Loss of SUMO1 in mice affects RanGAP1 localization and formation of PML nuclear bodies, but is not lethal as it can be compensated by SUMO2 or SUMO3

SUMO-4: A novel functional candidate in the human placental protein SUMOylation machinery

Background

Small ubiquitin-like modifiers (SUMOs) conjugate to proteins post-translationally, thereby affecting target localization, activity and stability. Functional SUMO family members identified in the human placenta include SUMO-1 to SUMO-3, which are elevated in pre-eclampsia. Whether the fourth isoform, SUMO-4, plays a role in placental development and function remains unknown.

Objectives

We tested the hypothesis that SUMO-4 is expressed in the human placenta and demonstrates altered SUMOylation in pre-eclamptic pregnancies.

Methods

SUMO-4 mRNA (qRT-PCR) and protein (Western blot and immunohistochemistry) were measured in Jar cells, BeWo cells, first trimester placental villous explants and placental tissues across normal gestation and in pre-eclampsia. SUMO-4 expression in response to oxidative stress (H₂O₂: 0, 0.1, 1 and 5mM), as well as, hypoxia-reperfusion (O₂: 1%, 8% and 20%) was measured. Lastly, SUMO-4 binding (covalently vs. non-covalently) to target proteins was investigated.

Results

SUMO-4 mRNA and protein were unchanged across gestation. SUMO-4 was present in the villous trophoblast layer throughout gestation. SUMO-4 mRNA expression and protein levels were increased ~2.2-fold and ~1.8-fold in pre-eclamptic placentas compared to age-matched controls, respectively (p<0.01). SUMO-4 mRNA and protein expression increased in Jars, BeWos and first trimester placental explants with 5mM H₂O₂ treatment, as well as with exposure to hypoxia-reperfusion. SUMO-1 to SUMO-3 did not show consistent trends across models. SUMO-4 hyper-SUMOylation was predominantly covalent in nature.

Conclusions

SUMO-4 is expressed in normal placental development. SUMO-4 expression was increased in pre-eclamptic placentas and in models of oxidative stress and hypoxic injury. These data suggests that SUMO-4 hyper-SUMOylation may be a potential post-translational mechanism in the stressed pre-eclamptic placenta.

Sumoylation in Craniofacial Disorders

Craniofacial development requires a complex series of coordinated and finely tuned events to take place, during a relatively short time frame. These events are set in motion by switching on and off transcriptional cascades that involve the use of numerous signalling pathways and a multitude of factors that act at the site of gene transcription. It is now well known that amidst the subtlety of this process lies the intricate world of protein modification, and the posttranslational addition of the small ubiquitin -like modifier, SUMO, is an example that has been implicated in this process. Many proteins that are required for formation of various structures in the embryonic head and face adapt specific functions with SUMO modification. Interestingly, the main clinical phenotype reported for a disruption of the SUMO1 locus is the common birth defect cleft lip and palate. In this chapter therefore, we discuss the role of SUMO1 in craniofacial development, with emphasis on orofacial clefts. We suggest that these defects can be a sensitive indication of down regulated SUMO modification at a critical stage during embryogenesis. As well as specific mutations affecting the ability of particular proteins to be sumoylated, non-genetic events may have the effect of down-regulating the SUMO pathway to give the same result. Enzymes regulating the SUMO pathway may become important therapeutic targets in the preventative and treatment therapies for craniofacial defects in the future.

Introduction to Sumoylation

Reversible post-translational modification is a rapid and efficient system to control the activity of pre-existing proteins. Modifiers range from small chemical moieties, such as phosphate groups, to proteins themselves as the modifier. The patriarch of the protein modifiers is ubiquitin which plays a central role in protein degradation and protein targeting. Over the last 20 years, the ubiquitin family has expanded to include a variety of ubiquitin-related small modifier proteins that are all covalently attached to a lysine residue on target proteins via series of enzymatic reactions. Of these more recently discovered ubiquitin-like proteins, the SUMO family has gained prominence as a major regulatory component that impacts numerous aspects of cell growth, differentiation, and response to stress. Unlike ubiquitinylation which often leads to proteins turn over, sumoylation performs a variety of function such as altering protein stability, modulating protein trafficking, directing protein-protein interactions, and regulating protein activity. This chapter will introduce the basic properties of SUMO proteins and the general tenets of sumoylation.

Neuron-specific SUMO knockdown suppresses global gene expression response and worsens functional outcome after transient forebrain ischemia in mice

Small ubiquitin-like modifier (SUMO) conjugation (SUMOylation) plays key roles in neurologic function in health and disease. Neuronal SUMOylation is essential for emotionality and cognition, and this pathway is dramatically activated in post-ischemic neurons, a neuroprotective response to ischemia. It is also known from cell culture studies that SUMOylation modulates gene expression. However, it remains unknown how SUMOylation regulates neuronal gene expression in vivo, in the physiologic state and after ischemia, and modulates post-ischemic recovery of neurologic function. To address these important questions, we used a SUMO1-3 knockdown (SUMO-KD) mouse in which a Thy-1 promoter drives expression of 3 distinct microRNAs against SUMO1-3 to silence SUMO expression specifically in neurons. Wild-type and SUMO-KD mice were subjected to transient forebrain ischemia. Microarray analysis was performed in hippocampal CA1 samples, and neurologic function was evaluated. SUMOylation had opposite effects on neuronal gene expression before and after ischemia. In the physiological state, most genes regulated by SUMOylation were up-regulated in SUMO-KD compared to wild-type mice. Brain ischemia/reperfusion significantly modulated the expression levels of more than 400 genes in wild-type mice, with a majority of those genes upregulated. The extent of this post-ischemic transcriptome change was suppressed in SUMO-KD mice. Moreover, SUMO-KD mice exhibited significantly worse functional outcome. This suggests that suppression of global gene expression response in post-ischemic brain due to SUMO knockdown has a negative effect on post-ischemic neurologic function. Together, our data provide a basis for future studies to mechanistically link SUMOylation to neurologic function in health and disease.

CACUL1/CAC1 attenuates p53 activity through PML post-translational modification

Promyelocytic leukaemia (PML) is a tumor suppressor protein covalently conjugated with SUMO family proteins, leading to the formation of PML nuclear bodies (NBs). PML-NBs provide a platform for efficient posttranslational modification of targets and protein-protein interaction, contributing to the adjustment of gene expression and chromatin integrity. Although PML SUMOylation is thought to play important roles in diverse cellular functions, the control mechanisms of adequate modification levels have remained unsolved. Here, we report that Cullin-related protein CACUL1/CAC1 (CACUL1) inhibits PML posttranslational modification. CACUL1 interacts with PML and suppresses PML SUMOylation, leading to the regulation of PML-NB size in the nucleus. We also found that Ubc9, a SUMO-conjugating enzyme, binds to CACUL1 and antagonizes the interaction between CACUL1 and PML. Furthermore, CACUL1 attenuates p53 transcriptional activity. These data suggest that CACUL1 is a novel regulator that negatively controls p53 activity through the regulation of PML SUMOylation.

