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Membrane-less organelles are condensates formed by phase separation whose functions often remain enigmatic. Upon oxidative stress, PML scaffolds Nuclear Bodies (NBs) to regulate senescence or stress adaptation (1). PML was the first identified protein of the SUMO-targeted Ubiquitin-mediated RNF4 ligase (2). We previously showed that PML NBs concentrates UBC9 together with PML partner proteins and thus proposed that PML NBs might favor partner sumoylation and subsequent poly-ubiquitination by NB-associated RNF4 (3). To establish whether PML controls partner sumoylation upon physiological stress response, we recently explored different models with distinct NB-status (4). SUMO2-conjugation is increased upon arsenic-induced PML NB re-organization in Acute Promyelocytic Leukemia (APL) mice, as well as in mouse Embryonic Stem Cells (mESCs) compared to Pml deficient mESCs. Differential in vivo SUMO2 proteomics identified as PML NB-dependent SUMO2-targets several members of the repressive KAP1 complex, as well as the DPPA2 transcription factor. Their sumoylation drives functional activation of this key epigenetic repressor and inhibition of DPPA2. Accordingly, Pml deficient mESCs re-express transposable elements and display features of 2-cell-like cells, a state of the mESCs reassembly to the 2-cell stage of embryogenesis. Finally, PML is required for adaptive stress responses in mESCs. Collectively, PML regulates mESC fate by orchestrating SUMO2-conjugation of key transcriptional or epigenetic regulators.

Sumo Modification in Cell Signalling

Exploration of PML in stress response and embryonic stem cell fate

kap1

sumo

SUMO (Small Ubiquitin-like Modifier) modification has recently emerged as a critical class of post translational modification (PTM), playing a major role in diverse cellular pathways in all eukaryotes. A key function of SUMO is to act as a vital counterpoise to Ubiquitination, adding a layer of control above ubiquitination including competition for targets and prevention of ubiquitin-dependent protein degradation. Phosphorylation is also regulated by SUMOylation of kinases and phosphatases. Crucially, unlike ubiquitination and phosphorylation which are controlled by thousands of genes, only 33-50 genes regulate the SUMO modification pathway in eukaryotes making SUMO an ideal experimental system to decipher the language and rules of PTMs.


Discovering how SUMO transduces environmental signals into specific physiological responses will be key to understanding this new set of fundamental ‘rules of life’ that regulate processes ranging from development to disease resistance in an ever-changing environment. This 2- day satellite meeting will highlight the established cellular role of SUMO in a range of eukaryotes in development to disease along with identifying knowledge gaps and horizon scanning. Particular emphasis will be paid to emerging technologies such as single cell techniques that will be crucial to decipher the SUMO code across eukaryotes. The meeting will consist of a series of talks from established to early career researchers who are using a variety of techniques to understand SUMO mediated cellular signalling in a range of systems (including yeast, plants and animals). Each session will end in a panel discussion with the audience hosted by the chair of the session.

This 2- day satellite meeting will highlight the established cellular role of SUMO in a range of eukaryotes in development to disease along with identifying knowledge gaps and horizon scanning. Particular emphasis will be paid to emerging technologies such as single cell techniques that will be crucial to decipher the SUMO code across eukaryotes.

technical paper

SLX4 (FANCP) is a multifunctional scaffold protein required for the maintenance of genome integrity. The functions of SLX4 depend on its BTB dimerization domain, but how these functions arise from SLX4 self-assembly remains elusive. SLX4 is predicted to be mainly disordered and includes SUMO interacting motifs. These characteristic features are conducive to the formation of biomolecular condensates that concentrate selectively proteins and nucleic acids into defined foci. Therefore, we used an optogenetic system to explore the capacity of SLX4 to form membraneless compartments and to understand their functions. We found that cooperative interactions mediated by the BTB dimerization domain, the SUMO interacting motifs, and the intrinsically disordered C terminal region drive the self-assembly of reversible SLX4 condensates. These micrometer-sized structures exhibited liquid-like properties. Optogenetic activation of SLX4 condensates occurred within minutes, in the absence of an exogenous source of DNA lesions. Therefore, we probed the functions that arise specifically from the condensation of SLX4. We found that the E3 SUMO ligases PIAS1, PIAS4, ZNF451, and the SUMO‐Targeted Ubiquitin Ligase RNF4 partition into SLX4 condensates and modify component proteins, including XPF, topoisomerase I, MDC1, and BRCA1. Furthermore, the control of SLX4 condensation by optogenetic activation induced the resection of newly synthesized DNA, as revealed by DNA fiber labeling and the removal of Top1-DPCs after CPT treatment. Our working hypothesis is that the nucleation of SLX4 condensates at DNA damage sites catalyzes protein group modification by the SUMO/STUBL pathway selectively, to enable the nucleolytic processing of DNA lesions and the initiation of repair.

