM) that is critically involved in many biological processes that are linked to cell stress responses. It is catalyzed by a class of enzymes known as poly-ADP-ribose polymerases (PARPs). In particular, PARP1 is a nuclear protein that is activated upon sensing nicked DNA. Once activated, PARP1 is responsible for the synthesis of a large number of PARylated proteins and initiation of the DNA damage response mechanisms. This observation provided the rationale for developing PARP1 inhibitors for the treatment of human malignancies. Indeed, three PARP1 inhibitors (Olaparib, Rucaparib, and Niraparib) have recently been approved by the Food and Drug Administration for the treatment of ovarian cancer. Moreover, in 2017, both Olaparib and Niraparib have also been approved for the treatment of fallopian tube cancer and primary peritoneal cancer. Despite this very exciting progress in the clinic, the basic signaling mechanism that connects PARP1 to a diverse array of biological processes is still poorly understood. This is, in large part, due to the inherent technical difficulty associated with the analysis of protein PARylation, which is a low-abundance, labile, and heterogeneous PTM. The study of PARylation has been greatly facilitated by the recent advances in mass spectrometry-based proteomic technologies tailored to the analysis of this modification. In this Perspective, we discuss these breakthroughs, including their technical development, and applications that provide a global view of the many biological processes regulated by this important protein modification.

ADPr-Peptide Synthesis

Synthetic mono-ADPr-peptides are useful for structural, biochemical, and proteomics studies. We describe here a protocol for the preparation of mono-ADPr-peptides based on a fairly standard Fmoc-based solid-phase synthesis. Phosphoribosylated precursor building blocks are introduced into the peptide chain on solid-phase and subsequently converted to ADPr-sites by chemical phosphorylation with adenosine phosphoramidite. Suitably protected phosphoribosylated glutamine, asparagine, and citrulline building blocks described in this protocol allow introduction of ADP-Gln, ADPr-Asn, and ADPr-Cit into peptide chains as demonstrated for three peptides. Trifunctional amino acids, for which base-sensitive side-chain protection is available, can be accommodated in the sequences flanking the ADPr-cites.

What is targeted proteomics? A concise revision of targeted acquisition and targeted data analysis in mass spectrometry

Targeted proteomics has gained significant popularity in mass spectrometry-based protein quantification as a method to detect proteins of interest with high sensitivity, quantitative accuracy and reproducibility. However, with the emergence of a wide variety of targeted proteomics methods, some of them with high-throughput capabilities, it is easy to overlook the essence of each method and to determine what makes each of them a targeted proteomics method. In this viewpoint, we revisit the main targeted proteomics methods and classify them in four categories differentiating those methods that perform targeted data acquisition from targeted data analysis, and those methods that are based on peptide ion data (MS1 targeted methods) from those that rely on the peptide fragments (MS2 targeted methods).

Proteomic analyses identify ARH3 as a serine mono-ADP-ribosylhydrolase

ADP-ribosylation is a posttranslational modification that exists in monomeric and polymeric forms. Whereas the writers (e.g. ARTD1/PARP1) and erasers (e.g. PARG, ARH3) of poly-ADP-ribosylation (PARylation) are relatively well described, the enzymes involved in mono-ADP-ribosylation (MARylation) have been less well investigated. While erasers for the MARylation of glutamate/aspartate and arginine have been identified, the respective enzymes with specificity for serine were missing. Here we report that, in vitro, ARH3 specifically binds and demodifies proteins and peptides that are MARylated. Molecular modeling and site-directed mutagenesis of ARH3 revealed that numerous residues are critical for both the mono- and the poly-ADP-ribosylhydrolase activity of ARH3. Notably, a mass spectrometric approach showed that ARH3-deficient mouse embryonic fibroblasts are characterized by a specific increase in serine-ADP-ribosylation in vivo under untreated conditions as well as following hydrogen peroxide stress. Together, our results establish ARH3 as a serine mono-ADP-ribosylhydrolase and as an important regulator of the basal and stress-induced ADP-ribosylome.

Mass spectrometry for serine ADP-ribosylation? Think o-glycosylation!

Protein ADP-ribosylation (ADPr), a biologically and clinically important post-translational modification, exerts its functions by targeting a variety of different amino acids. Its repertoire recently expanded to include serine ADPr, which is emerging as an important and widespread signal in the DNA damage response. Chemically, serine ADPr (and more generally o-glycosidic ADPr) is a form of o-glycosylation, and its extreme lability renders it practically invisible to standard mass spectrometry approaches, often leading to erroneous localizations. The knowledge from the mature field of o-glycosation and our own initial difficulties with mass spectrometric analyzes of serine ADPr suggest how to avoid these misidentifications and fully explore the scope of o-glycosidic ADPr in DNA damage response and beyond.

Application of selected reaction monitoring and parallel reaction monitoring for investigation of HL-60 cell line differentiation

Targeted mass spectrometry represents a powerful tool for investigation of biological processes. The convenient approach of selected reaction monitoring using stable isotope-labeled peptide standard (SIS) is widely applied for protein quantification. Along with this method, high-resolution parallel reaction monitoring has been increasingly used for protein targeted analysis. Here we applied two targeted approaches (selected reaction monitoring with SIS and label-free parallel reaction monitoring) to investigate expression of 11 proteins during all-trans retinoic acid-induced differentiation of HL-60 cells. In our experiments, we have determined the proteins expression ratio at 3, 24, 48, and 96 h after all-trans retinoic acid treatment in comparison with 0 h, respectively. Expression profiles of four proteins (VAV1, PRAM1, LYN, and CEBPB) were highly correlated ( r > 0.75) and FGR expression was detected on proteome level starting from 24 h by both techniques. For prominent differences (fold change ≥ 2) label-free parallel reaction monitoring is not inferior to selected reaction monitoring with isotopically labeled peptide standards. Differentially expressed proteins, that have been determined in our study, can be considered as potential drug targets for acute myeloid leukemia (AML) treatment.