Targeting the SUMO pathway for neuroprotection in brain ischaemia

Small ubiquitin-like modifier (SUMO) conjugation (SUMOylation) is a post-translational protein modification that modulates almost all major cellular processes, and has been implicated in many human diseases. A growing body of evidence from in vitro and in vivo studies demonstrates that increasing global levels of SUMO conjugated proteins (global SUMOylation) protects cells against ischaemia-induced damage, while suppressing global SUMOylation promotes cell injury after ischaemia. Indeed, SUMOylation has emerged as a potential therapeutic target for neuroprotection in brain ischaemia, including global brain ischaemia and focal brain ischaemia (ischaemic stroke). Here, we summarise findings on the role of SUMOylation in human diseases, brain ischaemia in particular, and review recent developments in drug discovery targeting SUMOylation with a major focus on its neuroprotective applications.

The Ubiquitin-Like SUMO System and Heart Function: From Development to Disease

SUMOylation is a ubiquitin-related transient posttranslational modification pathway catalyzing the conjugation of small ubiquitin-like modifier (SUMO) proteins (SUMO1, SUMO2, and SUMO3) to lysine residues of proteins. SUMOylation targets a wide variety of cellular regulators and thereby affects a multitude of different cellular processes. SUMO/sentrin-specific proteases are able to remove SUMOs from targets, contributing to a tight control of SUMOylated proteins. Genetic and cell biological experiments indicate a critical role of balanced SUMOylation/deSUMOylation for proper cardiac development, metabolism, and stress adaptation. Here, we review the current knowledge about SUMOylation/deSUMOylation in the heart and provide an integrated picture of cardiac functions of the SUMO system under physiologic or pathologic conditions. We also describe potential therapeutic approaches targeting the SUMO machinery to combat heart disease.

SENP1-modulated sumoylation regulates retinoblastoma protein (RB) and Lamin A/C interaction and stabilization

The retinoblastoma tumor suppressor protein (RB) plays a critical role in cell proliferation and differentiation and its inactivation is a frequent underlying factor in tumorigenesis. While the regulation of RB function by phosphorylation is well studied, proteasome-mediated RB protein degradation is emerging as an important regulatory mechanism. Although our understanding of RB turnover is currently limited, there is evidence that the nuclear lamina filament protein Lamin A/C protects RB from proteasomal degradation. Here we show that SUMO1 conjugation of RB and Lamin A/C is modulated by the SUMO protease SENP1 and that sumoylation of both proteins is required for their interaction. Importantly, this SUMO1-dependent complex protects both RB and Lamin A/C from proteasomal turnover.

SUMO5, a Novel Poly-SUMO Isoform, Regulates PML Nuclear Bodies

Promyelocytic leukemia nuclear bodies (PML-NBs) are PML-based nuclear structures that regulate various cellular processes. SUMOylation, the process of covalently conjugating small ubiquitin-like modifiers (SUMOs), is required for both the formation and the disruption of PML-NBs. However, detailed mechanisms of how SUMOylation regulates these processes remain unknown. Here we report that SUMO5, a novel SUMO variant, mediates the growth and disruption of PML-NBs. PolySUMO5 conjugation of PML at lysine 160 facilitates recruitment of PML-NB components, which enlarges PML-NBs. SUMO5 also increases polySUMO2/3 conjugation of PML, resulting in RNF4-mediated disruption of PML-NBs. The acute promyelocytic leukemia oncoprotein PML-RARα blocks SUMO5 conjugation of PML, causing cytoplasmic displacement of PML and disruption of PML-NBs. Our work not only identifies a new member of the SUMO family but also reveals the mechanistic basis of the PML-NB life cycle in human cells.

Uncoupling phototoxicity-elicited neural dysmorphology and death by insidious function and selective impairment of Ran-binding protein 2 (Ranbp2)

Morphological disintegration of neurons is coupled invariably to neural death. In particular, disruption of outer segments of photoreceptor neurons triggers photoreceptor death regardless of the pathological stressors. We show that Ranbp2(-/-)::Tg-Ranbp2(CLDm-HA) mice with mutations in SUMO-binding motif (SBM) of cyclophilin-like domain (CLD) of Ran-binding protein 2 (Ranbp2) expressed in a null Ranbp2 background lack untoward effects in photoreceptors in the absence of light-stress. However, compared to wild type photoreceptors, light-stress elicits profound disintegration of outer segments of Ranbp2(-/-)::Tg-Ranbp2(CLDm-HA) with paradoxical age-dependent resistance of photoreceptors to death and genotype-independent activation of caspases. Ranbp2(-/-)::Tg-Ranbp2(CLDm-HA) exhibit photoreceptor death-independent changes in ubiquitin-proteasome system (UPS), but death-dependent increase of ubiquitin carrier protein 9(ubc9) levels. Hence, insidious functional impairment of SBM of Ranbp2's CLD promotes neuroprotection and uncoupling of photoreceptor degeneration and death against phototoxicity.

Global Reprogramming of Host SUMOylation during Influenza Virus Infection

Dynamic nuclear SUMO modifications play essential roles in orchestrating cellular responses to proteotoxic stress, DNA damage, and DNA virus infection. Here, we describe a non-canonical host SUMOylation response to the nuclear-replicating RNA pathogen, influenza virus, and identify viral RNA polymerase activity as a major contributor to SUMO proteome remodeling. Using quantitative proteomics to compare stress-induced SUMOylation responses, we reveal that influenza virus infection triggers unique re-targeting of SUMO to 63 host proteins involved in transcription, mRNA processing, RNA quality control, and DNA damage repair. This is paralleled by widespread host deSUMOylation. Depletion screening identified ten virus-induced SUMO targets as potential antiviral factors, including C18orf25 and the SMC5/6 and PAF1 complexes. Mechanistic studies further uncovered a role for SUMOylation of the PAF1 complex component, parafibromin (CDC73), in potentiating antiviral gene expression. Our global characterization of influenza virus-triggered SUMO redistribution provides a proteomic resource to understand host nuclear SUMOylation responses to infection.