SLX4 compartmentalizes the SUMO/Ubiquitin system to drive repair

The small ubiquitin-like modifier (SUMO) family is composed of five members, SUMO1, the highly similar SUMO2/SUMO3, SUMO4 and tissue restricted SUMO5/SUMO1P1. Sequence variation in SUMO4 is thought to prevent its maturation, resulting in an un-conjugatable SUMO isoform and consequently, its functions are poorly understood. Here we show for the first time that SUMO4 promotes DNA double-strand break (DSB) signalling in a manner distinct from SUMO1 or SUMO2/3. We show that SUMO4 function depends on interaction with partner proteins through SUMO interacting motifs and, on its inability to be conjugated. We show that SUMO4 promotes the activity of the SUMO protease SENP1. In the absence of SUMO4, reduced SENP1 catalytic activity results in hyperSUMOylation that unbalances the normal recruitment of several DSB repair factors and reduces DNA repair efficiency. These data reveal that SUMO4 acts as buffer for the SUMOylation system.

SUMO4 regulates DNA double strand break repair independently of conjugation

More than two decades of basic research is finally beginning to bear fruit, as the first early phase trials are evaluating the safety and efficacy of SUMO pathway inhibition in cancer patients. In cancer, SUMO machinery components are often dysregulated at the genomic level and by conditions typically present in cancer, such as hypoxia. Many molecular mechanisms relevant to the pathogenesis of specific cancers involve SUMO, highlighting the relevance of SUMO machinery components as potential prognostic markers and therapeutic targets. Our research focuses on characterizing mechanisms involving SUMO that drive cancer growth, ultimately helping find subgroups of patients putatively benefitting of drugs targeting SUMOylation. Our results of SUMOylation in breast cancer, head and neck cancer and neuroblastoma will be discussed.

Targeting SUMO machinery in cancer – any promise for patients?

TAK-981, a first in class inhibitor of SUMOylation, exhibits strong synergy with genotoxics and epidrugs used for acute myeloid leukemia treatments, against AML cell lines and primary patient blast cells in cellulo and in vivo. TAK-981 and epidrugs combination increases transcription of genes involved in apoptosis, cell cycle control but also triggers an anti-tumoral immunity mediated by Natural Killer cells.

TAK-981, a first in class inhibitor of SUMOylation, synergizes with Acute Myeloid Leukemias treatments and induces anti-tumor immunity in preclinical models

SUMO conjugation is emerging as an important mechanism to transduce environmental cues into cellular signaling. We present evidence to show that SUMOylation of key transcriptional regulators such as DELLA and BZR1, provides a conduit for environmental influence of root growth and development during salt stress. SUMOylation stabilizes BZR1 in the nucleus by inhibiting its interaction with BIN2 kinase. During salt stress, Arabidopsis plants arrest growth through deSUMOylation of BZR1 in the cytoplasm by promoting the accumulation of the BZR1 targeting SUMO protease, ULP1a. ULP1a mutants are salt tolerant and insensitive to the BR inhibitor, brassinazole with regards to root growth. BR treatment stimulates ULP1a degradation, allowing SUMOylated BZR1 to accumulate and promote growth. This study uncovers a mechanism for integrating environmental cues into BR signaling to shape root growth. 
Intriguingly an interacting partner of BZR1, the growth repressing DELLA proteins are also SUMOylated. We demonstrate that a proportion of DELLAs are conjugated to SUMO, the extent of conjugation increases during salt stress. We identify a SUMO interacting motif (SIM) in the GA receptor GID1 and demonstrate that SUMO-conjugated DELLA binds to this motif in a GA-independent manner. The consequent sequestration of GID1 by SUMO-conjugated DELLAs leads to an accumulation of non-SUMOylated DELLAs resulting in repression of root growth in salinity. We present a model depicting how SUMO decodes environmental cues to shape root growth and development.

