MAPK/MAK/MRK overlapping kinase (MOK) controls microglial inflammatory/type-I IFN responses via Brd4 and is involved in ALS.

Amyotrophic lateral sclerosis (ALS) is a fatal and incurable neurodegenerative disease affecting motor neurons and characterized by microglia-mediated neurotoxic inflammation whose underlying mechanisms remain incompletely understood. In this work, we reveal that MAPK/MAK/MRK overlapping kinase (MOK), with an unknown physiological substrate, displays an immune function by controlling inflammatory and type-I interferon (IFN) responses in microglia which are detrimental to primary motor neurons. Moreover, we uncover the epigenetic reader bromodomain-containing protein 4 (Brd4) as an effector protein regulated by MOK, by promoting Ser492-phospho-Brd4 levels. We further demonstrate that MOK regulates Brd4 functions by supporting its binding to cytokine gene promoters, therefore enabling innate immune responses. Remarkably, we show that MOK levels are increased in the ALS spinal cord, particularly in microglial cells, and that administration of a chemical MOK inhibitor to ALS model mice can modulate Ser492-phospho-Brd4 levels, suppress microglial activation, and modify the disease course, indicating a pathophysiological role of MOK kinase in ALS and neuroinflammation.

SUMO control of nervous system development.

In the last decades, the post-translational modification system by covalent attachment of the SUMO polypeptide to proteins has emerged as an essential mechanism controlling virtually all the physiological processes in the eukaryotic cell. This includes vertebrate development. In the nervous system, SUMO plays crucial roles in synapse establishment and it has also been linked to a variety of neurodegenerative diseases. However, to date, the involvement of the modification of specific targets in key aspects of nervous system development, like patterning and differentiation, has remained largely elusive. A number of recent works confirm the participation of target-specific SUMO modification in critical aspects of nervous system development. Here, we review pioneering and new findings demonstrating the essential role SUMO plays in neurogenesis and other facets of neurodevelopment, which will help to precisely understand the variety of mechanisms SUMO utilizes to control most fundamental processes in the cell.

Structural diversification of vitamin D using microbial biotransformations.

Vitamin D deficiencies are linked to multiple human diseases. Optimizing its synthesis, physicochemical properties, and delivery systems while minimizing side effects is of clinical relevance and is of great medical and industrial interest. Biotechnological techniques may render new modified forms of vitamin D that may exhibit improved absorption, stability, or targeted physiological effects. Novel modified vitamin D derivatives hold promise for developing future therapeutic approaches and addressing specific health concerns related to vitamin D deficiency or impaired metabolism, such as avoiding hypercalcemic effects. Identifying and engineering key enzymes and biosynthetic pathways involved, as well as developing efficient cultures, are therefore of outmost importance and subject of intense research. Moreover, we elaborate on the critical role that microbial bioconversions might play in the a la carte design, synthesis, and production of novel, more efficient, and safer forms of vitamin D and its analogs. In summary, the novelty of this work resides in the detailed description of the physiological, medical, biochemical, and epidemiological aspects of vitamin D supplementation and the steps towards the enhanced and simplified industrial production of this family of bioactives relying on microbial enzymes. KEY POINTS: • Liver or kidney pathologies may hamper vitamin D biosynthesis • Actinomycetes are able to carry out 1α- or 25-hydroxylation on vitamin D precursors.

Exome sequencing links the SUMO protease SENP7 with fatal arthrogryposis multiplex congenita, early respiratory failure and neutropenia.

BACKGROUND: SUMOylation involves the attachment of small ubiquitin-like modifier (SUMO) proteins to specific lysine residues on thousands of substrates with target-specific effects on protein function. Sentrin-specific proteases (SENPs) are proteins involved in the maturation and deconjugation of SUMO. Specifically, SENP7 is responsible for processing polySUMO chains on targeted substrates including the heterochromatin protein 1α (HP1α). METHODS: We performed exome sequencing and segregation studies in a family with several infants presenting with an unidentified syndrome. RNA and protein expression studies were performed in fibroblasts available from one subject. RESULTS: We identified a kindred with four affected subjects presenting with a spectrum of findings including congenital arthrogryposis, no achievement of developmental milestones, early respiratory failure, neutropenia and recurrent infections. All died within four months after birth. Exome sequencing identified a homozygous stop gain variant in SENP7 c.1474C>T; p.(Gln492*) as the probable aetiology. The proband's fibroblasts demonstrated decreased mRNA expression. Protein expression studies showed significant protein dysregulation in total cell lysates and in the chromatin fraction. We found that HP1α levels as well as different histones and H3K9me3 were reduced in patient fibroblasts. These results support previous studies showing interaction between SENP7 and HP1α, and suggest loss of SENP7 leads to reduced heterochromatin condensation and subsequent aberrant gene expression. CONCLUSION: Our results suggest a critical role for SENP7 in nervous system development, haematopoiesis and immune function in humans.