SUMOylation-Mediated Regulation of Cell Cycle Progression and Cancer

Protein conjugation with Small ubiquitin-like modifier (SUMOylation) has critical roles during cell cycle progression. Many important cell cycle regulators, including many oncogenes and tumor suppressors, are functionally regulated via SUMOylation. The dynamic SUMOylation pattern observed throughout the cell cycle is ensured via distinct spatial and temporal regulation of the SUMO machinery. Additionally, SUMOylation cooperates with other post-translational modifications to mediate cell cycle progression. Deregulation of these SUMOylation and deSUMOylation enzymes causes severe defects in cell proliferation and genome stability. Different types of cancer were recently shown to be dependent on a functioning SUMOylation system, a finding that could be exploited in anticancer therapies.

Spatiotemporal distribution of SUMOylation components during mouse brain development

Posttranslational modification of proteins might play an important role in brain cellular dynamics via the rapid turnover or functional change of critical proteins controlling neuronal differentiation or synaptic transmission. Small ubiquitin-like modifier protein (SUMO) is a family of ubiquitin-like small proteins that are covalently attached to target proteins to modify their function posttranslationally. Many cellular processes, such as transcription and protein trafficking, are regulated by SUMOylation, but its functional significance in the brain remains unclear. Although developmental regulation of SUMOylation levels in rat brain was recently demonstrated, no comparative immunohistochemical analysis of the cellular distribution profiles of SUMOylation components, including SUMO1, SUMO2/3, and Ubc9, has been undertaken so far. The present study used immunohistochemical and immunoblot analysis with the different developmental stages of mice and demonstrated the developmentally regulated distribution of SUMO1, SUMO2/3, and Ubc9 in the brain. During embryonic development, SUMOylation by SUMO1 and SUMO2/3 occurred in the nucleoplasm of nestin-positive neural stem cells. Although the total amount of SUMO-modified proteins decreased during postnatal brain development, intense and persistent accumulation of SUMO2/3 was detected throughout life in neural progenitor populations in neurogenic regions, including the subventricular zone and the hippocampal subgranular zone. In contrast, many neurons in the adult brain accumulated SUMO1 rather than SUMO2/3. Heavy immunoreactivity of SUMO1 was found in large projection neurons in the brainstem, whereas SUMO2/3 was almost absent from these areas. This heterogeneous distribution implies that both proteins play a specific and unique role in the brain.

Mapping the SUMOylated landscape

SUMOylation is a post‐translational modification that regulates a multitude of cellular processes, including replication, cell‐cycle progression, protein transport and the DNA damage response. Similar to ubiquitin, SUMO (small ubiquitin‐like modifier) is covalently attached to target proteins in a reversible process via an enzymatic cascade. SUMOylation is essential for nearly all eukaryotic organisms, and deregulation of the SUMO system is associated with human diseases such as cancer and neurodegenerative diseases. Therefore, it is of great interest to understand the regulation and dynamics of this post‐translational modification. Within the last decade, mass spectrometry analyses of SUMO proteomes have overcome several obstacles, greatly expanding the number of known SUMO target proteins. In this review, we briefly outline the basic concepts of the SUMO system, and discuss the potential of proteomic approaches to decipher SUMOylation patterns in order to understand the role of SUMO in health and disease.

Proteotoxic stress reprograms the chromatin landscape of SUMO modification

The small ubiquitin-like modifier 2 (SUMO-2) is required for survival when cells are exposed to treatments that induce proteotoxic stress by causing the accumulation of misfolded proteins. Exposure of cells to heat shock or other forms of proteotoxic stress induces the conjugation of SUMO-2 to proteins in the nucleus. We investigated the chromatin landscape of SUMO-2 modifications in response to heat stress. Through chromatin immunoprecipitation assays coupled to high-throughput DNA sequencing and mRNA sequencing, we showed that in response to heat shock, SUMO-2 accumulated at nucleosome-depleted, active DNA regulatory elements, which represented binding sites for large protein complexes and were predominantly associated with active genes. However, SUMO did not act as a direct transcriptional repressor or activator of these genes during heat shock. Instead, integration of our results with published proteomics data on heat shock-induced SUMO-2 substrates supports a model in which the conjugation of SUMO-2 to proteins acts as an acute stress response that is required for the stability of protein complexes involved in gene expression and posttranscriptional modification of mRNA. We showed that the conjugation of SUMO-2 to chromatin-associated proteins is an integral component of the proteotoxic stress response, and propose that SUMO-2 fulfills its essential role in cell survival by contributing to the maintenance of protein complex homeostasis.

SUMOylation is developmentally regulated and required for cell pairing during conjugation in Tetrahymena thermophila

The covalent attachment of small ubiquitin-like modifier (SUMO) to target proteins regulates numerous nuclear events in eukaryotes, including transcription, mitosis and meiosis, and DNA repair. Despite extensive interest in nuclear pathways within the field of ciliate molecular biology, there have been no investigations of the SUMO pathway in Tetrahymena. The developmental program of sexual reproduction of this organism includes cell pairing, micronuclear meiosis, and the formation of a new somatic macronucleus. We identified the Tetrahymena thermophila SMT3 (SUMO) and UBA2 (SUMO-activating enzyme) genes and demonstrated that the corresponding green fluorescent protein (GFP) tagged gene products are found predominantly in the somatic macronucleus during vegetative growth. Use of an anti-Smt3p antibody to perform immunoblot assays with whole-cell lysates during conjugation revealed a large increase in SUMOylation that peaked during formation of the new macronucleus. Immunofluorescence using the same antibody showed that the increase was localized primarily within the new macronucleus. To initiate functional analysis of the SUMO pathway, we created germ line knockout cell lines for both the SMT3 and UBA2 genes and found both are essential for cell viability. Conditional Smt3p and Uba2p cell lines were constructed by incorporation of the cadmium-inducible metallothionein promoter. Withdrawal of cadmium resulted in reduced cell growth and increased sensitivity to DNA-damaging agents. Interestingly, Smt3p and Uba2p conditional cell lines were unable to pair during sexual reproduction in the absence of cadmium, consistent with a function early in conjugation. Our studies are consistent with multiple roles for SUMOylation in Tetrahymena, including a dynamic regulation associated with the sexual life cycle.

