3-Aminobenzamide Blocks MAMP-Induced Callose Deposition Independently of Its Poly(ADPribosyl)ation Inhibiting Activity

Antileukemic potential of methylated indolequinone MAC681 through immunogenic necroptosis and PARP1 degradation

BACKGROUND

Despite advancements in chronic myeloid leukemia (CML) therapy with tyrosine kinase inhibitors (TKIs), resistance and intolerance remain significant challenges. Leukemia stem cells (LSCs) and TKI-resistant cells rely on altered mitochondrial metabolism and oxidative phosphorylation. Targeting rewired energy metabolism and inducing non-apoptotic cell death, along with the release of damage-associated molecular patterns (DAMPs), can enhance therapeutic strategies and immunogenic therapies against CML and prevent the emergence of TKI-resistant cells and LSC persistence.

METHODS

Transcriptomic analysis was conducted using datasets of CML patients' stem cells and healthy cells. DNA damage was evaluated by fluorescent microscopy and flow cytometry. Cell death was assessed by trypan blue exclusion test, fluorescent microscopy, flow cytometry, colony formation assay, and in vivo Zebrafish xenografts. Energy metabolism was determined by measuring NAD+ and NADH levels, ATP production rate by Seahorse analyzer, and intracellular ATP content. Mitochondrial fitness was estimated by measurements of mitochondrial membrane potential, ROS, and calcium accumulation by flow cytometry, and morphology was visualized by TEM. Bioinformatic analysis, real-time qPCR, western blotting, chemical reaction prediction, and molecular docking were utilized to identify the drug target. The immunogenic potential was assessed by high mobility group box (HMGB)1 ELISA assay, luciferase-based extracellular ATP assay, ectopic calreticulin expression by flow cytometry, and validated by phagocytosis assay, and in vivo vaccination assay using syngeneic C57BL/6 mice.

RESULTS

Transcriptomic analysis identified metabolic alterations and DNA repair deficiency signatures in CML patients. CML patients exhibited enrichment in immune system, DNA repair, and metabolic pathways. The gene signature associated with BRCA mutated tumors was enriched in CML datasets, suggesting a deficiency in double-strand break repair pathways. Additionally, poly(ADP-ribose) polymerase (PARP)1 was significantly upregulated in CML patients' stem cells compared to healthy counterparts. Consistent with the CML patient DNA repair signature, treatment with the methylated indolequinone MAC681 induced DNA damage, mitochondrial dysfunction, calcium homeostasis disruption, metabolic catastrophe, and necroptotic-like cell death. In parallel, MAC681 led to PARP1 degradation that was prevented by 3-aminobenzamide. MAC681-treated myeloid leukemia cells released DAMPs and demonstrated the potential to generate an immunogenic vaccine in C57BL/6 mice. MAC681 and asciminib exhibited synergistic effects in killing both imatinib-sensitive and -resistant CML, opening new therapeutic opportunities.

CONCLUSIONS

Overall, increasing the tumor mutational burden by PARP1 degradation and mitochondrial deregulation makes CML suitable for immunotherapy.

The multifarious role of callose and callose synthase in plant development and environment interactions

Callose is an important linear form of polysaccharide synthesized in plant cell walls. It is mainly composed of β-1,3-linked glucose residues with rare amount of β-1,6-linked branches. Callose can be detected in almost all plant tissues and are widely involved in various stages of plant growth and development. Callose is accumulated on plant cell plates, microspores, sieve plates, and plasmodesmata in cell walls and is inducible upon heavy metal treatment, pathogen invasion, and mechanical wounding. Callose in plant cells is synthesized by callose synthases located on the cell membrane. The chemical composition of callose and the components of callose synthases were once controversial until the application of molecular biology and genetics in the model plant Arabidopsis thaliana that led to the cloning of genes encoding synthases responsible for callose biosynthesis. This minireview summarizes the research progress of plant callose and its synthetizing enzymes in recent years to illustrate the important and versatile role of callose in plant life activities.

ADP-Ribosylation and Antiviral Resistance in Plants

ADP-ribosylation (ADPRylation) is a versatile posttranslational modification in eukaryotic cells which is involved in the regulation of a wide range of key biological processes, including DNA repair, cell signalling, programmed cell death, growth and development and responses to biotic and abiotic stresses. Members of the poly(ADP-ribosyl) polymerase (PARP) family play a central role in the process of ADPRylation. Protein targets can be modified by adding either a single ADP-ribose moiety (mono(ADP-ribosyl)ation; MARylation), which is catalysed by mono(ADP-ribosyl) transferases (MARTs or PARP "monoenzymes"), or targets may be decorated with chains of multiple ADP-ribose moieties (PARylation), via the activities of PARP "polyenzymes". Studies have revealed crosstalk between PARylation (and to a lesser extent, MARylation) processes in plants and plant-virus interactions, suggesting that these tight links may represent a novel factor regulating plant antiviral immunity. From this perspective, we go through the literature linking PARylation-associated processes with other plant regulation pathways controlling virus resistance. Once unraveled, these links may serve as the basis of innovative strategies to improve crop resistance to viruses under challenging environmental conditions which could mitigate yield losses.

