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

Targeting the Ubiquitin-Proteasome System and Recent Advances in Cancer Therapy

Ubiquitination is a reversible post-translational modification based on the chemical addition of ubiquitin to proteins with regulatory effects on various signaling pathways. Ubiquitination can alter the molecular functions of tagged substrates with respect to protein turnover, biological activity, subcellular localization or protein-protein interaction. As a result, a wide variety of cellular processes are under ubiquitination-mediated control, contributing to the maintenance of cellular homeostasis. It follows that the dysregulation of ubiquitination reactions plays a relevant role in the pathogenic states of human diseases such as neurodegenerative diseases, immune-related pathologies and cancer. In recent decades, the enzymes of the ubiquitin-proteasome system (UPS), including E3 ubiquitin ligases and deubiquitinases (DUBs), have attracted attention as novel druggable targets for the development of new anticancer therapeutic approaches. This perspective article summarizes the peculiarities shared by the enzymes involved in the ubiquitination reaction which, when deregulated, can lead to tumorigenesis. Accordingly, an overview of the main pharmacological interventions based on targeting the UPS that are in clinical use or still in clinical trials is provided, also highlighting the limitations of the therapeutic efficacy of these approaches. Therefore, various attempts to circumvent drug resistance and side effects as well as UPS-related emerging technologies in anticancer therapeutics are discussed.

Targeting the NEDP1 enzyme to ameliorate ALS phenotypes through stress granule disassembly

The elimination of aberrant inclusions is regarded as a therapeutic approach in neurodegeneration. In amyotrophic lateral sclerosis (ALS), mutations in proteins found within cytoplasmic condensates called stress granules (SGs) are linked to the formation of pathological SGs, aberrant protein inclusions, and neuronal toxicity. We found that inhibition of NEDP1, the enzyme that processes/deconjugates the ubiquitin-like molecule NEDD8, promotes the disassembly of physiological and pathological SGs. Reduction in poly(ADP-ribose) polymerase1 activity through hyper-NEDDylation is a key mechanism for the observed phenotype. These effects are related to improved cell survival in human cells, and in C. elegans, nedp1 deletion ameliorates ALS phenotypes related to animal motility. Our studies reveal NEDP1 as potential therapeutic target for ALS, correlated to the disassembly of pathological SGs.

Intracellular mono-ADP-ribosyltransferases at the host-virus interphase

The innate immune system, the primary defense mechanism of higher organisms against pathogens including viruses, senses pathogen-associated molecular patterns (PAMPs). In response to PAMPs, interferons (IFNs) are produced, allowing the host to react swiftly to viral infection. In turn the expression of IFN-stimulated genes (ISGs) is induced. Their products disseminate the antiviral response. Among the ISGs conserved in many species are those encoding mono-ADP-ribosyltransferases (mono-ARTs). This prompts the question whether, and if so how, mono-ADP-ribosylation affects viral propagation. Emerging evidence demonstrates that some mono-ADP-ribosyltransferases function as PAMP receptors and modify both host and viral proteins relevant for viral replication. Support for mono-ADP-ribosylation in virus–host interaction stems from the findings that some viruses encode mono-ADP-ribosylhydrolases, which antagonize cellular mono-ARTs. We summarize and discuss the evidence linking mono-ADP-ribosylation and the enzymes relevant to catalyze this reversible modification with the innate immune response as part of the arms race between host and viruses.

PKD-dependent PARP12-catalyzed mono-ADP-ribosylation of Golgin-97 is required for E-cadherin transport from Golgi to plasma membrane

Adenosine diphosphate (ADP)-ribosylation is a posttranslational modification involved in key regulatory events catalyzed by ADP-ribosyltransferases (ARTs). Substrate identification and localization of the mono-ADP-ribosyltransferase PARP12 at the trans-Golgi network (TGN) hinted at the involvement of ARTs in intracellular traffic. We find that Golgin-97, a TGN protein required for the formation and transport of a specific class of basolateral cargoes (e.g., E-cadherin and vesicular stomatitis virus G protein [VSVG]), is a PARP12 substrate. PARP12 targets an acidic cluster in the Golgin-97 coiled-coil domain essential for function. Its mutation or PARP12 depletion, delays E-cadherin and VSVG export and leads to a defect in carrier fission, hence in transport, with consequent accumulation of cargoes in a trans-Golgi/Rab11-positive intermediate compartment. In contrast, PARP12 does not control the Golgin-245-dependent traffic of cargoes such as tumor necrosis factor alpha (TNFα). Thus, the transport of different basolateral proteins to the plasma membrane is differentially regulated by Golgin-97 mono-ADP-ribosylation by PARP12. This identifies a selective regulatory mechanism acting on the transport of Golgin-97- vs. Golgin-245-dependent cargoes. Of note, PARP12 enzymatic activity, and consequently Golgin-97 mono-ADP-ribosylation, depends on the activation of protein kinase D (PKD) at the TGN during traffic. PARP12 is directly phosphorylated by PKD, and this is essential to stimulate PARP12 catalytic activity. PARP12 is therefore a component of the PKD-driven regulatory cascade that selectively controls a major branch of the basolateral transport pathway. We propose that through this mechanism, PARP12 contributes to the maintenance of E-cadherin-mediated cell polarity and cell-cell junctions.