Global SUMOylation is a molecular mechanism underlying hypothermia-induced ischemic tolerance

The molecular mechanisms underlying hypothermic neuroprotection have yet to be fully elucidated. Herein we demonstrate that global SUMOylation, a form of post-translational modification with the Small Ubiquitin-like MOdifer, participates in the multimodal molecular induction of hypothermia-induced ischemic tolerance. Mild (32°C) to moderate (28°C) hypothermic treatment(s) during OGD (oxygen-glucose-deprivation) or ROG (restoration of oxygen/glucose) increased global SUMO-conjugation levels and protected cells (both SHSY5Y and E18 rat cortical neurons) from OGD and ROG-induced cell death. Hypothermic exposure either before or after permanent middle cerebral artery occlusion (pMCAO) surgery in wild type mice increased global SUMO-conjugation levels in the brain and in so doing protected these animals from pMCAO-induced ischemic damage. Of note, hypothermic exposure did not provide an additional increase in protection from pMCAO-induced ischemic brain damage in Ubc9 transgenic (Ubc9 Tg) mice, which overexpress the sole E2 SUMO conjugating enzyme and thereby display elevated basal levels of global SUMOylation under normothermic conditions. Such evidence suggests that increases in global SUMOylation are critical and may account for a substantial part of the observed increase in cellular tolerance to brain ischemia caused via hypothermia.

SUMO proteomics to decipher the SUMO-modified proteome regulated by various diseases

Small ubiquitin-like modifier (SUMO1-3) conjugation is a posttranslational protein modification whereby SUMOs are conjugated to lysine residues of target proteins. SUMO conjugation can alter the activity, stability, and function of target proteins, and thereby modulate almost all major cellular pathways. Many diseases are associated with SUMO conjugation, including heart failure, arthritis, cancer, degenerative diseases, and brain ischemia/stroke. It is, therefore, of major interest to characterize the SUMO-modified proteome regulated by these disorders. SUMO proteomics analysis is hampered by low levels of SUMOylated proteins. Several strategies have, therefore, been developed to enrich SUMOylated proteins from cell/tissue extracts. These include proteomics analysis on cells expressing epitope-tagged SUMO isoforms, use of monoclonal SUMO antibodies for immunoprecipitation and epitope-specific peptides for elution, and affinity purification with peptides containing SUMO interaction motifs to specifically enrich polySUMOylated proteins. Recently, two mouse models were generated and characterized that express tagged SUMO isoforms, and allow purification of SUMOylated proteins from complex organ extracts. Ultimately, these new analytical tools will help to decipher the SUMO-modified proteome regulated by various human diseases, and thereby, identify new targets for preventive and therapeutic purposes.

SUMO modification of TBK1 at the adaptor-binding C-terminal coiled-coil domain contributes to its antiviral activity

The non-canonical IKK kinase TBK1 serves as an important signal transmitter of the antiviral interferon response, but is also involved in the regulation of further processes such as autophagy. The activity of TBK1 is regulated by posttranslational modifications comprising phosphorylation and ubiquitination. This study identifies SUMOylation as a novel posttranslational TBK1 modification. TBK1 kinase activity is required to allow the attachment of SUMO1 or SUMO2/3 proteins. Since TBK1 does not bind to the E2 enzyme Ubc9, this modification most likely proceeds via trans-SUMOylation. Mass spectrometry allowed identifying K694 as the SUMO acceptor site, a residue located in the C-terminal coiled-coil domain which is exclusively responsible for the association with the adaptor proteins NAP1, Sintbad and TANK. SUMO modification at K694 contributes to the antiviral function of TBK1 and accordingly the viral protein Gam1 antagonizes this posttranslational modification.

Neuronal SUMOylation: mechanisms, physiology, and roles in neuronal dysfunction

Protein SUMOylation is a critically important posttranslational protein modification that participates in nearly all aspects of cellular physiology. In the nearly 20 years since its discovery, SUMOylation has emerged as a major regulator of nuclear function, and more recently, it has become clear that SUMOylation has key roles in the regulation of protein trafficking and function outside of the nucleus. In neurons, SUMOylation participates in cellular processes ranging from neuronal differentiation and control of synapse formation to regulation of synaptic transmission and cell survival. It is a highly dynamic and usually transient modification that enhances or hinders interactions between proteins, and its consequences are extremely diverse. Hundreds of different proteins are SUMO substrates, and dysfunction of protein SUMOylation is implicated in a many different diseases. Here we briefly outline core aspects of the SUMO system and provide a detailed overview of the current understanding of the roles of SUMOylation in healthy and diseased neurons.

SUMO2 is essential while SUMO3 is dispensable for mouse embryonic development

Small ubiquitin-like modifier (SUMO1-3) conjugation plays a critical role in embryogenesis. Embryos deficient in the SUMO-conjugating enzyme Ubc9 die at the early postimplantation stage. Sumo1(-/-) mice are viable, as SUMO2/3 can compensate for most SUMO1 functions. To uncover the role of SUMO2/3 in embryogenesis, we generated Sumo2- and Sumo3-null mutant mice. Here, we report that Sumo3(-/-) mice were viable, while Sumo2(-/-) embryos exhibited severe developmental delay and died at approximately embryonic day 10.5 (E10.5). We also provide evidence that SUMO2 is the predominantly expressed SUMO isoform. Furthermore, although Sumo2(+/-) and Sumo2(+/-);Sumo3(+/-) mice lacked any overt phenotype, only 2 Sumo2(+/-);Sumo3(-/-) mice were found at birth in 35 litters after crossing Sumo2(+/-);Sumo3(+/-) with Sumo3(-/-) mice, and these rare mice were considerably smaller than littermates of the other genotypes. Thus, our findings suggest that expression levels and not functional differences between SUMO2 and SUMO3 are critical for normal embryogenesis.

Characterization of nuclear localization and SUMOylation of the ATBF1 transcription factor in epithelial cells

ATBF1/ZFHX3 is a large transcription factor that functions in development, tumorigenesis and other biological processes. ATBF1 is normally localized in the nucleus, but is often mislocalized in the cytoplasm in cancer cells. The mechanism underlying the mislocalization of ATBF1 is unknown. In this study, we analyzed the nuclear localization of ATBF1, and found that ectopically expressed ATBF1 formed nuclear body (NB)-like dots in the nucleus, some of which indeed physically associated with promyelocytic leukemia (PML) NBs. We also defined a 3-amino acid motif, KRK^2615-2617, as the nuclear localization signal (NLS) for ATBF1. Interestingly, diffusely distributed nuclear SUMO1 proteins were sequestered into ATBF1 dots, which could be related to ATBF1's physical association with PML NBs, known SUMOylation hotspots. Furthermore, ATBF1 itself was SUMOylated. ATBF1 SUMOylation occurred at more than 3 lysine residues including K²³⁴⁹, K²⁸⁰⁶ and K³²⁵⁸ and was nuclear specific. Finally, the PIAS3 SUMO1 E3 ligase, which interacts with ATBF1 directly, diminished rather than enhanced ATBF1 SUMOylation, preventing the co-localization of ATBF1 with SUMO1 in the nucleus. These findings suggest that nuclear localization and SUMOylation are important for the transcription factor function of ATBF1, and that ATBF1 could cooperate with PML NBs to regulate protein SUMOylation in different biological processes.