Synthesis and interaction with growth factors of sulfated oligosaccharides containing an anomeric fluorinated tail.

Compounds that mimic the biological properties of glycosaminoglycans (GAGs) and can be more easily prepared than the native GAG oligosaccharides are highly demanded. Here, we present the synthesis of sulfated oligosaccharides displaying a perfluorinated aliphatic tag at the reducing end as GAG mimetics. The preparation of these molecules was greatly facilitated by the presence of the fluorinated tail since the reaction intermediates were isolated by simple fluorous solid-phase extraction. Fluorescence polarization competition assays indicated that the synthesized oligosaccharides interacted with two heparin-binding growth factors, midkine (MK) and FGF-2, showing higher binding affinities than the natural oligosaccharides, and can be therefore considered as useful GAG mimetics. Moreover, NMR experiments showed that the 3D structure of these compounds is similar to that of the native sequences, in terms of sugar ring and glycosidic linkage conformations. Finally, we also demonstrated that these derivatives are able to block the MK-stimulating effect on NIH3T3 cells growth.

Cancer-Associated Dysregulation of Sumo Regulators: Proteases and Ligases.

SUMOylation is a post-translational modification that has emerged in recent decades as a mechanism involved in controlling diverse physiological processes and that is essential in vertebrates. The SUMO pathway is regulated by several enzymes, proteases and ligases being the main actors involved in the control of sumoylation of specific targets. Dysregulation of the expression, localization and function of these enzymes produces physiological changes that can lead to the appearance of different types of cancer, depending on the enzymes and target proteins involved. Among the most studied proteases and ligases, those of the SENP and PIAS families stand out, respectively. While the proteases involved in this pathway have specific SUMO activity, the ligases may have additional functions unrelated to sumoylation, which makes it more difficult to study their SUMO-associated role in cancer process. In this review we update the knowledge and advances in relation to the impact of dysregulation of SUMO proteases and ligases in cancer initiation and progression.

Midkine Interaction with Chondroitin Sulfate Model Synthetic Tetrasaccharides and Their Mimetics: The Role of Aromatic Interactions.

Midkine (MK) is a neurotrophic factor that participates in the embryonic central nervous system (CNS) development and neural stem cell regulation, interacting with sulfated glycosaminoglycans (GAGs). Chondroitin sulfate (CS) is the natural ligand in the CNS. In this work, we describe the interactions between a library of synthetic models of CS-types and mimics. We did a structural study of this library by NMR and MD (Molecular Dynamics), concluding that the basic shape is controlled by similar geometry of the glycosidic linkages. Their 3D structures are a helix with four residues per turn, almost linear. We have studied the tetrasaccharide-midkine complexes by ligand observed NMR techniques and concluded that the shape of the ligands does not change upon binding. The ligand orientation into the complex is very variable. It is placed inside the central cavity of MK formed by the two structured beta-sheets domains linked by an intrinsically disordered region (IDR). Docking analysis confirmed the participation of aromatics residues from MK completed with electrostatic interactions. Finally, we test the biological activity by increasing the MK expression using CS tetrasaccharides and their capacity in enhancing the growth stimulation effect of MK in NIH3T3 cells.

The Systemic Administration of the Chemokine CCL1 Evokes Thermal Analgesia in Mice Through the Activation of the Endocannabinoid System.