Chemical synthesis of linear ADP-ribose oligomers up to pentamer and their binding to the oncogenic helicase ALC1

ADP-ribosylation is a pivotal post-translational modification that mediates various important cellular processes producing negatively charged biopolymer, poly (ADP-ribose), the functions of which need further elucidation. Toward this end, the availability of well-defined ADP-ribose (ADPr) oligomers in sufficient quantities is a necessity. In this work, we demonstrate the chemical synthesis of linear ADPr oligomers of defined, increasing length using a modified solid phase synthesis method. An advanced phosphoramidite building block temporarily protected with the base sensitive Fm-group was designed and implemented in the repeating pyrophosphate formation via a P(v)-P(iii) coupling procedure on Tentagel solid support. Linear ADPr oligomers up to a pentamer were successfully synthesized and their affinity for the poly-(ADP-ribose)-binding macrodomain of the human oncogenic helicase and chromatin remodeling enzyme ALC1 was determined. Our data reveal a length-dependent binding manner of the nucleic acid, with larger ADPr oligomers exhibiting higher binding enthalpies for ALC1, illustrating how the activity of this molecular machine is gated by PAR.

Post-translational modifications: Regulators of neurodegenerative proteinopathies

One of the hallmark features in the neurodegenerative disorders (NDDs) is the accumulation of aggregated and/or non-functional protein in the cellular milieu. Post-translational modifications (PTMs) are an essential regulator of non-functional protein aggregation in the pathogenesis of NDDs. Any alteration in the post-translational mechanism and the protein quality control system, for instance, molecular chaperone, ubiquitin-proteasome system, autophagy-lysosomal degradation pathway, enhances the accumulation of misfolded protein, which causes neuronal dysfunction. Post-translational modification plays many roles in protein turnover rate, accumulation of aggregate and can also help in the degradation of disease-causing toxic metabolites. PTMs such as acetylation, glycosylation, phosphorylation, ubiquitination, palmitoylation, SUMOylation, nitration, oxidation, and many others regulate protein homeostasis, which includes protein structure, functions and aggregation propensity. Different studies demonstrated the involvement of PTMs in the regulation of signaling cascades such as PI3K/Akt/GSK3β, MAPK cascade, AMPK pathway, and Wnt signaling pathway in the pathogenesis of NDDs. Further, mounting evidence suggests that targeting different PTMs with small chemical molecules, which acts as an inhibitor or activator, reverse misfolded protein accumulation and thus enhances the neuroprotection. Herein, we briefly discuss the protein aggregation and various domain structures of different proteins involved in the NDDs, indicating critical amino acid residues where PTMs occur. We also describe the implementation and involvement of various PTMs on signaling cascade and cellular processes in NDDs. Lastly, we implement our current understanding of the therapeutic importance of PTMs in neurodegeneration, along with emerging techniques targeting various PTMs.

Response to stress in biological disorders: Implications of stress granule assembly and function

It is indispensable for cells to adapt and respond to environmental stresses, in order for organisms to survive. Stress granules (SGs) are condensed membrane‐less organelles dynamically formed in the cytoplasm of eukaryotes cells to cope with diverse intracellular or extracellular stress factors, with features of liquid‐liquid phase separation. They are composed of multiple constituents, including translationally stalled mRNAs, translation initiation factors, RNA‐binding proteins and also non‐RNA‐binding proteins. SG formation is triggered by stress stimuli, viral infection and signal transduction, while aberrant assembly of SGs may contribute to tissue degenerative diseases. Recently, a growing body of evidence has emerged on SG response mechanisms for cells facing high temperatures, oxidative stress and osmotic stress. In this review, we aim to summarize factors affecting SGs assembly, present the impact of SGs on germ cell development and other biological processes. We particularly emphasize the significance of recently reported RNA modifications in SG stress responses. In parallel, we also review all current perspectives on the roles of SGs in male germ cells, with a particular focus on the dynamics of SG assembly.