Sumoylation: a regulatory protein modification in health and disease

Posttranslational modification with small ubiquitin-related modifier (SUMO) proteins is now established as one of the key regulatory protein modifications in eukaryotic cells. Hundreds of proteins involved in processes such as chromatin organization, transcription, DNA repair, macromolecular assembly, protein homeostasis, trafficking, and signal transduction are subject to reversible sumoylation. Hence, it is not surprising that disease links are beginning to emerge and that interference with sumoylation is being considered for intervention. Here, we summarize basic mechanisms and highlight recent developments in the physiology of sumoylation.

Decoding the SUMO signal

SUMO (small ubiquitin-like modifier) emerged from the shadow of the well-established ubiquitin some 15 years ago when it was shown that a distinct conjugation pathway was responsible for SUMO modification. Since then it has been established that SUMO modifies over a thousand substrates and plays diverse roles in many important biological processes. Recognition of SUMO is mediated by short peptide sequences known as SIMs (SUMO-interaction motifs) that allow effector proteins to engage SUMO-modified substrates. Like ubiquitin, SUMO can form polymeric chains, and these chains can be recognized by proteins containing multiple SIMs. One protein that contains such a sequence of SIMs also contains a RING (really interesting new gene) domain that is the hallmark of a ubiquitin E3 ligase. This ubiquitin ligase known as RNF4 (RING finger protein 4) has the unique property that it can recognize SUMO-modified proteins and target them for ubiquitin-mediated proteolysis. Structural and biochemical analyses of RNF4 has shed light on the long sought after mechanism of ubiquitin transfer and illustrates how its RING domain primes the ubiquitin-loaded E2 for catalysis.

SUMO rules: regulatory concepts and their implication in neurologic functions

Posttranslational modification of proteins by the small ubiquitin-like modifier (SUMO) is a potent regulator of various cellular events. Hundreds of substrates have been identified, many of them involved in vital processes like transcriptional regulation, signal transduction, protein degradation, cell cycle regulation, DNA repair, chromatin organization, and nuclear transport. In recent years, protein sumoylation increasingly attracted attention, as it could be linked to heart failure, cancer, and neurodegeneration. However, underlying mechanisms involving how modification by SUMO contributes to disease development are still scarce thus necessitating further research. This review aims to critically discuss currently available concepts of the SUMO pathway, thereby highlighting regulation in the healthy versus diseased organism, focusing on neurologic aspects. Better understanding of differential regulation in health and disease may finally allow to uncover pathogenic mechanisms and contribute to the development of disease-specific therapies.

Protein sumoylation in brain development, neuronal morphology and spinogenesis

Small ubiquitin-like modifiers (SUMOs) are polypeptides resembling ubiquitin that are covalently attached to specific lysine residue of target proteins through a specific enzymatic pathway. Sumoylation is now seen as a key posttranslational modification involved in many biological processes, but little is known about how this highly dynamic protein modification is regulated in the brain. Disruption of the sumoylation enzymatic pathway during the embryonic development leads to lethality revealing a pivotal role for this protein modification during development. The main aim of this review is to briefly describe the SUMO pathway and give an overview of the sumoylation regulations occurring in brain development, neuronal morphology and synapse formation.

SUMO and ischemic tolerance

Hibernating squirrels slow blood flow to a crawl, but sustain no damage to brain or other tissues. This phenomenon provides an excellent model of natural tolerance to ischemia. Small ubiquitin-like modifier (SUMO) is a 100-residue peptide that modifies other proteins by being attached to the epsilon amino group of specific lysine residues. The discovery of massive SUMOylation (by both SUMO-1 and SUMO-2/3) occurring in the brains of 13-lined ground squirrels (Ictidomys tridecemlineatus) during hibernation torpor had opened the door to the studies on SUMO and ischemic tolerance reviewed here. Ischemic stress was shown to increase the levels of SUMO conjugation, especially SUMO-2/3, mostly during reperfusion in animal models and during restoration of oxygen and glucose in cell culture systems. Over-expression or depletion of SUMOs and/or Ubc9 (the SUMO E2 conjugating enzyme) increases or decreases (respectively) the levels of SUMO conjugates. Elevated global SUMO conjugations were shown to cytoprotect from ischemic insults; conversely, depressed SUMOylation sensitized cells. Global protein conjugation not only by SUMOs, but also by other ubiquitin-like modifiers (ULMs) including NEDD8, ISG15, UFM1 and FUB1 was shown to be significantly increased in the brains of hibernating ground squirrels during torpor. These increases in multiple ULM conjugations may orchestrate the cellular events in hibernating ground squirrels that induce a state of natural tolerance through their multipronged effects. Certain miRNAs such as the miR-200 family and the miR-182 family were shown, at least partly, to control the levels of these ULM conjugations. Lowering the levels of these miRNAs leads to an increase in global SUMOylation/ULM conjugation, thereby providing the tolerance to ischemia. This suggests that these miRNAs may be good targets for therapeutic intervention in stroke.

It takes two to tango: Ubiquitin and SUMO in the DNA damage response

The complexity of living cells is primarily determined by the genetic information encoded in DNA and gets fully disclosed upon translation. A major determinant of complexity is the reversible post-translational modification (PTM) of proteins, which generates variants displaying distinct biological properties such as subcellular localization, enzymatic activity and the ability to assemble in complexes. Decades of work on phosphorylation have unambiguously proven this concept. In recent years, the covalent attachment of Ubiquitin or Small Ubiquitin-like Modifiers (SUMO) to amino acid residues of target proteins has been recognized as another crucial PTM, re-directing protein fate and protein-protein interactions. This review focuses on the role of ubiquitylation and sumoylation in the control of DNA damage response proteins. To lay the ground, we begin with a description of ubiquitylation and sumoylation, providing established examples of DNA damage response elements that are controlled through these PTMs. We then examine in detail the role of PTMs in the cellular response to DNA double-strand breaks illustrating hierarchy, cross-talk, synergism or antagonism between phosphorylation, ubiquitylation and sumoylation. We conclude offering a perspective on Ubiquitin and SUMO pathways as targets in cancer therapy.

