A nucleolar targeting signal in PML-I addresses PML to nucleolar caps in stressed or senescent cells

PML Body Biogenesis: A Delicate Balance of Interactions

PML bodies are subnuclear protein complexes that play a crucial role in various physiological and pathological cellular processes. One of the general structural proteins of PML bodies is a member of the tripartite motif (TRIM) family-promyelocytic leukemia protein (PML). It is known that PML interacts with over a hundred partners, and the protein itself is represented by several major isoforms, differing in their variable and disordered C-terminal end due to alternative splicing. Despite nearly 30 years of research, the mechanisms underlying PML body formation and the role of PML proteins in this process remain largely unclear. In this review, we examine the literature and highlight recent progress in this field, with a particular focus on understanding the role of individual domains of the PML protein, its post-translational modifications, and polyvalent nonspecific interactions in the formation of PML bodies. Additionally, based on the available literature, we propose a new hypothetical model of PML body formation.

Learning consistent subcellular landmarks to quantify changes in multiplexed protein maps

Highly multiplexed imaging holds enormous promise for understanding how spatial context shapes the activity of the genome and its products at multiple length scales. Here, we introduce a deep learning framework called CAMPA (Conditional Autoencoder for Multiplexed Pixel Analysis), which uses a conditional variational autoencoder to learn representations of molecular pixel profiles that are consistent across heterogeneous cell populations and experimental perturbations. Clustering these pixel-level representations identifies consistent subcellular landmarks, which can be quantitatively compared in terms of their size, shape, molecular composition and relative spatial organization. Using high-resolution multiplexed immunofluorescence, this reveals how subcellular organization changes upon perturbation of RNA synthesis, RNA processing or cell size, and uncovers links between the molecular composition of membraneless organelles and cell-to-cell variability in bulk RNA synthesis rates. By capturing interpretable cellular phenotypes, we anticipate that CAMPA will greatly accelerate the systematic mapping of multiscale atlases of biological organization to identify the rules by which context shapes physiology and disease.

PML and PML-like exonucleases restrict retrotransposons in jawed vertebrates

We have uncovered a role for the promyelocytic leukemia (PML) gene and novel PML-like DEDDh exonucleases in the maintenance of genome stability through the restriction of LINE-1 (L1) retrotransposition in jawed vertebrates. Although the mammalian PML protein forms nuclear bodies, we found that the spotted gar PML ortholog and related proteins in fish function as cytoplasmic DEDDh exonucleases. In contrast, PML proteins from amniote species localized both to the cytoplasm and formed nuclear bodies. We also identified the PML-like exon 9 (Plex9) genes in teleost fishes that encode exonucleases. Plex9 proteins resemble TREX1 but are unique from the TREX family and share homology to gar PML. We also characterized the molecular evolution of TREX1 and the first non-mammalian TREX1 homologs in axolotl. In an example of convergent evolution and akin to TREX1, gar PML and zebrafish Plex9 proteins suppressed L1 retrotransposition and could complement TREX1 knockout in mammalian cells. Following export to the cytoplasm, the human PML-I isoform also restricted L1 through its conserved C-terminus by enhancing ORF1p degradation through the ubiquitin-proteasome system. Thus, PML first emerged as a cytoplasmic suppressor of retroelements, and this function is retained in amniotes despite its new role in the assembly of nuclear bodies.

PML Alternative Splice Products Differentially Regulate HAdV Productive Infection

Promyelocytic leukemia nuclear bodies (PML-NBs) were considered to maintain antiviral capacity, as these spherical complexes are antagonized by viruses. Actual work provides evidence, that PML-NB-associated factors might also be beneficial for distinct viral processes indicating why genomes and replication centers of nuclear replicating viruses are often found juxtaposed to PML-NBs. Several early HAdV proteins target PML-NBs, such as E4orf3 that promotes redistribution into track-like structures. PML-associated dependency factors that enhance viral gene expression, such as Sp100A remain in the nuclear tracks while restrictive factors, such as Daxx, are inhibited by either proteasomal degradation or relocalization to repress antiviral functions. Here, we did a comprehensive analysis of nuclear PML isoforms during HAdV infection. Our results show cell line specific differences as PML isoforms differentially regulate productive HAdV replication and progeny production. Here, we identified PML-II as a dependency factor that supports viral progeny production, while PML-III and PML-IV suppress viral replication. In contrast, we identified PML-I as a positive regulator and PML-V as a restrictive factor during HAdV infection. Solely PML-VI was shown to repress adenoviral progeny production in both model systems. We showed for the first time, that HAdV can reorganize PML-NBs that contain PML isoforms other then PML-II. Intriguingly, HAdV was not able to fully disrupt PML-NBs composed out of the PML isoforms that inhibit viral replication, while PML-NBs composed out of PML isoforms with beneficial influence on the virus formed tracks in all examined cells. In sum, our findings clearly illustrate the crucial role of PML-track formation in efficient viral replication. IMPORTANCE Actual work provides evidence that PML-NB-associated factors might also be beneficial for distinct viral processes indicating why genomes and replication centers of nuclear replicating viruses are often found juxtaposed to PML-NBs. Alternatively spliced PML isoforms I-VII are expressed from one single pml gene containing nine exons and their transcription is tightly controlled and stimulated by interferons and p53. Several early HAdV proteins target PML-NBs, such as E4orf3, promoting redistribution into track-like structures. Our comprehensive studies indicate a diverging role of PML isoforms throughout the course of productive HAdV infection in either stably transformed human lung (H1299) or liver (HepG2) cells, in which we observed a multivalent regulation of HAdV by all six PML isoforms. PML-I and PML-II support HAdV-mediated track formation and efficient formation of viral replication centers, thus promoting HAdV productive infection. Simultaneously, PML-III, -IV,-V, and -VI antagonize viral gene expression and particle production.

Running 'LAPS' Around nLD: Nuclear Lipid Droplet Form and Function

The nucleus harbours numerous protein subdomains and condensates that regulate chromatin organization, gene expression and genomic stress. A novel nuclear subdomain that is formed following exposure of cells to excess fatty acids is the nuclear lipid droplet (nLD), which is composed of a neutral lipid core surrounded by a phospholipid monolayer and associated regulatory and lipid biosynthetic enzymes. While structurally resembling cytoplasmic LDs, nLDs are formed by distinct but poorly understood mechanisms that involve the emergence of lipid droplets from the lumen of the nucleoplasmic reticulum and de novo lipid synthesis. Luminal lipid droplets that emerge into the nucleoplasm do so at regions of the inner nuclear membrane that become enriched in promyelocytic leukemia (PML) protein. The resulting nLDs that retain PML on their surface are termed lipid-associated PML structures (LAPS), and are distinct from canonical PML nuclear bodies (NB) as they lack key proteins and modifications associated with these NBs. PML is a key regulator of nuclear signaling events and PML NBs are sites of gene regulation and post-translational modification of transcription factors. Therefore, the subfraction of nLDs that form LAPS could regulate lipid stress responses through their recruitment and retention of the PML protein. Both nLDs and LAPS have lipid biosynthetic enzymes on their surface suggesting they are active sites for nuclear phospholipid and triacylglycerol synthesis as well as global lipid regulation. In this review we have summarized the current understanding of nLD and LAPS biogenesis in different cell types, their structure and composition relative to other PML-associated cellular structures, and their role in coordinating a nuclear response to cellular overload of fatty acids.