Deciphering the SUMO code for adaptive responses in roots

Post-translational modifications by SUMO (Small Ubiquitin-like MOdifier) modulate protein activity through regulation of subcellular localization, protein activity and stability, and protein-protein interactions. In plants, SUMO is essential during seed development, modulates hormone signaling, and biotic/abiotic stress responses. In the cell, SUMO conjugates are highly enriched in the nucleus, consistently with a preferential nuclear localization of the SUMOylation machinery components. In this scenario, a major unsolved question is to uncover the mechanism that facilitates the conjugation of the cytosolic SUMO targets.
We will present our findings pointing to a major role of the large subunit of the SUMO activating enzyme, SAE2, controlling nucleocytoplasmic partitioning of SUMOylation. We have found that the C-terminus of SAE2 is processed, and this modification facilitates partial localization of SAE2 to the cytosol. Accumulation of SAE2 in the cytosol promotes cytosolic SUMOylation and behave as a signal for growth arrest. We will also discuss the potential implications of this mechanism in seed appearance during plant evolution.

Seed proteolytic proteoforms of SUMO-activating enzyme 2 (SAE2) mediate nucleocytoplasmic partitioning of SUMOylation.

In the model plant Arabidopsis, two closely related members of the plant specific SYP12 clade of secretory syntaxins (PEN1 and SYP122) are indispensable for the formation of evolutionarily conserved defense structures that provide general immunity to filamentous pathogens. However, in addition to the conserved SYP12 clade function shared with SYP122, PEN1 also provides resistance to attack by powdery mildew fungi. To elucidate the molecular properties that enables PEN1, but not SYP122, to mediate powdery mildew resistance, we made a series of chimeric constructs in which domains and individual amino acids from the two syntaxins were interchanged. In plants lacking native PEN1 and SYP122, we analyzed these chimeric syntaxins for their ability to restore the conserved SYP12 function and the specific function mediated by PEN1 alone. Here we have uncovered SUMOylation to be a critical modifier of PEN1 functionality and its localization in response to powdery mildew attack. To our knowledge, this is the first example in which SUMOylation positively regulates the function of a secretory syntaxin. Accordingly, PEN1 SUMOylation could represent a new mechanistic function for this post-translational modification.

SUMOylation regulates plant syntaxin function in innate immunity

Adrenal Cortex; SENP2; cellular differentiation; WNT/Beta-catenin

Loss of SUMO-specific protease 2 blocks adrenocortex transdifferentiation

TRIM28-dependent SUMOylation protects the adult ovary from activation of the testicular pathway

Massively increased global SUMO conjugation is detected in the brains of hibernating animals, leading to the concept that protein SUMOylation is essential for protecting cells from otherwise lethally reduced blood flow. Initially inspired by this naturally occurring phenomenon, we have focused our efforts on investigating the roles of protein SUMOylation and deSUMOylation in cell stress response implicated in various diseased conditions. Two examples will be presented in this regard. The first one is our finding about an adaptive pathway following ischemia, where deSUMO-2/3-ylation of dynamin-related protein 1 (Drp1) mediated by the SUMO protease SENP3 selectively promotes Drp1 binding to the mitochondrial fission factor (Mff), thereby enhancing Drp1 partitioning to the mitochondrial outer membrane to cause cytochrome c release and caspase 3-dependent cell death (1, 2). Interestingly, Bcl-2 family protein Bcl-xL appears to be associated with this pathway by interacting with Drp1. The Drp1-Bcl-xL interaction is primed by Mff and promoted by SENP3-mediated deSUMO-2/3-lyation of Drp1, and contributes to cell death following ischemia (3). The second one is our most recent finding about a previously uncharacterized role for SENP3 in mitochondrial quality control program in stressed cells, where SENP3-mediated deSUMO-2/3-ylation of mitochondrial fission 1 protein (Fis1) promotes mitophagy induced by the iron chelator deferiprone (4, 5). These findings offer new insights into the mechanisms underlying ischemic cell death and dysfunctional mitochondrial disposal upon iron chelation, and open the possibility of identifying new targets and developing therapeutic strategies against ischemia/reperfusion injury and a number of age-related disorders such as Parkinson's disease.

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Exploration of PML in stress response and embryonic stem cell fate

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