Murine gammaherpesvirus 68 ORF75c contains ubiquitin E3 ligase activity and requires PML SUMOylation but not other known cellular PML regulators, CK2 and E6AP, to mediate PML degradation

All gammaherpsviruses encode at least one gene related to the cellular formylglycinamide ribonucleotide amidotransferase (FGARAT) enzyme but their biological roles are relatively unknown. The murine gammaherpesvirus 68 (MHV68) vFGARAT, ORF75c, mediates a proteasome-dependent degradation of the antiviral promyelocytic leukemia (PML) protein by an unknown mechanism, which is addressed in this study. We found that ORF75c interacts weakly with PML and SUMO-modified forms of PML are important for its degradation by ORF75c. ORF75c-mediated PML degradation was not dependent on two known cellular regulators of PML stability, Casein kinase II (CK2) and human papilloma virus E6-associated protein (E6AP). Finally, ORF75c had self-ubiquitination activity in vitro and its expression increased levels of ubiquitinated PML in transfected cells. Taken together, the evidence accumulated in this study provides new insights into the function of a vFGARAT and is consistent with a model in which ORF75c could mediate direct ubiquitination of PML resulting in its degradation by the proteasome.

Senp1 is essential for desumoylating Sumo1-modified proteins but dispensable for Sumo2 and Sumo3 deconjugation in the mouse embryo

Posttranslational modification with small ubiquitin-like modifier (Sumo) regulates numerous cellular and developmental processes. Sumoylation is dynamic with deconjugation by Sumo-specific proteases (Senps) regulating steady-state levels. Different Senps are found in distinct subcellular domains, which may limit their deconjugation activity to colocalizing Sumo-modified proteins. In vitro, Senps can discriminate between the different Sumo paralogs: Sumo1 versus the highly related Sumo2 and Sumo3 (Sumo2/3), which can form poly-Sumo chains. However, a full understanding of Senp specificity in vivo is still lacking. Here, using biochemical and genetic approaches, we establish that Senp1 has an essential, nonredundant function to desumoylate Sumo1-modified proteins during mouse embryonic development. Senp1 specificity for Sumo1 conjugates represents an intrinsic function and not simply a product of colocalization. In contrast, Senp1 has only a limited role in Sumo2/3 desumoylation, although it may regulate Sumo1-mediated termination of poly-Sumo2/3 chains.

Differential Roles of PML Isoforms

The tumor suppressor promyelocytic leukemia (PML) protein is fused to the retinoic acid receptor alpha in patients suffering from acute promyelocytic leukemia (APL). Treatment of APL patients with arsenic trioxide (As2O3) reverses the disease phenotype by a process involving the degradation of the fusion protein via its PML moiety. Several PML isoforms are generated from a single PML gene by alternative splicing. They share the same N-terminal region containing the RBCC/tripartite motif but differ in their C-terminal sequences. Recent studies of all the PML isoforms reveal the specific functions of each. Here, we review the nomenclature and structural organization of the PML isoforms in order to clarify the various designations and classifications found in different databases. The functions of the PML isoforms and their differential roles in antiviral defense also are reviewed. Finally, the key players involved in the degradation of the PML isoforms in response to As2O3 or other inducers are discussed.

Viral manipulation of cellular protein conjugation pathways: The SUMO lesson

Small ubiquitin-like modifier (SUMO)ylation is a key post-translational modification mechanism that controls the function of a plethora of proteins and biological processes. Given its central regulatory role, it is not surprising that it is widely exploited by viruses. A number of viral proteins are known to modify and/or be modified by the SUMOylation system to exert their function, to create a cellular environment more favorable for virus survival and propagation, and to prevent host antiviral responses. Since the SUMO pathway is a multi-step cascade, viral proteins engage with it at many levels, to advance and favor each stage of a typical infection cycle: replication, viral assembly and immune evasion. Here we review the current knowledge on the interplay between the host SUMO system and viral lifecycle.

SUMO-1 regulates body weight and adipogenesis via PPARγ in male and female mice

Properly functioning adipose tissue is essential for normal insulin sensitivity of the body. When mice are kept on high-fat diet (HFD), adipose tissue expands, adipocytes increase in size and number, and the mice become obese. Many of these changes are mediated by the nuclear receptor peroxisome proliferator-activated receptor γ (PPARγ), the activity of which is regulated by multiple posttranslational modifications, including SUMOylation. To address the role of small ubiquitin-like modifier-1 (SUMO-1) in PPARγ function in vivo, particularly in fat cell biology, we subjected Sumo1-knockout mice to HFD. Sumo1-null mice gained less weight and had smaller and fewer adipocytes in their gonadal fat tissue on HFD, but their glucose tolerance was similar to that of wild-type littermates. Adipogenesis was impaired in Sumo1-null cells, and expression of PPARγ target genes was attenuated. In addition, both Sumo1-null cells and Sumo1-null mice responded less efficiently to rosiglitazone, a PPARγ agonist. These findings indicate that SUMO-1 is important also for transcriptional activation by the PPARγ signaling pathway and not only for trans-repressive functions of PPARγ as previously reported.

Post-translational modifications of PML: consequences and implications

The tumor suppressor promyelocytic leukemia protein (PML) predominantly resides in a structurally distinct sub-nuclear domain called PML nuclear bodies. Emerging evidences indicated that PML actively participates in many aspects of cellular processes, but the molecular mechanisms underlying PML regulation in response to stress and environmental cues are not complete. Post-translational modifications, such as SUMOylation, phosphorylation, acetylation, and ubiquitination of PML add a complex layer of regulation to the physiological function of PML. In this review, we discuss the fast-moving horizon of post-translational modifications targeting PML.

SUMOylation of Pax7 is essential for neural crest and muscle development

Regulatory transcription factors of the Pax family play fundamental roles in the function of multipotent cells during vertebrate development, post-natal regeneration, and cancer. Pax7 and its homologue Pax3 are important players in neural crest and muscle development. Both genes are coexpressed in various tissues and are thought to provide similar, but not identical, functions. The mechanisms that allow specific regulation of Pax7 remain largely unknown. Here, we report for the first time that Pax7 is regulated by SUMOylation. We identify the interaction of Pax7 with Ubc9, the SUMO conjugating enzyme, and reveal that SUMOylation machinery is enriched in neural crest precursors and plays a critical role in NC development. We demonstrate that Pax7 becomes SUMOylated and identify an essential role for lysine 85 (K85) in Pax7-SUMOylation. Despite high conservation surrounding K85 amongst Pax genes, we were unable to identify SUMOylation of other Pax proteins tested, including Pax3. Using a non-SUMOylatable Pax7 variant (K85 X R), we demonstrate that SUMOylation is essential for the function of Pax7 in neural crest development, C2C12 myogenic differentiation, and transcriptional transactivation. Our study provides new mechanistic insight into the molecular regulation of Pax7's function by SUMOylation in neural crest and muscle development.