Apart from its involvement in immune functions, the chemokine CCL1 can participate in the modulation of nociceptive processing. Previous studies have demonstrated the hypernociceptive effect produced by CCL1 in the spinal cord, but its possible action on peripheral nociception has not yet been characterized. We describe here that the subcutaneous administration of CCL1 (1-10 µg/kg) produces dose-dependent and long-lasting increases in thermal withdrawal latencies measured by the unilateral hot plate test in mice. The antinociceptive nature of this effect is further supported by the reduction of spinal neurons expressing Fos protein in response to a noxious thermal stimulus observed after the administration of 10 µg/kg of CCL1. CCL1-induced antinociception was inhibited after systemic, but not spinal administration of the selective antagonist R243 (0.1-1 mg/kg), demonstrating the participation of peripheral CCR8 receptors. The absence of this analgesic effect in mice treated with a dose of cyclophosphamide that produces a drastic depletion of leukocytes suggests its dependency on white blood cells. Furthermore, whereas the antinociceptive effect of CCL1 was unaffected after the treatment with either the antagonist of opioid receptors naloxone or the cannabinoid type 1 receptor blocker AM251, it was dose-dependently inhibited after the administration of the CB2 receptor antagonist SR144528 (0.1-1 mg/kg). The detection by ELISA of an increased presence of the endocannabinoid 2-arachidonoylglycerol after the administration of an analgesic dose of CCL1 supports the notion that CCL1 can evoke thermal analgesia through the release of this endocannabinoid from circulating leukocytes.

The Sumo protease Senp7 is required for proper neuronal differentiation.

Covalent attachment of the Small ubiquitin-like modifier (Sumo) polypeptide to proteins regulates many processes in the eukaryotic cell. In the nervous system, Sumo has been associated with the synapsis and with neurodegenerative diseases. However, its involvement in regulating neuronal differentiation remains largely unknown. Here we show that net Sumo deconjugation is observed during neurogenesis and that Sumo overexpression impairs this process. In an attempt to shed light on the underlying mechanisms, we have analyzed the expression profile of genes coding for components of the sumoylation pathway following induction of neuronal differentiation. Interestingly, we observed strong upregulation of the Senp7 protease at both mRNA and protein levels under differentiation conditions. Sumo proteases, by removing Sumo from targets, are key regulators of sumoylation. Strikingly, loss-of-function analysis demonstrated that Senp7 is required for neuronal differentiation not only in a model cell line, but also in the developing neural tube. Finally, reporter-based analysis of the Senp7 promoter indicated that Senp7 was transiently activated at early stages of neuronal differentiation. Thus, the Sumo protease Senp7 adds to the list of factors involved in vertebrate neurogenesis.

Hyperalgesic and hypoalgesic mechanisms evoked by the acute administration of CCL5 in mice.

We show here that the intraplantar administration of CCL5 in mice produces hyperalgesia at low doses but activates compensatory antinociceptive mechanisms at doses slightly higher. Thus, the injection of 3-10ng of CCL5 evoked thermal hyperalgesia through the activation of CCR1 and CCR5 receptors, as demonstrated by the inhibitory effect exerted by the selective antagonists J113863 (0.01-0.1µg) and DAPTA (0.3-3µg), respectively. The prevention of this hyperalgesia by diclofenac (1-10µg), the inhibitors of COX-1 SC-560 (0.1-1µg) or COX-2 celecoxib (1-5µg), the TRPV1 antagonist capsazepine (0.03-0.3µg) or the TRPA1 antagonist HC030031 (10-50µg) demonstrates the involvement of prostaglandin synthesis and TRP sensitization in CCL5-evoked hyperalgesia. Doses of CCL5 higher than 17µg did not evoke hyperalgesia. However, this effect was restored by the administration of naloxone-methiodide (5µg), nor-binaltorphimine (10mg/kg) or an anti-dynorphin A antibody (0.62-2.5ng). The administration of 30ng of CCL5 also induced hyperalgesia in mice with reduced number of circulating white blood cells in response to cyclophosphamide or with selective neutrophil depletion induced by an anti-Ly6G antibody. In fact, the number of neutrophils present in paws treated with 30ng of CCL5 was greater than in paws receiving the administration of the hyperalgesic dose of 10ng. Finally, the expression of the endogenous opioid peptide dynorphin A was demonstrated by double immunofluorescence assays in these neutrophils attracted by CCL5. These results support previous data describing the hyperalgesic properties of CCL5 and constitute the first indication that a chemokine of the CC group can activate endogenous analgesic mechanisms.

Pleiotrophin antagonizes Brd2 during neuronal differentiation.