SUMO: Glue or Solvent for Phase-Separated Ribonucleoprotein Complexes and Molecular Condensates?

Spatial organization of cellular processes in membranous or membrane-less organelles (MLOs, alias molecular condensates) is a key concept for compartmentalizing biochemical pathways. Prime examples of MLOs are the nucleolus, PML nuclear bodies, nuclear splicing speckles or cytosolic stress granules. They all represent distinct sub-cellular structures typically enriched in intrinsically disordered proteins and/or RNA and are formed in a process driven by liquid-liquid phase separation. Several MLOs are critically involved in proteostasis and their formation, disassembly and composition are highly sensitive to proteotoxic insults. Changes in the dynamics of MLOs are a major driver of cell dysfunction and disease. There is growing evidence that post-translational modifications are critically involved in controlling the dynamics and composition of MLOs and recent evidence supports an important role of the ubiquitin-like SUMO system in regulating both the assembly and disassembly of these structures. Here we will review our current understanding of SUMO function in MLO dynamics under both normal and pathological conditions.

RNA phase separation-mediated direction of molecular trafficking under conditions of molecular crowding

Living cells are highly crowded with large and small biomolecules. The total concentration of biomolecules can reach 400 mg/ml, and 40% of the cell volume is occupied by biomolecules. Droplet formation in cells via liquid-liquid phase separation may play a role in controlling biochemical reactions in this complex molecular environment. Liquid-liquid phase separation generally involves nucleic acids and proteins as anionic and cationic components, respectively. Significant characteristics of droplets, which make them different from protein aggregation or fibril formation, are reversibility of formation and responsiveness to the molecular environment. In this review, we quantitatively describe the molecular environment inside cells and droplets that participate in controlling central dogma reactions. Finally, we discuss the importance of droplets under conditions of molecular crowding within living cells.

Dynamic PML protein nucleolar associations with persistent DNA damage lesions in response to nucleolar stress and senescence-inducing stimuli

Diverse stress insults trigger interactions of PML with nucleolus, however, the function of these PML nucleolar associations (PNAs) remains unclear. Here we show that during induction of DNA damage-induced senescence in human non-cancerous cells, PML accumulates at the nucleolar periphery simultaneously with inactivation of RNA polymerase I (RNAP I) and nucleolar segregation. Using time-lapse and high-resolution microscopy, we followed the genesis, structural transitions and destiny of PNAs to show that: 1) the dynamic structural changes of the PML-nucleolar interaction are tightly associated with inactivation and reactivation of RNAP I-mediated transcription, respectively; 2) the PML-nucleolar compartment develops sequentially under stress and, upon stress termination, it culminates in either of two fates: disappearance or persistence; 3) all PNAs stages can associate with DNA damage markers; 4) the persistent, commonly long-lasting PML multi-protein nucleolar structures (PML-NDS) associate with markers of DNA damage, indicating a role of PNAs in persistent DNA damage response characteristic for senescent cells. Given the emerging evidence implicating PML in homologous recombination-directed DNA repair, we propose that PNAs contribute to sequestration and faithful repair of the highly unstable ribosomal DNA repeats, a fundamental process to maintain a precise balance between DNA repair mechanisms, with implications for genomic integrity and aging.

PML Bodies in Mitosis

Promyelocytic leukemia (PML) bodies are dynamic intracellular structures that recruit and release a variety of different proteins in response to stress, virus infection, DNA damage and cell cycle progression. While PML bodies primarily are regarded as nuclear compartments, they are forced to travel to the cytoplasm each time a cell divides, due to breakdown of the nuclear membrane at entry into mitosis and subsequent nuclear exclusion of nuclear material at exit from mitosis. Here we review the biochemical and biophysical transitions that occur in PML bodies during mitosis and discuss this in light of post-mitotic nuclear import, cell fate decision and acute promyelocytic leukemia therapy.

APE1 polymorphic variants cause persistent genomic stress and affect cancer cell proliferation

Apurinic/apyrimidinic endonuclease 1 (APE1) is the main mammalian AP-endonuclease responsible for the repair of endogenous DNA damage through the base excision repair (BER) pathway. Molecular epidemiological studies have identified several genetic variants associated with human diseases, but a well-defined functional connection between mutations in APE1 and disease development is lacking. In order to understand the biological consequences of APE1 genetic mutations, we examined the molecular and cellular consequences of the selective expression of four non-synonymous APE1 variants (L104R, R237C, D148E and D283G) in human cells. We found that D283G, L104R and R237C variants have reduced endonuclease activity and impaired ability to associate with XRCC1 and DNA polymerase β, which are enzymes acting downstream of APE1 in the BER pathway. Complementation experiments performed in cells, where endogenous APE1 had been silenced by shRNA, showed that the expression of these variants resulted in increased phosphorylation of histone H2Ax and augmented levels of poly(ADP-ribosyl)ated (PAR) proteins. Persistent activation of DNA damage response markers was accompanied by growth defects likely due to combined apoptotic and autophagic processes. These phenotypes were observed in the absence of exogenous stressors, suggesting that chronic replication stress elicited by the BER defect may lead to a chronic activation of the DNA damage response. Hence, our data reinforce the concept that non-synonymous APE1 variants present in the human population may act as cancer susceptibility alleles.

Acute Promyelocytic Leukemia: A Paradigm for Oncoprotein-Targeted Cure

Recent clinical trials have demonstrated that the immense majority of acute promyelocytic leukemia (APL) patients can be definitively cured by the combination of two targeted therapies: retinoic acid (RA) and arsenic. Mouse models have provided unexpected insights into the mechanisms involved. Restoration of PML nuclear bodies upon RA- and/or arsenic-initiated PML/RARA degradation is essential, while RA-triggered transcriptional activation is dispensable for APL eradication. Mutations of the arsenic-binding site of PML/RARA, but also PML, have been detected in therapy-resistant patients, demonstrating the key role of PML in APL cure. PML nuclear bodies are druggable and could be harnessed in other conditions.

Promyelocytic Leukemia Protein, a Protein at the Crossroad of Oxidative Stress and Metabolism

SIGNIFICANCE

Cellular metabolic activity impacts the production of reactive oxygen species (ROS), both positively through mitochondrial oxidative processes and negatively by promoting the production of reducing agents (including NADPH and reduced glutathione). A defined metabolic state in cancer cells is critical for cell growth and long-term self-renewal, and such state is intrinsically associated with redox balance. Promyelocytic leukemia protein (PML) regulates several biological processes, at least in part, through its ability to control the assembly of PML nuclear bodies (PML NBs). Recent Advances: PML is oxidation-prone, and oxidative stress promotes NB biogenesis. These nuclear subdomains recruit many nuclear proteins and regulate their SUMOylation and other post-translational modifications. Some of these cargos-such as p53, SIRT1, AKT, and mammalian target of rapamycin (mTOR)-are key regulators of cell fate. PML was also recently shown to regulate oxidation.

