Highlights d SG assembly is driven by the collective interactions of a core protein-RNA network d The central node G3BP encodes a molecular switch that regulates RNA-dependent LLPS d Interplay between 3 distinct IDRs in G3BP tunes the intrinsic propensity for LLPS d Extrinsic factors regulate SG assembly through positive or negative cooperativity

Summary Expansion of a hexanucleotide repeat GGGGCC (G4C2) in C9ORF72 is the most common cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Transcripts carrying (G4C2) expansions undergo unconventional, non-ATG-dependent translation, generating toxic dipeptide repeat (DPR) proteins thought to contribute to disease. Here we identify the interactome of all DPRs and find that arginine-containing DPRs, polyGly-Arg (GR) and polyPro-Arg (PR), interact with RNA-binding proteins and proteins with low complexity sequence domains (LCDs) that often mediate the assembly of membrane-less organelles. Indeed, most GR/PR interactors are components of membrane-less organelles such as nucleoli, the nuclear pore complex and stress granules. Genetic analysis in Drosophila demonstrated the functional relevance of these interactions to DPR toxicity. Furthermore, we show that GR and PR altered phase separation of LCD-containing proteins, insinuating into their liquid assemblies and changing their material properties, resulting in perturbed dynamics and/or functions of multiple membrane-less organelles.

SUMMARY Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are age-related neurodegenerative disorders with shared genetic etiologies and overlapping clinical and pathological features. Here we studied a novel ALS/FTD family and identified the P362L mutation in the low complexity domain (LCD) of T-cell-restricted intracellular antigen-1 (TIA1). Subsequent genetic association analyses showed an increased burden of TIA1 LCD mutations in ALS patients compared to controls (P = 8.7×10−6). Postmortem neuropathology of five TIA1 mutations carriers showed a consistent pathological signature with numerous round, hyaline, TAR DNA-binding protein 43 (TDP-43)-positive inclusions. TIA1 mutations significantly increased the propensity of TIA1 protein to undergo phase transition. In live cells, TIA1 mutations delayed stress granule (SG) disassembly and promoted the accumulation of non-dynamic SGs that harbored TDP-43. Moreover, TDP-43 in SGs became less mobile and insoluble. The identification of TIA1 mutations in ALS/FTD reinforces the importance of RNA metabolism and SG dynamics in ALS/FTD pathogenesis.

Amyotrophic lateral sclerosis (ALS) is a rapidly progressing neurodegenerative disease characterized by motor neuron loss, leading to paralysis and death 2–5 years following disease onset1. Nearly all ALS patients contain aggregates of the RNA-binding protein TDP-43 in the brain and spinal cord2, and rare mutations in the gene encoding TDP-43 can cause ALS3. There are no effective TDP-43-directed therapies for ALS or related TDP-43 proteinopathies, such as frontotemporal dementia (FTD). Antisense oligonucleotides (ASOs) and RNA interference approaches are emerging as attractive therapeutic strategies in neurological diseases4. Indeed, treating a rodent model of inherited ALS (caused by a mutation in SOD1) with ASOs to SOD1 significantly slowed disease progression5. But since SOD1 mutations account for only ~2–5% of ALS cases, additional therapeutic strategies are needed. Silencing TDP-43 itself is probably not warranted given its critical cellular functions1,6 Here we present an unexpectedly powerful alternative therapeutic strategy for ALS, by targeting ataxin 2. Lowering ataxin 2 suppresses TDP-43 toxicity in yeast and flies7, and intermediate-length polyglutamine expansions in the ataxin 2 gene increase risk of ALS7,8. We used two independent approaches to test whether reducing ataxin 2 levels could mitigate disease in a mouse model of TDP-43 proteinopathy9. First, we crossed ataxin 2 knockout mice to TDP-43 transgenic mice. Lowering ataxin 2 reduced TDP-43 aggregation, had a dramatic effect on survival and improved motor function. Second, in a more therapeutically applicable approach, we administered ASOs targeting ataxin 2 to the central nervous system of TDP-43 mice. This single treatment markedly extended survival. Because TDP-43 aggregation is a component of nearly all ALS cases6, targeting ataxin 2 could represent a broadly effective therapeutic strategy.

Stress granules (SGs) are non-membrane-bound RNA-protein granules that assemble through phase separation in response to cellular stress. Disturbances in SG dynamics have been implicated as a primary driver of neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), suggesting the hypothesis that these diseases reflect an underlying disturbance in the dynamics and material properties of SGs. However, this concept has remained largely untestable in available models of SG assembly, which require the confounding variable of exogenous stressors. Here we introduce a light-inducible SG system, termed OptoGranules, based on optogenetic multimerization of G3BP1, which is an essential scaffold protein for SG assembly. In this system, which permits experimental control of SGs in living cells in the absence of exogenous stressors, we demonstrate that persistent or repetitive assembly of SGs is cytotoxic and is accompanied by the evolution of SGs to cytoplasmic inclusions that recapitulate the pathology of ALS-FTD.Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (<xref ref-type="decision-letter" rid="SA1">see decision letter</xref>).

Summary Disturbances in autophagy and stress granule dynamics have been implicated as potential mechanisms underlying inclusion body myopathy (IBM) and related disorders. Yet, the roles of core autophagy proteins in IBM and stress granule dynamics remain poorly characterized. Here, we demonstrate that disrupted expression of the core autophagy proteins ULK1/2 in mice causes a vacuolar myopathy with ubiquitin and TDP-43–positive inclusions, similar to that caused by mutations in VCP, the most common cause of familial IBM. Mechanistically, we show that ULK1/2 localize to stress granules and phosphorylate VCP, thereby increasing VCP’s activity and ability to disassemble stress granules. These data suggest that VCP dysregulation and defective stress granule disassembly contribute to IBM-like disease in Ulk7/2-deficient mice. In addition, stress granule disassembly is accelerated by an ULK1/2 agonist, suggesting ULK1/2 as targets for exploiting higher-order regulation of stress granules for therapeutic intervention of IBM and related disorders.

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