Bromodomain-containing protein 2 (Brd2) is a BET family chromatin adaptor required for expression of cell-cycle-associated genes and therefore involved in cell cycle progression. Brd2 is expressed in proliferating neuronal progenitors, displays cell-cycle-stimulating activity and, when overexpressed, impairs neuronal differentiation. Paradoxically, Brd2 is also detected in differentiating neurons. To shed light on the role of Brd2 in the transition from cell proliferation to differentiation, we had previously looked for proteins that interacted with Brd2 upon induction of neuronal differentiation. Surprisingly, we identified the growth factor pleiotrophin (Ptn). Here, we show that Ptn antagonized the cell-cycle-stimulating activity associated with Brd2, thus enhancing induced neuronal differentiation. Moreover, Ptn knockdown reduced neuronal differentiation. We analyzed Ptn-mediated antagonism of Brd2 in a cell differentiation model and in two embryonic processes associated with the neural tube: spinal cord neurogenesis and neural crest migration. Finally, we investigated the mechanisms of Ptn-mediated antagonism and determined that Ptn destabilizes the association of Brd2 with chromatin. Thus, Ptn-mediated Brd2 antagonism emerges as a modulation system accounting for the balance between cell proliferation and differentiation in the vertebrate nervous system.

Control of neuronal differentiation by sumoylation of BRAF35, a subunit of the LSD1-CoREST histone demethylase complex.

The LSD1-CoREST histone demethylase complex is required to repress neuronal genes in nonneuronal tissues. Here we show that sumoylation of Braf35, one of the subunits of the complex, is required to maintain full repression of neuron-specific genes and for occupancy of the LSD1-CoREST complex at its gene targets. Interestingly, expression of Braf35 was sufficient to prevent neuronal differentiation induced by bHLH neurogenic transcription factors in P19 cells and in neuronal progenitors of the chicken embryo neural tube. Sumoylation of Braf35 is required for this antineurogenic activity. We also show that iBraf, a paralogue of Braf35, forms heterodimers with Braf35. Braf35-iBraf heterodimerization impairs Braf35 interaction with the LSD1-CoREST complex and inhibits Braf35 sumoylation. Consistent with these results, iBraf prevents the antineurogenic activity of Braf35 in vivo. Our data uncover a mechanism of regulation of the LSD1-CoREST complex and provide a molecular explanation for the antagonism between Braf35 and iBraf in neuronal differentiation.

Association of bromodomain BET proteins with chromatin requires dimerization through the conserved motif B.

BET (bromodomain and extra terminal domain) family proteins are unique among bromodomain-containing proteins in that they not only associate with acetylated chromatin in interphase, but also remain attached to chromosomes during mitosis. Although the two tandem bromodomains are essential to display this behaviour, they do not suffice. In this work we report that a small conserved domain, motif B, is also required. A deletion mutant of this domain dissociates from mitotic chromosomes. However, inhibition of histone deacetylases alleviates dissociation. We also show that motif-B-dependent association with chromosomes is not restricted to mitosis. Interestingly, our results indicate that motif B constitutes a surface for homo- and hetero-dimerization between BET proteins. Finally, linked to the prominent role BET proteins play in cell proliferation, we report that ectopic expression of the family member Brd2 interferes with neuronal differentiation in P19 cells and in the vertebrate neural tube, probably because of preservation of adequate levels of cyclin A2 and cyclin D1. By contrast, a deletion mutant of motif B fails to perform in this way, highlighting the relevance of this domain for Brd2 function.

Topological regulation of the estrogen transcriptional response by ZATT-mediated inhibition of TOP2B activity.

Human type-II topoisomerases, TOP2A and TOP2B, remove transcription associated DNA supercoiling, thereby affecting gene-expression programs, and have recently been associated with 3D genome architecture. Here, we study the regulatory roles of TOP2 paralogs in response to estrogen, which triggers an acute transcriptional induction that involves rewiring of genome organization. We find that, whereas TOP2A facilitates transcription, as expected for a topoisomerase, TOP2B limits the estrogen response. Consistent with this, TOP2B activity is locally downregulated upon estrogen treatment to favor the establishment and stabilization of regulatory chromatin contacts, likely through an accumulation of DNA supercoiling. We show that estrogen-mediated inhibition of TOP2B requires estrogen receptor α (ERα), a non-catalytic function of TOP2A, and the action of the atypical SUMO-ligase ZATT. This mechanism of topological transcriptional-control, which may be shared by additional gene-expression circuits, highlights the relevance of DNA topoisomerases as central actors of genome dynamics.