CRITICAL ISSUES

While it was long considered primarily as a tumor suppressor protein, PML-regulated metabolic switch uncovered that this protein could promote survival and/or stemness of some normal or cancer cells. In this study, we review the recent findings on this multifunctional protein.

FUTURE DIRECTIONS

Studying PML scaffolding functions as well as its fine role in the activation of p53 or fatty acid oxidation will bring new insights in how PML could bridge oxidative stress, senescence, cell death, and metabolism. Antioxid. Redox Signal. 26, 432-444.

Human promyelocytic leukemia protein is targeted to distinct subnuclear domains in plant nuclei and colocalizes with nucleolar constituents in a SUMO-dependent manner

Eukaryotic nuclei are subdivided into subnuclear structures. Among the most prominent of these structures are the nucleolus and the PML nuclear bodies (PML‐NBs). PML‐NBs are spherical multiprotein aggregates of varying size localized in the interchromosomal area. PML‐NB formation is dependent on the presence of the promyelocytic leukemia protein (PML) as well as on post‐translational modification of core components by covalent attachment of the small ubiquitin‐like modifier (SUMO). So far, PML‐NBs as well as PML have been described in mammalian cells only, whereas no orthologs are known in the plant kingdom. In order to investigate conserved mechanisms in PML targeting, we expressed human PML (hPML) fused to the GFP in Nicotiana benthamiana. Using confocal laser scanning microscopy and coimmunoprecipitation followed by mass spectrometric analysis, we found the fusion protein in association with nucleolar constituents. Importantly, mutants of hPML, which are no longer SUMOylated, showed altered localizations, implying SUMO‐dependent targeting of hPML in plants as has previously been shown for mammalian cells. Interestingly, in the presence of proteasome inhibitors, hPML could also be found in the nucleolus of mammalian cells suggesting conserved targeting mechanisms of PML across kingdoms. Finally, Solanum tuberosum COP1, a proposed PML‐like protein from plants, was fused to the red fluorescent protein (RFP) and coexpressed with hPML::eGFP. Microscopic analysis confirmed the localization of COP1::RFP in nuclear speckles. However, hPML::eGFP did not colocalize with COP1::RFP. Hence, we conclude that plants do not possess specialized PML‐NBs, but that their functions may be covered by other subnuclear structures like the nucleolus.

Database Proteomics data have been deposited to the ProteomeXchange Consortium with the identifier PXD004254.

Nucleolar Reorganization Upon Site-Specific Double-Strand Break Induction

DNA damage response (DDR) in ribosomal genes and mechanisms of DNA repair in embryonic stem cells (ESCs) are less explored nuclear events. DDR in ESCs should be unique due to their high proliferation rate, expression of pluripotency factors, and specific chromatin signature. Given short population doubling time and fast progress through G1 phase, ESCs require a sustained production of rRNA, which leads to the formation of large and prominent nucleoli. Although transcription of rRNA in the nucleolus is relatively well understood, little is known about DDR in this nuclear compartment. Here, we directed formation of double-strand breaks in rRNA genes with I- PpoI endonuclease, and we studied nucleolar morphology, DDR, and chromatin modifications. We observed a pronounced formation of I- PpoI-induced nucleolar caps, positive on BRCA1, NBS1, MDC1, γH2AX, and UBF1 proteins. We showed interaction of nucleolar protein TCOF1 with HDAC1 and TCOF1 with CARM1 after DNA injury. Moreover, H3R17me2a modification mediated by CARM1 was found in I- PpoI-induced nucleolar caps. Finally, we report that heterochromatin protein 1 is not involved in DNA repair of nucleolar caps.

A Promyelocytic Leukemia Protein-Thrombospondin-2 Axis and the Risk of Relapse in Neuroblastoma

PURPOSE

Neuroblastoma is a childhood malignancy originating from the sympathetic nervous system with a complex biology, prone to metastasize and relapse. High-risk, metastatic cases are explained in part by amplification or mutation of oncogenes, such as MYCN and ALK, and loss of tumor suppressor genes in chromosome band 1p. However, it is fundamental to identify other pathways responsible for the large portion of neuroblastomas with no obvious molecular alterations.

EXPERIMENTAL DESIGN

Neuroblastoma cell lines were used for the assessment of tumor growth in vivo and in vitro Protein expression in tissues and cells was assessed using immunofluorescence and IHC. The association of promyelocytic leukemia (PML) expression with neuroblastoma outcome and relapse was calculated using log-rank and Mann-Whitney tests, respectively. Gene expression was assessed using chip microarrays.

RESULTS

PML is detected in the developing and adult sympathetic nervous system, whereas it is not expressed or is low in metastatic neuroblastoma tumors. Reduced PML expression in patients with low-risk cancers, that is, localized and negative for the MYCN proto-oncogene, is strongly associated with tumor recurrence. PML-I, but not PML-IV, isoform suppresses angiogenesis via upregulation of thrombospondin-2 (TSP2), a key inhibitor of angiogenesis. Finally, PML-I and TSP2 expression inversely correlates with tumor angiogenesis and recurrence in localized neuroblastomas.

CONCLUSIONS

Our work reveals a novel PML-I-TSP2 axis for the regulation of angiogenesis and cancer relapse, which could be used to identify patients with low-risk, localized tumors that might benefit from chemotherapy. Clin Cancer Res; 22(13); 3398-409. ©2016 AACR.

Phase separation in biology; functional organization of a higher order

Inside eukaryotic cells, macromolecules are partitioned into membrane-bounded compartments and, within these, some are further organized into non-membrane-bounded structures termed membrane-less organelles. The latter structures are comprised of heterogeneous mixtures of proteins and nucleic acids and assemble through a phase separation phenomenon similar to polymer condensation. Membrane-less organelles are dynamic structures maintained through multivalent interactions that mediate diverse biological processes, many involved in RNA metabolism. They rapidly exchange components with the cellular milieu and their properties are readily altered in response to environmental cues, often implicating membrane-less organelles in responses to stress signaling. In this review, we discuss: (1) the functional roles of membrane-less organelles, (2) unifying structural and mechanistic principles that underlie their assembly and disassembly, and (3) established and emerging methods used in structural investigations of membrane-less organelles.

