Role of ADP-ribosylation in Stress Granules

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Project Summary Membrane-less structures are prevalent in cells and executing unique functions (e.g., DNA repair foci and nucleoli for making ribosomal subunits). The mechanisms by which their formation, size, number, and dynamics are regulated, however, remain unclear. Emergent studies revealed that their formation can be partly explained by a biophysical phenomenon known as liquid-liquid phase separation, whereby the nucleoplasm and cytoplasm are considered complex fluids that stably segregate like oil and water. Phase separation is often triggered when proteins bind to a common scaffold such as the nucleic acids DNA and RNA, resulting in the condensation of proteins to form higher-order structures. We previously discovered that an under-studied nucleic acid called poly(ADP-ribose) (PAR) is critical for the formation of a class of membrane-less organelles implicated in cancer, virus infection and neurodegeneration called stress granules. Stress granules are cytoplasmic RNA-protein assemblies formed in different sizes in response to stressors such as hypoxia, oxidative stress and heat shock. Most granule components dynamically exchange with the surrounding cytoplasm, and individual granules grow in size over time through fusion. Notably, stress granules in models of neuropathological diseases, such as amyotrophic lateral sclerosis (ALS), have slower exchange dynamics and are less able to fuse. However, the molecular factors that control the stress granule dynamics and fusion (which affects size and number of granules) remain poorly understood. In this proposal, we will (1) determine how PAR regulates phase separation in stress granules using innovative techniques to define critical parameters of ADP- ribosylation for stress granule formation in cells and in vitro, and (2) determine whether PAR- protein interactions regulate stress granule fusion using mutagenesis, live-cell imaging, biophysical methods and proteomics. Projected Impact: Besides its role in stress granules, PAR is also critical for the formation of time- and location-specific membrane-less structures, including DNA repair complexes and nucleoli. This proposal will thus advance the field by defining critical parameters for physiologically relevant PAR-mediated phase separation and by identifying ADP-ribosylated proteins required for these phenomena. Given that PARPs are druggable and actively targeted by pharmaceutical companies, next-generation inhibitors may be designed to modulate the formation and dynamics of physiological and pathological membrane-less structures in neurological or other diseases.