PMR4-dependent cell wall depositions are a consequence but not the cause of temperature-induced autoimmunity

Plants possess a well-balanced immune system that is required for defense against pathogen infections. In autoimmune mutants or necrotic crosses, an intrinsic temperature-dependent imbalance leads to constitutive immune activation, resulting in severe damage or even death of plants. Recently, cell wall depositions were described as one of the symptoms following induction of the autoimmune phenotype in Arabidopsis saul1-1 mutants. However, the regulation and function of these depositions remained unclear. Here, we show that cell wall depositions, containing lignin and callose, were a common autoimmune feature and were deposited in proportion to the severity of the autoimmune phenotype at reduced ambient temperatures. When plants were exposed to reduced temperature for periods insufficient to induce an autoimmune phenotype, the cell wall depositions were not present. After low temperature intervals, sufficient to induce autoimmune responses, cell wall depositions correlated with a point of no return in saul1-1 autoimmunity. Although cell wall depositions were largely abolished in saul1-1 pmr4-1 double mutants lacking SAUL1 and the callose synthase gene GSL5/PMR4, their phenotype remained unchanged compared to that of the saul1-1 single mutant. Our data showed that cell wall depositions generally occur in autoimmunity, but appear not to be the cause of autoimmune phenotypes.

Crosstalk between Ubiquitination and Other Post-translational Protein Modifications in Plant Immunity

Post-translational modifications (PTMs) are central to the modulation of protein activity, stability, subcellular localization, and interaction with partners. They greatly expand the diversity and functionality of the proteome and have taken the center stage as key players in regulating numerous cellular and physiological processes. Increasing evidence indicates that in addition to a single regulatory PTM, many proteins are modified by multiple different types of PTMs in an orchestrated manner to collectively modulate the biological outcome. Such PTM crosstalk creates a combinatorial explosion in the number of proteoforms in a cell and greatly improves the ability of plants to rapidly mount and fine-tune responses to different external and internal cues. While PTM crosstalk has been investigated in depth in humans, animals, and yeast, the study of interplay between different PTMs in plants is still at its infant stage. In the past decade, investigations showed that PTMs are widely involved and play critical roles in the regulation of interactions between plants and pathogens. In particular, ubiquitination has emerged as a key regulator of plant immunity. This review discusses recent studies of the crosstalk between ubiquitination and six other PTMs, i.e., phosphorylation, SUMOylation, poly(ADP-ribosyl)ation, acetylation, redox modification, and glycosylation, in the regulation of plant immunity. The two basic ways by which PTMs communicate as well as the underlying mechanisms and diverse outcomes of the PTM crosstalk in plant immunity are highlighted.

ATP Hydrolysis Determines Cold Tolerance by Regulating Available Energy for Glutathione Synthesis in Rice Seedling Plants

BACKGROUND

Glutathione (GSH) is important for plants to resist abiotic stress, and a large amount of energy is required in the process. However, it is not clear how the energy status affects the accumulation of GSH in plants under cold stress.

RESULTS

Two rice pure lines, Zhongzao39 (ZZ39) and its recombinant inbred line 82 (RIL82) were subjected to cold stress for 48 h. Under cold stress, RIL82 suffered more damages than ZZ39 plants, in which higher increases in APX activity and GSH content were showed in the latter than the former compared with their respective controls. This indicated that GSH was mainly responsible for the different cold tolerance between these two rice plants. Interestingly, under cold stress, greater increases in contents of carbohydrate, NAD(H), NADP(H) and ATP as well as the expression levels of GSH1 and GSH2 were showed in RIL82 than ZZ39 plants. In contrast, ATPase content in RIL82 plants was adversely inhibited by cold stress while it increased significantly in ZZ39 plants. This indicated that cold stress reduced the accumulation of GSH in RIL82 plants mainly due to the inhibition on ATP hydrolysis rather than energy deficit.

CONCLUSION

We inferred that the energy status determined by ATP hydrolysis involved in regulating the cold tolerance of plants by controlling GSH synthesis.

Poly(ADP-Ribose) Polymerases in Plants and Their Human Counterparts: Parallels and Peculiarities

Poly(ADP-ribosyl)ation is a rapid and transient post-translational protein modification that was described first in mammalian cells. Activated by the sensing of DNA strand breaks, poly(ADP-ribose)polymerase1 (PARP1) transfers ADP-ribose units onto itself and other target proteins using NAD⁺ as a substrate. Subsequently, DNA damage responses and other cellular responses are initiated. In plants, poly(ADP-ribose) polymerases (PARPs) have also been implicated in responses to DNA damage. The Arabidopsis genome contains three canonical PARP genes, the nomenclature of which has been uncoordinated in the past. Albeit assumptions concerning the function and roles of PARP proteins in planta have often been inferred from homology and structural conservation between plant PARPs and their mammalian counterparts, plant-specific roles have become apparent. In particular, PARPs have been linked to stress responses of plants. A negative role under abiotic stress has been inferred from studies in which a genetic or, more commonly, pharmacological inhibition of PARP activity improved the performance of stressed plants; in response to pathogen-associated molecular patterns, a positive role has been suggested. However, reports have been inconsistent, and the effects of PARP inhibitors appear to be more robust than the genetic abolition of PARP gene expression, indicating the presence of alternative targets of those drugs. Collectively, recent evidence suggests a conditionality of stress-related phenotypes of parp mutants and calls for a reconsideration of PARP inhibitor studies on plants. This review critically summarizes our current understanding of poly(ADP-ribosylation) and PARP proteins in plants, highlighting similarities and differences to human PARPs, areas of controversy, and requirements for future studies.