PML IV/ARF interaction enhances p53 SUMO-1 conjugation, activation, and senescence

Promyelocytic leukemia protein (PML) nuclear bodies (NBs) recruit multiple partners, including p53 and many of its regulators. NBs are believed to facilitate several posttranslational modifications and are key regulators of senescence. PML, the organizer of NBs, is expressed as a number of splice variants that all efficiently recruit p53 partners. However, overexpression of only one of them, PML IV, triggers p53-driven senescence. Here, we show that PML IV specifically binds ARF, a key p53 regulator. Similar to ARF, PML IV enhances global SUMO-1 conjugation, particularly that of p53, resulting in p53 stabilization and activation. ARF interacts with and stabilizes the NB-associated UBC9 SUMO-conjugating enzyme, possibly explaining PML IV-enhanced SUMOylation. These results unexpectedly link two key tumor suppressors, highlighting their convergence for global control of SUMO conjugation, p53 activation, and senescence induction.

LMNA Mutation c.917T>G (p.L306R) Leads to Deleterious Hyper-Assembly of Lamin A/C and Associates with Severe Right Ventricular Cardiomyopathy and Premature Aging

Mutations in the LMNA gene coding for the nuclear lamina proteins lamin A and its smaller splice form lamin C associate with a heterogeneous group of diseases collectively called laminopathies. Here, we describe a 2-year-old patient with a previously undescribed phenotype including right ventricular cardiomyopathy, progeroid features, and premature death. Sequencing of LMNA revealed a novel heterozygous de novo mutation p.L306R located in the α-helical rod domain of A-type lamins. Fibroblasts from the patient showed reduced proliferation and early premature replicative senescence, as characterized by progressive hyperlobulation of the nuclei, abnormally clustered centromeres, loss of lamin B1, and reorganization of promyelocytic leukemia nuclear bodies. Furthermore, the patient cells were more sensitive to double-strand DNA breaks. Similar structural and phenotypic defects were observed in normal fibroblasts transfected with FLAG-tagged p.L306R lamin A. Correspondingly, in vitro assembly studies revealed that the p.L306R generates a "hyper-assembly" mutant of lamin A that forms extensive fiber arrays under physiological conditions where wild-type lamin A is still largely soluble. In summary, we report a novel LMNA p.L306R mutation that leads to previously undescribed hyper-assembly of lamin A, heavy distortion of nuclear shape and that manifests as right ventricular cardiomyopathy and premature aging.

Cmr1/WDR76 defines a nuclear genotoxic stress body linking genome integrity and protein quality control

DNA replication stress is a source of genomic instability. Here we identify changed mutation rate 1 (Cmr1) as a factor involved in the response to DNA replication stress in Saccharomyces cerevisiae and show that Cmr1--together with Mrc1/Claspin, Pph3, the chaperonin containing TCP1 (CCT) and 25 other proteins--define a novel intranuclear quality control compartment (INQ) that sequesters misfolded, ubiquitylated and sumoylated proteins in response to genotoxic stress. The diversity of proteins that localize to INQ indicates that other biological processes such as cell cycle progression, chromatin and mitotic spindle organization may also be regulated through INQ. Similar to Cmr1, its human orthologue WDR76 responds to proteasome inhibition and DNA damage by relocalizing to nuclear foci and physically associating with CCT, suggesting an evolutionarily conserved biological function. We propose that Cmr1/WDR76 plays a role in the recovery from genotoxic stress through regulation of the turnover of sumoylated and phosphorylated proteins.

A Phospho-SIM in the Antiviral Protein PML is Required for Its Recruitment to HSV-1 Genomes

Herpes simplex virus type 1 (HSV-1) is a significant human pathogen that infects a large portion of the human population. Cells deploy a variety of defenses to limit the extent to which the virus can replicate. One such factor is the promyelocytic leukemia (PML) protein, the nucleating and organizing factor of nuclear domain 10 (ND10). PML responds to a number of stimuli and is implicated in intrinsic and innate cellular antiviral defenses against HSV-1. While the role of PML in a number of cellular pathways is controlled by post-translational modifications, the effects of phosphorylation on its antiviral activity toward HSV-1 have been largely unexplored. Consequently, we mapped phosphorylation sites on PML, mutated these and other known phosphorylation sites on PML isoform I (PML-I), and examined their effects on a number of PML's activities. Our results show that phosphorylation at most sites on PML-I is dispensable for the formation of ND10s and colocalization between PML-I and the HSV-1 regulatory protein, ICP0, which antagonizes PML-I function. However, inhibiting phosphorylation at sites near the SUMO-interaction motif (SIM) of PML-I impairs its ability to respond to HSV-1 infection. Overall, our data suggest that PML phosphorylation regulates its antiviral activity against HSV-1.

A Dialog on the First 20 Years of PML Research and the Next 20 Ahead

This introductory article has been written in the form of a conversation between Pier Paolo Pandolfi, Director of the Cancer Center of Beth Israel Deaconess Medical Center in Boston, and Rosa Bernardi, a former post-doctoral fellow in the laboratory of Dr. Pandolfi, now principal investigator at San Raffaele Scientific Institute in Milan, Italy. We have chosen this atypical review format because we want to offer to our readers a more direct and personal perspective on the first 20 years of research over the promyelocytic leukemia gene. This article begins as an interview, but soon transforms into a dialog where we exchange our thoughts on a number of issues around the past, present, and future research over the biology of PML. We were particularly keen on emphasizing the aspects that we find most interesting or challenging, therefore, we warn our readers that this will not be a comprehensive essay but rather a very personal view of what has been, is, and will be exciting and interesting in the PML world, in our opinion.

PML isoforms in response to arsenic: high-resolution analysis of PML body structure and degradation

Arsenic is a clinically effective treatment for acute promyelocytic leukaemia (APL) in which the promyelocytic leukaemia (PML) protein is fused to retinoic receptor alpha (RARα). PML-RARα is degraded by the proteasome by a SUMO-dependent, ubiquitin-mediated pathway in response to arsenic treatment, curing the disease. Six major PML isoforms are expressed as a result of alternative splicing, each of which encodes a unique C-terminal region. Using a system in which only a single EYFP-linked PML isoform is expressed, we demonstrate that PMLI, PMLII and PMLVI accumulate in the cytoplasm following arsenic treatment, whereas PMLIII, PMLIV and PMLV do not. 3D structured illumination was used to obtain super-resolution images of PML bodies, revealing spherical shells of PML along with associated SUMO. Arsenic treatment results in dramatic isoform-specific changes to PML body ultrastructure. After extended arsenic treatment most PML isoforms are degraded, leaving SUMO at the core of the nuclear bodies. A high-content imaging assay identifies PMLV as the isoform most readily degraded following arsenic treatment, and PMLIV as relatively resistant to degradation. Immunoprecipitation analysis demonstrates that all PML isoforms are modified by SUMO and ubiquitin after arsenic treatment, and by using siRNA, we demonstrate that arsenic-induced degradation of all PML isoforms is dependent on the ubiquitin E3 ligase RNF4. Intriguingly, depletion of RNF4 results in marked accumulation of PMLV, suggesting that this isoform is an optimal substrate for RNF4. Thus the variable C-terminal domain influences the rate and location of degradation of PML isoforms following arsenic treatment.