Cancer cell adaptability: turning ribonucleoprotein granules into targets

Stress granules (SGs) and processing bodies (P-bodies) are membraneless cytoplasmic condensates of ribonucleoproteins (RNPs). They both regulate RNA fate under physiological and pathological conditions, and are thereby involved in the regulation and maintenance of cellular integrity. During tumorigenesis, cancer cells use these granules to thrive, to adapt to the harsh conditions of the tumor microenvironment (TME), and to protect themselves from anticancer treatments. This ability to provide multiple outcomes not only makes RNP granules promising targets for cancer therapy but also emphasizes the need for more knowledge about the biology of these granules to achieve clinical use. In this review we focus on the role of RNP granules in cancer, and on how their composition and regulation might be used to elaborate therapeutic strategies.

Functional Roles of Poly(ADP-Ribose) in Stress Granule Formation and Dynamics

Stress granules (SGs) are highly dynamic cytoplasmic foci formed in response to stress. The formation of SGs is reported to be regulated by diverse post-translational protein modifications (PTMs). Among them, ADP-ribosylation is of emerging interest due to its recently identified roles in SG organization. In this review, we summarized the latest advances on the roles of poly(ADP-ribose) (PAR) in the regulation of SG formation and dynamics, including its function in modulating nucleocytoplasmic trafficking and SG recruitment of SG components, as well as its effects on protein phase separation behavior. Moreover, the functional role of PAR chain diversity on dynamic of SG composition is also introduced. Potential future developments on investigating global ADP-ribosylation networks, individual roles of different PARPs, and interactions between ADP-ribosylation and other PTMs in SGs are also discussed.

Poly(ADP-Ribosyl) Code Functions

Poly(ADP-ribosyl)ation plays a key role in cellular metabolism. Covalent poly(ADP-ribosyl)ation affects the activity of the proteins engaged in DNA repair, chromatin structure regulation, gene expression, RNA processing, ribosome biogenesis, and protein translation. Non-covalent PAR-dependent interactions are involved in the various types of cellular response to stress and viral infection, such as inflammation, hormonal signaling, and the immune response. The review discusses how structurally different poly(ADP-ribose) (PAR) molecules composed of identical monomers can differentially participate in various cellular processes acting as the so-called "PAR code." The article describes the ability of PAR polymers to form functional biomolecular clusters through a phase-separation in response to various signals. This phase-separation contributes to rapid spatial segregation of biochemical processes and effective recruitment of the necessary components. The cellular PAR level is tightly controlled by a network of regulatory proteins: PAR code writers, readers, and erasers. Impaired PAR metabolism is associated with the development of pathological processes causing oncological, cardiovascular, and neurodegenerative diseases. Pharmacological correction of the PAR level may represent a new approach to the treatment of various diseases.

Mono(ADP-ribosyl)ation Enzymes and NAD+ Metabolism: A Focus on Diseases and Therapeutic Perspectives

Mono(ADP-ribose) transferases and mono(ADP-ribosyl)ating sirtuins use NAD⁺ to perform the mono(ADP-ribosyl)ation, a simple form of post-translational modification of proteins and, in some cases, of nucleic acids. The availability of NAD⁺ is a limiting step and an essential requisite for NAD⁺ consuming enzymes. The synthesis and degradation of NAD⁺, as well as the transport of its key intermediates among cell compartments, play a vital role in the maintenance of optimal NAD⁺ levels, which are essential for the regulation of NAD⁺-utilizing enzymes. In this review, we provide an overview of the current knowledge of NAD⁺ metabolism, highlighting the functional liaison with mono(ADP-ribosyl)ating enzymes, such as the well-known ARTD10 (also named PARP10), SIRT6, and SIRT7. To this aim, we discuss the link of these enzymes with NAD⁺ metabolism and chronic diseases, such as cancer, degenerative disorders and aging.

A global analysis of low-complexity regions in the Trypanosoma brucei proteome reveals enrichment in the C-terminus of nucleic acid binding proteins providing potential targets of phosphorylation

Background: Low-complexity regions (LCRs) on proteins have attracted increasing attention recently due to their role in the assembly of membraneless organelles or granules by liquid-liquid phase separation. Several examples of such granules have been shown to sequester RNA and proteins in an inactive state, providing an important mechanism for dynamic post-transcriptional gene regulation. In trypanosome parasites, post-transcriptional control overwhelmingly dominates gene regulation due to the organisation of their genome into polycistronic transcription units. The purpose of the current study was to generate a substantially more comprehensive genome-wide survey of LCRs on trypanosome proteins than currently available . Methods: Using the Shannon's entropy method, provided in the R package 'entropy', we identified LCRs in the proteome of Trypanosoma brucei. Our analysis predicts LCRs and their positional enrichment in distinct protein cohorts and superimposes on this a range of post-translational modifications derived from available experimental datasets. Results: We have identified 8162 LCRs present on 4914 proteins, representing 42% of the proteome, placing Trypanosoma brucei among the eukaryotes with the highest percentage of LCRs . Our results highlight the enrichment of LCRs in the C-terminal region of predicted nucleic acid binding proteins, these acting as favoured sites for potential phosphorylation. Phosphorylation represents 51% of the post-translational modifications present on LCRs compared to 16% on the rest of the proteome. Conclusions: The post-translational modifications of LCRs, and in particular phosphorylation events, could contribute to post-transcriptional gene expression control and the dynamics of protein targeting to membraneless organelles in kinetoplastid parasites.