The transcription factor Krox20 is an E3 ligase that sumoylates its Nab coregulators.

Covalent attachment of small ubiquitin-like modifier (SUMO) to proteins regulates many processes in the eukaryotic cell. This reaction is similar to ubiquitination and usually requires an E3 ligase for substrate modification. However, only a few SUMO ligases have been described so far, which frequently facilitate sumoylation by bringing together the SUMO-conjugating enzyme Ubc9 and the target protein. Ubc9 is an interaction partner of the transcription factor Krox20, a key regulator of hindbrain development. Here, we show that Krox20 functions as a SUMO ligase for its coregulators--the Nab proteins--and that Nab sumoylation negatively modulates Krox20 transcriptional activity in vivo.

Relevance of BET Family Proteins in SARS-CoV-2 Infection.

The recent pandemic we are experiencing caused by the coronavirus disease 2019 (COVID-19) has put the world's population on the rack, with more than 191 million cases and more than 4.1 million deaths confirmed to date. This disease is caused by a new type of coronavirus, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). A massive proteomic analysis has revealed that one of the structural proteins of the virus, the E protein, interacts with BRD2 and BRD4 proteins of the Bromodomain and Extra Terminal domain (BET) family of proteins. BETs are essential to cell cycle progression, inflammation and immune response and have also been strongly associated with infection by different types of viruses. The fundamental role BET proteins play in transcription makes them appropriate targets for the propagation strategies of some viruses. Recognition of histone acetylation by BET bromodomains is essential for transcription control. The development of drugs mimicking acetyl groups, and thereby able to displace BET proteins from chromatin, has boosted interest on BETs as attractive targets for therapeutic intervention. The success of these drugs against a variety of diseases in cellular and animal models has been recently enlarged with promising results from SARS-CoV-2 infection studies.

BETting on a Transcriptional Deficit as the Main Cause for Cornelia de Lange Syndrome.

Cornelia de Lange Syndrome (CdLS) is a human developmental syndrome with complex multisystem phenotypic features. It has been traditionally considered a cohesinopathy together with other phenotypically related diseases because of their association with mutations in subunits of the cohesin complex. Despite some overlap, the clinical manifestations of cohesinopathies vary considerably and, although their precise molecular mechanisms are not well defined yet, the potential pathomechanisms underlying these diverse developmental defects have been theoretically linked to alterations of the cohesin complex function. The cohesin complex plays a critical role in sister chromatid cohesion, but this function is not affected in CdLS. In the last decades, a non-cohesion-related function of this complex on transcriptional regulation has been well established and CdLS pathoetiology has been recently associated to gene expression deregulation. Up to 70% of CdLS cases are linked to mutations in the cohesin-loading factor NIPBL, which has been shown to play a prominent function on chromatin architecture and transcriptional regulation. Therefore, it has been suggested that CdLS can be considered a transcriptomopathy. Actually, CdLS-like phenotypes have been associated to mutations in chromatin-associated proteins, as KMT2A, AFF4, EP300, TAF6, SETD5, SMARCB1, MAU2, ZMYND11, MED13L, PHIP, ARID1B, NAA10, BRD4 or ANKRD11, most of which have no known direct association with cohesin. In the case of BRD4, a critical highly investigated transcriptional coregulator, an interaction with NIPBL has been recently revealed, providing evidence on their cooperation in transcriptional regulation of developmentally important genes. This new finding reinforces the notion of an altered gene expression program during development as the major etiological basis for CdLS. In this review, we intend to integrate the recent available evidence on the molecular mechanisms underlying the clinical manifestations of CdLS, highlighting data that favors a transcription-centered framework, which support the idea that CdLS could be conceptualized as a transcriptomopathy.

Kappa-opioid receptor-mediated thermal analgesia evoked by the intrathecal administration of the chemokine CCL1 in mice.