Protein SUMOylation in spine structure and function

The active regulation of spine structure and function is of fundamental importance for information storage in the brain. Many proteins involved in spine development and activity-dependent remodelling are potential or validated substrates for modification by the Small Ubiquitin-like Modifier (SUMO). The functional consequences of neuronal protein SUMOylation appear diverse and, in many cases, have not yet been determined. However, for several proteins SUMOylation has been shown to be a key regulator, which has a profound impact on spine dynamics and protein trafficking and function. Here we provide an overview of neuronal SUMOylation and discuss how greater understanding of this relatively recently discovered posttranslational modification will provide insight into the complexity of protein interactions that control synaptic activity and dysfunction.

Ubiquitin-like proteins

The eukaryotic ubiquitin family encompasses nearly 20 proteins that are involved in the posttranslational modification of various macromolecules. The ubiquitin-like proteins (UBLs) that are part of this family adopt the β-grasp fold that is characteristic of its founding member ubiquitin (Ub). Although structurally related, UBLs regulate a strikingly diverse set of cellular processes, including nuclear transport, proteolysis, translation, autophagy, and antiviral pathways. New UBL substrates continue to be identified and further expand the functional diversity of UBL pathways in cellular homeostasis and physiology. Here, we review recent findings on such novel substrates, mechanisms, and functions of UBLs.

SUMO-2/3 conjugates accumulating under heat shock or MG132 treatment result largely from new protein synthesis

Small ubiquitin-related modifiers 1, 2 and 3 (SUMO-1, -2, -3), members of the ubiquitin-like protein family, can be conjugated to various cellular proteins. Conjugates of SUMO-2 and SUMO-3 (SUMO-2/3) accumulate in cells exposed to various stress stimuli or to MG132 treatment. Although the proteins modified by SUMO-2/3 during heat shock or under MG132 treatment have been identified, the significance of this modification remains unclear. Our data show that the inhibition of translation by puromycin or cycloheximide blocks both the heat shock and MG132 induced accumulation of SUMO-2/3 conjugates in HEK 293T and U2OS cells. However, the heat shock induced accumulation of SUMO-2/3 conjugates was restored by proteasome inhibition, which suggests that the inhibition of translation did not abolish SUMOylation itself. Furthermore, we show that some of the proteins truncated due to the treatment by low concentration of puromycin are SUMOylated in HEK 293T cells. We suggest that the SUMO-2/3 conjugates accumulating under the heat shock or MG132 treatment result largely from new protein synthesis and that portion of them is incorrectly folded.

Determination of posttranslational modifications of photoreceptor differentiation factor NRL: focus on SUMOylation

Conjugation of SUMO (small ubiquitin-related modifier 1) is a critical posttranslational modification, with significant impact on protein function/activity. Here, we describe direct SUMOylation of GST (glutathione S-transferase)-fusion protein and immunoprecipitation assays for investigating SUMOylation of any protein of interest. We have employed these methods to examine SUMOylation of the basic-motif leucine zipper transcription factor NRL.

BubR1 is modified by sumoylation during mitotic progression

BubR1 functions as a crucial component that monitors proper chromosome congression and mitotic timing during cell division. We investigated molecular regulation of BubR1 and found that BubR1 was modified by an unknown post-translation mechanism during the cell cycle, resulting in a significant mobility shift on denaturing gels. We termed it BubR1-M as the nature of modification was not characterized. Extended (>24 h) treatment of HeLa cells with a microtubule disrupting agent including nocodazole and taxol or release of mitotic shake-off cells into fresh medium induced BubR1-M. BubR1-M was derived from neither phosphorylation nor acetylation. Ectopic expression coupled with pulling down analyses showed that BubR1-M was derived from SUMO modification. Mutation analysis revealed that lysine 250 was a crucial site for sumoylation. Significantly, compared with the wild-type control, ectopic expression of a sumoylation-deficient mutant of BubR1 induced chromosomal missegregation and mitotic delay. Combined, our study identifies a new type of post-translational modification that is essential for BubR1 function during mitosis.

Enhanced desumoylation in murine hearts by overexpressed SENP2 leads to congenital heart defects and cardiac dysfunction

Sumoylation is a posttranslational modification implicated in a variety of cellular activities, and its role in a number of human pathogeneses such as cleft lip/palate has been well documented. However, the importance of the SUMO conjugation pathway in cardiac development and functional disorders is newly emerging. We previously reported that knockout of SUMO-1 in mice led to congenital heart diseases (CHDs). To further investigate the effects of imbalanced SUMO conjugation on heart development and function and its underlying mechanisms, we generated transgenic (Tg) mice with cardiac-specific expression of SENP2, a SUMO-specific protease that deconjugates sumoylated proteins, to evaluate the impact of desumoylation on heart development and function. Overexpression of SENP2 resulted in premature death of mice with CHDs-atrial septal defects (ASDs) and/or ventricular septal defects (VSDs). Immunobiochemistry revealed diminished cardiomyocyte proliferation in SENP2-Tg mouse hearts compared with that in wild type (WT) hearts. Surviving SENP2-Tg mice showed growth retardation, and developed cardiomyopathy with impaired cardiac function with aging. Cardiac-specific overexpression of the SUMO-1 transgene reduced the incidence of cardiac structural phenotypes in the sumoylation defective mice. Moreover, cardiac overexpression of SENP2 in the mice with Nkx2.5 haploinsufficiency promoted embryonic lethality and severity of CHDs, indicating the functional interaction between SENP2 and Nkx2.5 in vivo. Our findings indicate the indispensability of a balanced SUMO pathway for proper cardiac development and function. This article is part of a Special Issue entitled 'Post-translational Modification SI'.

Emerging roles of the SUMO pathway in development

Sumoylation is a reversible post-translational modification that targets a variety of proteins mainly within the nucleus, but also in the plasma membrane and cytoplasm of the cell. It controls diverse cellular mechanisms such as subcellular localization, protein-protein interactions, or transcription factor activity. In recent years, the use of several developmental model systems has unraveled many critical functions for the sumoylation system in the early life of diverse species. In particular, detailed analyses of mutant organisms in both the components of the SUMO pathway and their targets have established the importance of the SUMO system in early developmental processes, such as cell division, cell lineage commitment, specification, and/or differentiation. In addition, an increasing number of developmental proteins, including transcription factors and epigenetic regulators, have been identified as sumoylation substrates. Sumoylation acts on these targets through various mechanisms. For example, this modification has been involved in converting a transcription factor from an activator to a repressor or in regulating the localization and/or stability of numerous transcription factors. This review will summarize current information on the function of sumoylation in embryonic development in different species from yeast to mammals.