A global analysis of low-complexity regions in the Trypanosoma brucei proteome reveals enrichment in the C-terminus of nucleic acid binding proteins providing potential targets of phosphorylation

Background: Low-complexity regions (LCRs) on proteins have attracted increasing attention recently due to their role in the assembly of membraneless organelles or granules by liquid-liquid phase separation. Several examples of such granules have been shown to sequester RNA and proteins in an inactive state, providing an important mechanism for dynamic post-transcriptional gene regulation. In trypanosome parasites, post-transcriptional control overwhelmingly dominates gene regulation due to the organisation of their genome into polycistronic transcription units. The purpose of the current study was to generate a substantially more comprehensive genome-wide survey of LCRs on trypanosome proteins than currently available . Methods: Using the Shannon's entropy method, provided in the R package 'entropy', we identified LCRs in the proteome of Trypanosoma brucei. Our analysis predicts LCRs and their positional enrichment in distinct protein cohorts and superimposes on this a range of post-translational modifications derived from available experimental datasets. Results: Our results highlight the enrichment of LCRs in the C-terminal region of predicted nucleic acid binding proteins, these acting as favoured sites for potential phosphorylation. Conclusions: The post-translational modifications of LCRs, and in particular the phosphorylation events, could contribute to post-transcriptional gene expression control and the dynamics of protein targeting to membraneless organelles in kinetoplastid parasites.

The involvement of stress granules in aging and aging-associated diseases

Stress granules (SGs) are nonmembrane assemblies formed in cells in response to stress conditions. SGs mainly contain untranslated mRNA and a variety of proteins. RNAs and scaffold proteins with intrinsically disordered regions or RNA-binding domains are essential for the assembly of SGs, and multivalent macromolecular interactions among these components are thought to be the driving forces for SG assembly. The SG assembly process includes regulation through post-translational modification and involvement of the cytoskeletal system. During aging, many intracellular bioprocesses become disrupted by factors such as cellular environmental changes, mitochondrial dysfunction, and decline in the protein quality control system. Such changes could lead to the formation of aberrant SGs, as well as alterations in their maintenance, disassembly, and clearance. These aberrant SGs might in turn promote aging and aging-associated diseases. In this paper, we first review the latest progress on the molecular mechanisms underlying SG assembly and SG functioning under stress conditions. Then, we provide a detailed discussion of the relevance of SGs to aging and aging-associated diseases.

(ADP-ribosyl)hydrolases: structure, function, and biology

ADP-ribosylation is an intricate and versatile posttranslational modification involved in the regulation of a vast variety of cellular processes in all kingdoms of life. Its complexity derives from the varied range of different chemical linkages, including to several amino acid side chains as well as nucleic acids termini and bases, it can adopt. In this review, we provide an overview of the different families of (ADP-ribosyl)hydrolases. We discuss their molecular functions, physiological roles, and influence on human health and disease. Together, the accumulated data support the increasingly compelling view that (ADP-ribosyl)hydrolases are a vital element within ADP-ribosyl signaling pathways and they hold the potential for novel therapeutic approaches as well as a deeper understanding of ADP-ribosylation as a whole.

The Role of PARPs in Inflammation-and Metabolic-Related Diseases: Molecular Mechanisms and Beyond

Poly(ADP-ribosyl)ation (PARylation) is an essential post-translational modification catalyzed by poly(ADP-ribose) polymerase (PARP) enzymes. Poly(ADP-ribose) polymerase 1 (PARP1) is a well-characterized member of the PARP family. PARP1 plays a crucial role in multiple biological processes and PARP1 activation contributes to the development of various inflammatory and malignant disorders, including lung inflammatory disorders, cardiovascular disease, ovarian cancer, breast cancer, and diabetes. In this review, we will focus on the role and molecular mechanisms of PARPs enzymes in inflammation- and metabolic-related diseases. Specifically, we discuss the molecular mechanisms and signaling pathways that PARP1 is associated with in the regulation of pathogenesis. Recently, increasing evidence suggests that PARP inhibition is a promising strategy for intervention of some diseases. Thus, our in-depth understanding of the mechanism of how PARPs are activated and how their signaling downstream effecters can provide more potential therapeutic targets for the treatment of the related diseases in the future is crucial.

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.