Activation of the tumor suppressor p53 upon impairment of ribosome biogenesis

Errors in ribosome biogenesis can result in quantitative or qualitative defects in protein synthesis and consequently lead to improper execution of the genetic program and the development of specific diseases. Evidence has accumulated over the last decade suggesting that perturbation of ribosome biogenesis triggers a p53-activating checkpoint signaling pathway, often referred to as the ribosome biogenesis stress checkpoint pathway. Although it was originally suggested that p53 has a prominent role in preventing diseases by monitoring the fidelity of ribosome biogenesis, recent work has demonstrated that p53 activation upon impairment of ribosome biogenesis also mediates pathological manifestations in humans. Perturbations of ribosome biogenesis can trigger a p53-dependent checkpoint signaling pathway independent of DNA damage and the tumor suppressor ARF through inhibitory interactions of specific ribosomal components with the p53 negative regulator, Mdm2. Here we review the recent advances made toward understanding of this newly-recognized checkpoint signaling pathway, its role in health and disease, and discuss possible future directions in this exciting research field. This article is part of a Special Issue entitled: Role of the Nucleolus in Human Disease.

The translation initiation factor 3 subunit eIF3K interacts with PML and associates with PML nuclear bodies

The promyelocytic leukemia protein (PML) is a tumor suppressor protein that regulates a variety of important cellular processes, including gene expression, DNA repair and cell fate decisions. Integral to its function is the ability of PML to form nuclear bodies (NBs) that serve as hubs for the interaction and modification of over 90 cellular proteins. There are seven canonical isoforms of PML, which encode diverse C-termini generated by alternative pre-mRNA splicing. Recruitment of specific cellular proteins to PML NBs is mediated by protein-protein interactions with individual PML isoforms. Using a yeast two-hybrid screen employing peptide sequences unique to PML isoform I (PML-I), we identified an interaction with the eukaryotic initiation factor 3 subunit K (eIF3K), and in the process identified a novel eIF3K isoform, which we term eIF3K-2. We further demonstrate that eIF3K and PML interact both in vitro via pull-down assays, as well as in vivo within human cells by co-immunoprecipitation and co-immunofluorescence. In addition, eIF3K isoform 2 (eIF3K-2) colocalizes to PML bodies, particularly those enriched in PML-I, while eIF3K isoform 1 associates poorly with PML NBs. Thus, we report eIF3K as the first known subunit of the eIF3 translation pre-initiation complex to interact directly with the PML protein, and provide data implicating alternative splicing of both PML and eIF3K as a possible regulatory mechanism for eIF3K localization at PML NBs.

PML-mediated signaling and its role in cancer stem cells

The promyelocytic leukemia (PML) protein, initially discovered as a part of the PML/retinoic acid receptor alpha fusion protein, has been found to be a critical player in oncogenesis and tumor progression. Multiple cellular activities, including DNA repair, alternative lengthening of telomeres, transcriptional control, apoptosis and senescence, are regulated by PML and its featured subcellular structure, the PML nuclear body. In correspondence with its role in many important life processes, PML mediates several complex downstream signaling pathways. The determinant function of PML in tumorigenesis and cancer progression raises the interest in its involvement in cancer stem cells (CSCs), a subpopulation of cancer cells that share properties with stem cells and are critical for tumor propagation. Recently, there are exciting discoveries concerning the requirement of PML in CSC maintenance. Growing evidences strongly suggest a positive role of PML in regulating CSCs in both hematopoietic cancers and solid tumors, whereas the underlying mechanisms may be different and remain elusive. Here we summarize and discuss the PML-mediated signaling pathways in cancers and their potential roles in regulating CSCs.

Vitiligo: a possible model of degenerative diseases

Vitiligo is characterized by the progressive disappearance of pigment cells from skin and hair follicle. Several in vitro and in vivo studies show evidence of an altered redox status, suggesting that loss of cellular redox equilibrium might be the pathogenic mechanism in vitiligo. However, despite the numerous data supporting a pathogenic role of oxidative stress, there is still no consensus explanation underlying the oxidative stress-driven disappear of melanocytes from the epidermis. In this study, in vitro characterization of melanocytes cultures from non-lesional vitiligo skin revealed at the cellular level aberrant function of signal transduction pathways common with neurodegenerative diseases including modification of lipid metabolism, hyperactivation of mitogen-activated protein kinase (MAPK) and cAMP response element-binding protein (CREB), constitutive p53-dependent stress signal transduction cascades, and enhanced sensibility to pro-apoptotic stimuli. Notably, these long-term effects of subcytotoxic oxidative stress are also biomarkers of pre-senescent cellular phenotype. Consistent with this, vitiligo cells showed a significant increase in p16 that did not correlate with the chronological age of the donor. Moreover, vitiligo melanocytes produced many biologically active proteins among the senescence-associated secretory phenotype (SAPS), such as interleukin-6 (IL-6), matrix metallo proteinase-3 (MMP3), cyclooxygenase-2 (Cox-2), insulin-like growth factor-binding protein-3 and 7 (IGFBP3, IGFBP7). Together, these data argue for a complicated pathophysiologic puzzle underlying melanocytes degeneration resembling, from the biological point of view, neurodegenerative diseases. Our results suggest new possible targets for intervention that in combination with current therapies could correct melanocytes intrinsic defects.

Mutual protection of ribosomal proteins L5 and L11 from degradation is essential for p53 activation upon ribosomal biogenesis stress

Impairment of ribosomal biogenesis can activate the p53 protein independently of DNA damage. The ability of ribosomal proteins L5, L11, L23, L26, or S7 to bind Mdm2 and inhibit its ubiquitin ligase activity has been suggested as a critical step in p53 activation under these conditions. Here, we report that L5 and L11 are particularly important for this response. Whereas several other newly synthesized ribosomal proteins are degraded by proteasomes upon inhibition of Pol I activity by actinomycin D, L5 and L11 accumulate in the ribosome-free fraction where they bind to Mdm2. This selective accumulation of free L5 and L11 is due to their mutual protection from proteasomal degradation. Furthermore, the endogenous, newly synthesized L5 and L11 continue to be imported into nucleoli even after nucleolar disruption and colocalize with Mdm2, p53, and promyelocytic leukemia protein. This suggests that the disrupted nucleoli may provide a platform for L5- and L11-dependent p53 activation, implying a role for the nucleolus in p53 activation by ribosomal biogenesis stress. These findings may have important implications with respect to understanding the pathogenesis of diseases caused by impaired ribosome biogenesis.