Distinctive properties of Arabidopsis SUMO paralogues support the in vivo predominant role of AtSUMO1/2 isoforms

Protein modification by SUMO (small ubiquitin-related modifier) has emerged as an essential regulatory mechanism in eukaryotes. Even though the molecular mechanisms of SUMO conjugation/deconjugation are conserved, the number of SUMO machinery components and their degree of conservation are specific to each organism. In the present paper, we show data contributing to the notion that the four expressed Arabidopsis SUMO paralogues, AtSUMO1, 2, 3 and 5, have functionally diverged to a higher extent than their human orthologues. We have explored the degree of conservation of these paralogues and found that the surfaces involved in E1-activating enzyme recognition, and E2-conjugating enzyme and SIM (SUMO-interacting motif) non-covalent interactions are well conserved in AtSUMO1/2 isoforms, whereas AtSUMO3 shows a lower degree of conservation, and AtSUMO5 is the most divergent isoform. These differences are functionally relevant, since AtSUMO3 and 5 are deficient in establishing E2 non-covalent interactions, which has not been reported for any naturally occurring SUMO orthologue. In addition, AtSUMO3 is less efficiently conjugated than AtSUMO1/2, and AtSUMO5 shows the lowest conjugation level. A mutagenesis analysis revealed that decreases in conjugation rate and thioester-bond formation are the result of the non-conserved residues involved in E1-activating enzyme recognition that are present in AtSUMO3 and 5. The results of the present study support a role for the E1-activating enzyme in SUMO paralogue discrimination, providing a new mechanism to favour conjugation of the essential AtSUMO1/2 paralogues.

Silencing nuclear pore protein Tpr elicits a senescent-like phenotype in cancer cells

BACKGROUND

Tpr is a large coiled-coil protein located in the nuclear basket of the nuclear pore complex for which many different functions were proposed from yeast to human.

METHODOLOGY/PRINCIPAL FINDINGS

Here we show that depletion of Tpr by RNA interference triggers G0-G1 arrest and ultimately induces a senescent-like phenotype dependent on the presence of p53. We also found that Tpr depletion impairs the NES [nuclear export sequence]-dependent nuclear export of proteins and causes partial co-depletion of Nup153. In addition Tpr depletion impacts on level and function of the SUMO-protease SENP2 thus affecting SUMOylation regulation at the nuclear pore and overall SUMOylation in the cell.

CONCLUSIONS

Our data for the first time provide evidence that a nuclear pore component plays a role in controlling cellular senescence. Our findings also point to new roles for Tpr in the regulation of SUMO-1 conjugation at the nuclear pore and directly confirm Tpr involvement in the nuclear export of NES-proteins.

Comparative proteomic analysis identifies a role for SUMO in protein quality control

The small ubiquitin-like modifiers (SUMOs) alter the functions of diverse cellular proteins by covalent posttranslational modification and thus influence many cellular functions, including gene transcription, cell cycle, and DNA repair. Although conjugation by ubiquitin and SUMO-2/3 are largely functionally and mechanistically independent from one another, both appear to increase under conditions of proteasome inhibition. To better understand the relationship between SUMO and protein degradation by the proteasome, we performed a quantitative proteomic analysis of SUMO-2 substrates after short- and long-term inhibition of the proteasome with MG132. Comparisons with changes to the SUMO-2 conjugate subproteome in response to heat stress revealed qualitative and quantitative parallels between both conditions; however, in contrast to heat stress, the MG132-triggered increase in SUMO-2 conjugation depended strictly on protein synthesis, implying that the accumulation of newly synthesized, misfolded proteins destined for degradation by the proteasome triggered the SUMO conjugation response. Furthermore, proteasomal inhibition resulted in the accumulation of conjugated forms of all SUMO paralogs in insoluble protein inclusions and in the accumulation on SUMO-2 substrates of lysine-63-linked polyubiquitin chains, which are not thought to serve as signals for proteasome-mediated degradation. Together, these findings suggest multiple, proteasome-independent roles for SUMOs in the cellular response to the accumulation of misfolded proteins.

Defective sumoylation pathway directs congenital heart disease

Congenital heart defects (CHDs) are the most common of all birth defects, yet molecular mechanism(s) underlying highly prevalent atrial septal defects (ASDs) and ventricular septal defects (VSDs) have remained elusive. We demonstrate the indispensability of "balanced" posttranslational small ubiquitin-like modifier (SUMO) conjugation-deconjugation pathway for normal cardiac development. Both hetero- and homozygous SUMO-1 knockout mice exhibited ASDs and VSDs with high mortality rates, which were rescued by cardiac reexpression of the SUMO-1 transgene. Because SUMO-1 was also involved in cleft lip/palate in human patients, the previous findings provided a powerful rationale to question whether SUMO-1 was mutated in infants born with cleft palates and ASDs. Sequence analysis of DNA from newborn screening blood spots revealed a single 16 bp substitution in the SUMO-1 regulatory promoter of a patient displaying both oral-facial clefts and ASDs. Diminished sumoylation activity whether by genetics, environmental toxins, and/or pharmaceuticals may significantly contribute to susceptibility to the induction of congenital heart disease worldwide. Birth Defects Research (Part A) 2011. © 2011 Wiley-Liss, Inc.

Role of promyelocytic leukemia protein in host antiviral defense

Several pathways have been implicated in the establishment of antiviral state in response to interferon (IFN), one of which implicates the promyelocytic leukemia (PML) protein. The PML gene has been discovered 20 years ago and has led to new insights into oncogenesis, apoptosis, cell senescence, and antiviral defense. PML is induced by IFN, leading to a marked increase of expression of PML isoforms and the number of PML nuclear bodies (NBs). PML is the organizer of the NBs that contains at least 2 permanent NB-associated proteins, the IFN-stimulated gene product Speckled protein of 100 kDa (Sp100) and death-associated dead protein (Daxx), as well as numerous other transient proteins recruited in these structures in response to different stimuli. Accumulating reports have implicated PML in host antiviral defense and revealed various strategies developed by viruses to disrupt PML NBs. This review will focus on the regulation of PML and the implication of PML NBs in conferring resistance to DNA and RNA viruses. The role of PML in mediating an IFN-induced antiviral state will also be discussed.