BACKGROUND: The chemokine CC motif ligand 1 (CCL1) participates in immune cell recruitment and, as other chemokines, is also involved in nociceptive processing. In contrast with previous reports indicating its participation in allodynia and cold hypernociception when spinally administered, its ability to evoke heat thermal analgesia, mediated by circulating leukocytes and endocannabinoids, after systemic administration has recently been reported. OBJECTIVES: Aiming to explore the role played by CCL1 on spinal nociception, we study here the effect of its intrathecal administration on thermal nociception in mice. METHODS: Behavioral nociceptive assays, immunohistochemical experiments, white cell blood depletion procedures and qRT-PCR experiments were performed. RESULTS: The intrathecal administration of CCL1 (0.3-30 ng) produced analgesia as measured by the unilateral hot plate test. This effect peaked 1 h after injection, was prevented by the CCR8 antagonist R243 and was accompanied by a reduction of c-Fos expression in spinal neurons. Whereas blood leukocyte depletion did not modify it, analgesia was abolished by the microglial inhibitor minocycline, but not the astroglial inhibitor aminoadipate. Furthermore, antinociception remained unmodified by the coadministration of cannabinoid type 1 or 2 receptors antagonists. However, it was reversed by naloxone but not by selective blockade of mu- or delta-opioid receptors. The inhibitory effect induced by the selective kappa-opioid receptor antagonist, nor-binaltorphimine, and by an anti-dynorphin A 1-17 antibody indicates that analgesia evoked by spinal CCL1 is mediated by endogenous dynorphins acting on kappa-opioid receptors. CONCLUSIONS: Endogenous dynorphin and microglia behave as key players in heat thermal analgesia evoked by spinal CCL1 in mice.

The Sumo proteome of proliferating and neuronal-differentiating cells reveals Utf1 among key Sumo targets involved in neurogenesis.

Post-translational modification by covalent attachment of the Small ubiquitin-like modifier (Sumo) polypeptide regulates a multitude of processes in vertebrates. Despite demonstrated roles of Sumo in the development and function of the nervous system, the identification of key factors displaying a sumoylation-dependent activity during neurogenesis remains elusive. Through a SILAC (stable isotope labeling by/with amino acids in cell culture)-based proteomic approach, we have identified the Sumo proteome of the model cell line P19 under proliferation and neuronal differentiation conditions. More than 300 proteins were identified as putative Sumo targets differentially associated with one or the other condition. A group of proteins of interest were validated and investigated in functional studies. Among these, Utf1 was revealed as a new Sumo target. Gain-of-function experiments demonstrated marked differences between the effects on neurogenesis of overexpressing wild-type and sumoylation mutant versions of the selected proteins. While sumoylation of Prox1, Sall4a, Trim24, and Utf1 was associated with a positive effect on neurogenesis in P19 cells, sumoylation of Kctd15 was associated with a negative effect. Prox1, Sall4a, and Kctd15 were further analyzed in the vertebrate neural tube of living embryos, with similar results. Finally, a detailed analysis of Utf1 showed the sumoylation dependence of Utf1 function in controlling the expression of bivalent genes. Interestingly, this effect seems to rely on two mechanisms: sumoylation modulates binding of Utf1 to the chromatin and mediates recruitment of the messenger RNA-decapping enzyme Dcp1a through a conserved SIM (Sumo-interacting motif). Altogether, our results indicate that the combined sumoylation status of key proteins determines the proper progress of neurogenesis.

SUMO association with repressor complexes, emerging routes for transcriptional control.

Covalent attachment of the small ubiquitin-like modifier (SUMO) to proteins constitutes a posttranslational modification intensely studied during the last decade. From the many proteins that serve as SUMO substrates, multiple functions have been assigned to this 100-amino acid polypeptide. Among them, a salient role in transcriptional regulation, and essentially in repression, prevails. Association of histone deacetylases (HDACs) with SUMO closely ties sumoylation with transcriptional repression. However, repressive effects linked to SUMO modification are not exclusively attributable to HDAC recruitment. Recently, several reports have revealed the importance of SUMO in the function of a variety of repressor complexes. In this respect, sumoylation is usually coupled to the establishment of heterochromatic states in the DNA. In this review, we summarize these recent reports and previous results concerning SUMO-mediated transcriptional repression. The analysis of the available data uncovers the importance not only of the covalent attachment of SUMO to proteins, but also of the presence in many proteins of SUMO interacting motifs (SIMs) that mediate non-covalent association with SUMO. In light of these findings we raise key questions and discuss why SUMO adopts a prominent role in establishing transcriptional repression in context of the activity, localization and architecture of chromatin-associated repressor complexes.