Requirement of PML SUMO interacting motif for RNF4- or arsenic trioxide-induced degradation of nuclear PML isoforms

PML, the organizer of nuclear bodies (NBs), is expressed in several isoforms designated PMLI to VII which differ in their C-terminal region due to alternative splicing of a single gene. This variability is important for the function of the different PML isoforms. PML NB formation requires the covalent linkage of SUMO to PML. Arsenic trioxide (As₂O₃) enhances PML SUMOylation leading to an increase in PML NB size and promotes its interaction with RNF4, a poly-SUMO-dependent ubiquitin E3 ligase responsible for proteasome-mediated PML degradation. Furthermore, the presence of a bona fide SUMO Interacting Motif (SIM) within the C-terminal region of PML seems to be required for recruitment of other SUMOylated proteins within PML NBs. This motif is present in all PML isoforms, except in the nuclear PMLVI and in the cytoplasmic PMLVII. Using a bioluminescence resonance energy transfer (BRET) assay in living cells, we found that As₂O₃ enhanced the SUMOylation and interaction with RNF4 of nuclear PML isoforms (I to VI). In addition, among the nuclear PML isoforms, only the one lacking the SIM sequence, PMLVI, was resistant to As₂O₃-induced PML degradation. Similarly, mutation of the SIM in PMLIII abrogated its sensitivity to As₂O₃-induced degradation. PMLVI and PMLIII-SIM mutant still interacted with RNF4. However, their resistance to the degradation process was due to their inability to be polyubiquitinated and to recruit efficiently the 20S core and the β regulatory subunit of the 11S complex of the proteasome in PML NBs. Such resistance of PMLVI to As₂O₃-induced degradation was alleviated by overexpression of RNF4. Our results demonstrate that the SIM of PML is dispensable for PML SUMOylation and interaction with RNF4 but is required for efficient PML ubiquitination, recruitment of proteasome components within NBs and proteasome-dependent degradation of PML in response to As₂O₃.

Contribution of the C-terminal regions of promyelocytic leukemia protein (PML) isoforms II and V to PML nuclear body formation

Promyelocytic leukemia protein (PML) nuclear bodies are dynamic and heterogeneous nuclear protein complexes implicated in various important functions, most notably tumor suppression. PML is the structural component of PML nuclear bodies and has several nuclear splice isoforms that share a common N-terminal region but differ in their C termini. Previous studies have suggested that the coiled-coil motif within the N-terminal region is sufficient for PML nuclear body formation by mediating homo/multi-dimerization of PML molecules. However, it has not been investigated whether any of the C-terminal variants of PML may contribute to PML body assembly. Here we report that the unique C-terminal domains of PML-II and PML-V can target to PML-NBs independent of their N-terminal region. Strikingly, both domains can form nuclear bodies in the absence of endogenous PML. The C-terminal domain of PML-II interacts transiently with unknown binding sites at PML nuclear bodies, whereas the C-terminal domain of PML-V exhibits hyperstable binding to PML bodies via homo-dimerization. This strong interaction is mediated by a putative α-helix in the C terminus of PML-V. Moreover, nuclear bodies assembled from the C-terminal domain of PML-V also recruit additional PML body components, including Daxx and Sp100. These observations establish the C-terminal domain of PML-V as an additional important contributor to the assembly mechanism(s) of PML bodies.

Herpes simplex virus 1 ubiquitin ligase ICP0 interacts with PML isoform I and induces its SUMO-independent degradation

Herpes simplex virus 1 (HSV-1) immediate-early protein ICP0 localizes to cellular structures known as promyelocytic leukemia protein (PML) nuclear bodies or ND10 and disrupts their integrity by inducing the degradation of PML. There are six PML isoforms with different C-terminal regions in ND10, of which PML isoform I (PML.I) is the most abundant. Depletion of all PML isoforms increases the plaque formation efficiency of ICP0-null mutant HSV-1, and reconstitution of expression of PML.I and PML.II partially reverses this improved replication. ICP0 also induces widespread degradation of SUMO-conjugated proteins during HSV-1 infection, and this activity is linked to its ability to counteract cellular intrinsic antiviral resistance. All PML isoforms are highly SUMO modified, and all such modified forms are sensitive to ICP0-mediated degradation. However, in contrast to the situation with the other isoforms, ICP0 also targets PML.I that is not modified by SUMO, and PML in general is degraded more rapidly than the bulk of other SUMO-modified proteins. We report here that ICP0 interacts with PML.I in both yeast two-hybrid and coimmunoprecipitation assays. This interaction is dependent on PML.I isoform-specific sequences and the N-terminal half of ICP0 and is required for SUMO-modification-independent degradation of PML.I by ICP0. Degradation of the other PML isoforms by ICP0 was less efficient in cells specifically depleted of PML.I. Therefore, ICP0 has two distinct mechanisms of targeting PML: one dependent on SUMO modification and the other via SUMO-independent interaction with PML.I. We conclude that the ICP0-PML.I interaction reflects a countermeasure to PML-related antiviral restriction.

The cell biology of disease: Acute promyelocytic leukemia, arsenic, and PML bodies

Acute promyelocytic leukemia (APL) is driven by a chromosomal translocation whose product, the PML/retinoic acid (RA) receptor α (RARA) fusion protein, affects both nuclear receptor signaling and PML body assembly. Dissection of APL pathogenesis has led to the rediscovery of PML bodies and revealed their role in cell senescence, disease pathogenesis, and responsiveness to treatment. APL is remarkable because of the fortuitous identification of two clinically effective therapies, RA and arsenic, both of which degrade PML/RARA oncoprotein and, together, cure APL. Analysis of arsenic-induced PML or PML/RARA degradation has implicated oxidative stress in the biogenesis of nuclear bodies and SUMO in their degradation.

Intracellular transport of recombinant adeno-associated virus vectors

Recombinant adeno-associated viral vectors (rAAVs) have been widely used for gene delivery in animal models, and are currently evaluated for human gene therapy after successful clinical trials in the treatment of inherited, degenerative or acquired diseases, such as Leber congenital amaurosis, Parkinson disease or heart failure. However, limitations in vector tropism, such as limited tissue specificity and insufficient transduction efficiencies of particular tissues and cell types, still preclude therapeutic applications in certain tissues. Wild-type adeno-associated viruses (AAVs) are defective viruses that require the presence of a helper virus to complete their life cycle. On the one hand, this unique property makes AAV vectors one of the safest available viral vectors for gene delivery. On the other, it also represents a potential obstacle because rAAV vectors have to overcome several biological barriers in the absence of a helper virus to transduce successfully a cell. Consequently, a better understanding of the cellular roadblocks that limit rAAV gene delivery is crucial and, during the last 15 years, numerous studies resulted in an expanding body of knowledge of the intracellular trafficking pathways of rAAV vectors. This review describes our current understanding of the mechanisms involved in rAAV attachment to target cells, endocytosis, intracellular trafficking, capsid processing, nuclear import and genome release with an emphasis on the most recent discoveries in the field and the emerging strategies used to improve the efficiency of AAV-derived vectors.

