Characterizing the function of SUMO-targeted ubiquitin networks in coordinating nuclear and cytosolic protein quality control pathways

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Compartmentalizing cellular processes in membranous or membrane-less organelles, such as nuclear PML bodies (NBs) and paraspeckles or cytosolic stress granules (SGs), is a key concept of biochemical pathways. Membrane-less organelles function as RNA or protein quality control centers and their formation, disassembly and composition are highly sensitive to cellular stress. An example are cytosolic stress granules, which are composed of untranslated mRNAs and RNA-binding proteins and assemble in the cytosol in response to heat or redox stress. Nuclear RNA-binding proteins, including FUS and TDP-43, are recruited to SGs, but upon stress release and SG disassembly are relocated to the nucleus. In neurodegenerative disorders, such as amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD), pathogenic mutants of TDP-43 or FUS can induce formation of persistant stress granules. Proteotoxic stress and expression of TDP-43 or FUS mutants also trigger enrichment of the nuclear pool of both proteins in nuclear granules demonstrating that the dynamics of both nuclear and cytosolic membrane less-organelles is tightly connected to the cellular stress response. However, many aspects, such as involvement of post-translational modifications in these processes, have remained unclear. Further, interconnections of cytosolic stress granules with nuclear condensates are emerging, but the underlying molecular basis of these interconnections under normal and pathological conditions have remained enigmatic. We demonstrated that the SUMO-targeted ubiquitin ligase (StUbL) pathway is critically involved in the disassembly of stress granules upon stress release. In the StUbL pathway, which is induced in response to cellular stress, proteins are initially marked with polymeric SUMO chains thereby triggering their subsequent ubiquitylation by the Ub ligase RNF4. The pathway is considered as a nuclear protein quality control system, which is associated with PML NBs. We now found that the proper disassembly of SGs upon release from stress requires key component of the SUMO pathway, such as the modifier SUMO2, the SUMO ligase PIAS1, the StUbL RNF4 as well as the NB scaffold protein PML. The data reveal a so far unknown interconnection of SUMO signaling with SG clearance and an unprecedented interplay between SGs and PML NBs. Additionally, we demonstrate that StUbL signaling controls ubiquitylation and compartmentalization of ALS/FTDL-associated mutants of FUS and TDP-43. Here we aim to (i) dissect the SUMO/StUbL-regulated dynamics of SGs and PML NB (ii) understand the SUMO-RNF4-dependent crosstalk of SGs and NBs in proteostasis networks (iii) Define stress regulation of SUMO and Ub deconjugases in the StUbL pathway (iv) unveil the impact of SUMO signaling on pathogenic TDP-43 mutants.A better knowledge of these processes will improve our understanding of disrupted proteostasis in neurodegeneration and open up new avenues for therapeutic intervention.