Immunofluorescent localization of ubiquitin and proteasomes in nucleolar vacuoles of soybean root meristematic cells

In this study, using the immunofluorescent method, the immunopositive signals to ubiquitin and proteasomes in nucleoli of root meristematic cells of soybean seedlings have been observed. In fact, those signals were present exclusively in nucleolar vacuoles. No signals were observed in the nucleolar territory out of the nucleolar vacuoles or in the nucleoli without vacuoles. The ubiquitin-proteasome system (UPS) may act within the nucleoli of plants with high metabolic activities and may provide an additional level of regulation of intracellular proteolysis via compartment-specific activities of their components. It is suggested that the presence of the UPS solely in vacuolated nucleoli serves as a mechanism that enhances the speed of ribosome subunit production in very actively transcribing nucleoli. On the other hand, nucleolar vacuoles in a cell/nucleus could play additional roles associated with temporary sequestration or storage of some cellular factors, including components of the ubiquitin-proteasome system.

Interferon-β induces cellular senescence in cutaneous human papilloma virus-transformed human keratinocytes by affecting p53 transactivating activity

Interferon (IFN)-β inhibits cell proliferation and affects cell cycle in keratinocytes transformed by both mucosal high risk Human Papilloma Virus (HPV) and cutaneous HPV E6 and E7 proteins. In particular, upon longer IFN-β treatments, cutaneous HPV38 expressing cells undergo senescence. IFN-β appears to induce senescence by upregulating the expression of the tumor suppressor PML, a well known IFN-induced gene. Indeed, experiments in gene silencing via specific siRNAs have shown that PML is essential in the execution of the senescence programme and that both p53 and p21 pathways are involved. IFN-β treatment leads to a modulation of p53 phosphorylation and acetylation status and a reduction in the expression of the p53 dominant negative ΔNp73. These effects allow the recovery of p53 transactivating activity of target genes involved in the control of cell proliferation. Taken together, these studies suggest that signaling through the IFN pathway might play an important role in cellular senescence. This additional understanding of IFN antitumor action and mechanisms influencing tumor responsiveness or resistance appears useful in aiding further promising development of biomolecular strategies in the IFN therapy of cancer.

PML nuclear bodies and other TRIM-defined subcellular compartments

Tripartite motif (TRIM) proteins are defined by their possession of a RING, B-box and predicted coiled coil (RBCC) domain. The coiled-coil region facilitates the oligomerisation of TRIMs and contributes to the formation of high molecular weight complexes that show interesting subcellular compartmentalisations and structures. TRIM protein compartments include both nuclear and cytoplasmic filaments and aggregates (bodies), as well as diffuse subcellular distributions. TRIM 19, otherwise known as promyelocytic leukaemia (PML) protein forms nuclear aggregates termed PML nuclear bodies (PML NBs), at which a number of functionally diverse proteins transiently or covalently associate. PML NBs are therefore implicated in a wide variety of cellular functions such as transcriptional regulation, viral response, apoptosis and nuclear protein storage.

Defective DNA-damage repair induced by nuclear lamina dysfunction is a key mediator of smooth muscle cell aging

Accumulation of DNA damage is a major driving force of normal cellular aging and has recently been demonstrated to hasten the development of vascular diseases such as atherosclerosis. VSMCs (vascular smooth muscle cells) are essential for vessel wall integrity and repair, and maintenance of their proliferative capacity is essential for vascular health. The signalling pathways that determine VSMC aging remain poorly defined; however, recent evidence implicates persistent DNA damage and the A-type nuclear lamins as key regulators of this process. In the present review, we discuss the importance of the nuclear lamina in the spatial organization of nuclear signalling events, including the DNA-damage response. In particular, we focus on the evidence suggesting that prelamin A accumulation interferes with nuclear spatial compartmentalization by disrupting chromatin organization and DNA-damage repair pathways to promote VSMC aging and senescence.

Ring-like distribution of constitutive heterochromatin in bovine senescent cells

Background

Cells that reach “Hayflick limit” of proliferation, known as senescent cells, possess a particular type of nuclear architecture. Human senescent cells are characterized by the presence of highly condensed senescent associated heterochromatin foci (SAHF) that can be detected both by immunostaining for histone H3 three-methylated at lysine 9 (H3K9me3) and by DAPI counterstaining.

Methods

We have studied nuclear architecture in bovine senescent cells using a combination of immunofluorescence and 3D fluorescent in-situ hybridization (FISH).

Results

Analysis of heterochromatin distribution in bovine senescent cells using fluorescent in situ hybridization for pericentric chromosomal regions, immunostaining of H3K9me3, centromeric proteins CENP A/B and DNA methylation showed a lower level of heterochromatin condensation as compared to young cells. No SAHF foci were observed. Instead, we observed fibrous ring-like or ribbon-like heterochromatin patterns that were undetectable with DAPI counterstaining. These heterochromatin fibers were associated with nucleoli.

Conclusions

Constitutive heterochromatin in bovine senescent cells is organized in ring-like structures.

Mapping signals that are important for nuclear and nucleolar localization in MCRS2

MCRS2 is an oncoprotein that is sequestered in the nucleolus. When in the nucleolus, it promotes the transcription of the rRNA gene. MCRS2 also brings proteins into the nucleolus to change their function. This study analyzes the sequence of MCRS2 and determines that the nuclear localization signal, which has the sequence KRKK, is situated between amino acids 66 and 69. Meanwhile, MCRS2 contains a bipartite nucleolar localization signal, which comprises a KKSK motif, located between amino acids 133 and 136, and a downstream 152-amino acid region, from amino acid 314 to 465. The results of this study are important to understand the function of MCRS2.

PML isoforms I and II participate in PML-dependent restriction of HSV-1 replication

Intrinsic antiviral resistance mediated by constitutively expressed cellular proteins is one arm of defence against virus infection. Promyelocytic leukaemia nuclear bodies (PML-NBs, also known as ND10) contribute to host restriction of herpes simplex virus type 1 (HSV-1) replication via mechanisms that are counteracted by viral regulatory protein ICP0. ND10 assembly is dependent on PML, which comprises several different isoforms, and depletion of all PML isoforms decreases cellular resistance to ICP0-null mutant HSV-1. We report that individual expression of PML isoforms I and II partially reverses the increase in ICP0-null mutant HSV-1 plaque formation that occurs in PML-depleted cells. This activity of PML isoform I is dependent on SUMO modification, its SUMO interaction motif (SIM), and each element of its TRIM domain. Detailed analysis revealed that the punctate foci formed by individual PML isoforms differ subtly from normal ND10 in terms of composition and/or Sp100 modification. Surprisingly, deletion of the SIM motif from PML isoform I resulted in increased colocalisation with other major ND10 components in cells lacking endogenous PML. Our observations suggest that complete functionality of PML is dependent on isoform-specific C-terminal sequences acting in concert.

The role of PML in the nervous system

The promyeloctic leukemia protein PML is a tumor suppressor that was originally identified due to its involvement in the (15;17) translocation of acute promyelocytic leukemia. While the majority of early research has focused upon the role of PML in the pathogenesis of leukemia, more recent evidence has identified important roles for PML in tissues outside the hemopoietic system, including the central nervous system (CNS). Here, we review recent literature on the role of PML in the CNS, with particular focus on the processes of neurodevelopment and neurodegeneration, and propose new lines of investigation.

Subcellular distribution of nuclear import-defective isoforms of the promyelocytic leukemia protein

Background

The promyelocytic leukemia (PML) protein participates in a number of cellular processes, including transcription regulation, apoptosis, differentiation, virus defense and genome maintenance. This protein is structurally organized into a tripartite motif (TRIM) at its N-terminus, a nuclear localization signal (NLS) at its central region and a C-terminus that varies between alternatively spliced isoforms. Most PML splice variants target the nucleus where they define sub-nuclear compartments termed PML nuclear bodies (PML NBs). However, PML variants that lack the NLS are also expressed, suggesting the existence of PML isoforms with cytoplasmic functions. In the present study we expressed PML isoforms with a mutated NLS in U2OS cells to identify potential cytoplasmic compartments targeted by this protein.

Results

Expression of NLS mutated PML isoforms in U2OS cells revealed that PML I targets early endosomes, PML II targets the inner nuclear membrane (partially due to an extra NLS at its C-terminus), and PML III, IV and V target late endosomes/lysosomes. Clustering of PML at all of these subcellular locations depended on a functional TRIM domain.

Conclusions

This study demonstrates the capacity of PML to form macromolecular protein assemblies at several different subcellular sites. Further, it emphasizes a role of the variable C-terminus in subcellular target selection and a general role of the N-terminal TRIM domain in promoting protein clustering.

Post-translational modification of cellular proteins by ubiquitin and ubiquitin-like molecules: role in cellular senescence and aging

Ubiquitination ofendogenous proteins is one of the key regulatory steps that guides protein degradation through regulation of proteasome activity. During the last years evidence has accumulated that proteasome activity is decreased during the aging process in various model systems and that these changes might be causally related to aging and age-associated diseases. Since in most instances ubiquitination is the primary event in target selection, the system ofubiquitination and deubiquitination might be of similar importance. Furthermore, ubiquitination and proteasomal degradation are not completely congruent, since ubiquitination confers also functions different from targeting proteins for degradation. Depending on mono- and polyubiquitination and on how ubiquitin chains are linked together, post-translational modifications of cellular proteins by covalent attachment of ubiquitin and ubiquitin-like proteins are involved in transcriptional regulation, receptor internalization, DNA repair, stabilization of protein complexes and autophagy. Here, we summarize the current knowledge regarding the ubiquitinome and the underlying ubiquitin ligases and deubiquitinating enzymes in replicative senescence, tissue aging as well as in segmental progeroid syndromes and discuss potential causes and consequences for aging.

At the stem of youth and health

Cellular senescence is a specialized form of growth arrest, confined to mitotic cells, induced by various stressful stimuli and characterized by a permanent growth arrest, resistance to apoptosis, an altered pattern of gene expression and the expression of some markers that are characteristic, although not exclusive, to the senescent state. Senescent cells profoundly modify neighboring and remote cells through the production of an altered secretome, eventually leading to inflammation, fibrosis and possibly growth of neoplastic cells. Mammalian aging has been defined as a reduction in the capacity to adequately maintain tissue homeostasis or to repair tissues after injury. Tissue homeostasis and regenerative capacity are nowadays considered to be related to the stem cell pool present in every tissue. For this reason, pathological and patho-physiological conditions characterized by altered tissue homeostasis and impaired regenerative capacity can be viewed as a consequence of the reduction in stem cell number and/or function. Last, cellular senescence is a double-edged sword, since it may inhibit the growth of transformed cells, preventing the occurrence of cancer, while it may facilitate growth of preneoplastic lesions in a paracrine fashion; therefore, interventions targeting this cell response to stress may have a profound impact on many age-related pathologies, ranging from cardiovascular disease to oncology. Aim of this review is to discuss both molecular mechanisms associated with stem cell senescence and interventions that may attenuate or reverse this process.

Dysregulation of promyelocytic leukemia (PML) protein expression in preeclamptic placentae

Promyelocytic leukemia (PML) protein is a nucleoprotein that can regulate a variety of cellular stress responses. The aim of this study was to determine qualitative and quantitative changes in PML expression in preeclamptic placentae. Immunoblot, quantitative reverse transcription polymerase chain reaction (qRT-PCR), and immunohistochemistry techniques were used to determine PML gene expression and localization in normal (n = 6) and preeclamptic (n = 6) placentae and primary cells. Promyelocytic leukemia protein was immunolocalized within nuclei of villus mesenchyme, but largely absent in trophoblast nuclei, with a trend for increased PML reactivity in preeclamptic placenta. Immunoblot analyses of nuclear extracts confirmed relative increases (approximately 3-fold) of PML expression in preeclamptic placentae (P < .05). Conversely, less PML messenger RNA (mRNA; approximately 2-fold) was detected in preeclamptic versus normal placental samples. In vitro, PML expression could be increased by hypoxia in cultured endothelial cells but not trophoblast. Increased PML protein expression in preeclamptic villi suggests it could contribute to decreased vascularity and placental growth and/or function.

PML nuclear bodies

PML nuclear bodies are matrix-associated domains that recruit an astonishing variety of seemingly unrelated proteins. Since their discovery in the early 1960s, PML bodies have fascinated cell biologists because of their beauty and their tight association with cellular disorders. The identification of PML, a gene involved in an oncogenic chromosomal translocation, as the key organizer of these domains drew instant interest onto them. The multiple levels of PML body regulation by a specific posttranslational modification, sumoylation, have raised several unsolved issues. Functionally, PML bodies may sequester, modify or degrade partner proteins, but in many ways, PML bodies still constitute an enigma.

N4BP1 is a newly identified nucleolar protein that undergoes SUMO-regulated polyubiquitylation and proteasomal turnover at promyelocytic leukemia nuclear bodies

A number of proteins can be conjugated with both ubiquitin and the small ubiquitin-related modifier (SUMO), with crosstalk between these two post-translational modifications serving to regulate protein function and stability. We previously identified N4BP1 as a substrate for monoubiquitylation by the E3 ubiquitin ligase Nedd4. Here, we describe Nedd4-mediated polyubiquitylation and proteasomal degradation of N4BP1. In addition, we show that N4BP1 can be conjugated with SUMO1 and that this abrogates N4BP1 ubiquitylation. Consistent with this, endogenous N4BP1 is stabilized in primary embryonic fibroblasts from mutants of the desumoylating enzyme SENP1, which show increased steady-state sumoylation levels. We have localized endogenous N4BP1 predominantly to the nucleolus in primary cells. However, a small fraction is found at promyelocytic leukemia (PML) nuclear bodies (NBs). In cells deficient for SENP1 or in wild-type cells treated with the proteasome inhibitor MG132, there is considerable accumulation of N4BP1 at PML NBs. These findings suggest a dynamic interaction between subnuclear compartments, and a role for post-translational modification by ubiquitin and SUMO in the regulation of nucleolar protein turnover.