SUMMARY Stress granules are membrane-less organelles composed of RNA-binding proteins (RBPs) and RNA. Functional impairment of stress granules has been implicated in amyotrophic lateral sclerosis, frontotemporal dementia and multisystem proteinopathy - diseases that are characterized by fibrillar inclusions of RBPs. Genetic evidence suggests a link between persistent stress granules and the accumulation of pathological inclusions. Here we demonstrate that the RBP hnRNPA1 undergoes liquid-liquid phase separation (LLPS) into protein-rich droplets mediated by a low complexity sequence domain (LCD). While the LCD of hnRNPA1 is sufficient to mediate LLPS, the RNA recognition motifs contribute to LLPS in the presence of RNA, giving rise to several mechanisms for regulating assembly. Importantly, while not required for LLPS, fibrillization is enhanced in protein-rich droplets. We suggest that LCD-mediated LLPS contributes to the assembly of stress granules and their liquid properties, and provides a mechanistic link between persistent stress granules and fibrillar protein pathology in disease.

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 During apoptosis, the BCL-2 protein family controls Mitochondrial Outer Membrane Permeabilization (MOMP), but the dynamics of this regulation remains controversial. We employed chimeric proteins composed of exogenous BH3 domains inserted into a tBID backbone that can activate the pro-apoptotic effectors BAX and BAK to permeabilize membranes without being universally sequestered by all anti-apoptotic BCL-2 proteins. We thus identified two “modes” whereby pro-survival BCL-2 proteins can block MOMP, by sequestering direct activator BH3-only proteins (“MODE 1”) or by binding active BAX and BAK (“MODE 2”). Notably, we found that MODE 1 sequestration is less efficient and more easily de-repressed to promote MOMP than MODE 2. Further, MODE 2 sequestration prevents mitochondrial fusion. We provide a unified model of BCL-2 family function that helps to explain otherwise paradoxical observations relating to MOMP, apoptosis, and mitochondrial dynamics.

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.

MCL-1, an anti-apoptotic BCL-2 family member that is essential for the survival of multiple cell lineages, is also among the most highly amplified genes in cancer. Although MCL-1 is known to oppose cell death, precisely how it functions to promote survival of normal and malignant cells is poorly understood. Here, we report that different forms of MCL-1 reside in distinct mitochondrial locations and exhibit separable functions. On the outer mitochondrial membrane, a MCL-1 isoform acts like other anti-apoptotic BCL-2 molecules to antagonize apoptosis, whereas an amino-terminally truncated isoform of MCL-1 that is imported into the mitochondrial matrix is necessary to facilitate normal mitochondrial fusion, ATP production, membrane potential, respiration, cristae ultrastructure, and maintenance of oligomeric ATP synthase. Our results provide insight into how MCL-1's surprisingly diverse salutary functions may control the survival of both normal and cancer cells.

SUMMARY The inflammasome is an intracellular signaling complex, which on recognition of pathogens and physiological aberration, drives activation of caspase-1, pyroptosis, and the release of the pro-inflammatory cytokines IL-1β and IL-18. Bacterial ligands must secure entry into the cytoplasm to activate inflammasomes, however, the mechanism by which concealed ligands are liberated in the cytoplasm have remained unclear. Here, we showed that the interferon-inducible protein IRGB10 is essential for activation of the DNA-sensing AIM2 inflammasome by Francisella novicida, and contributed to the activation of the LPS-sensing caspase-11 and NLRP3 inflammasome by Gram-negative bacteria. IRGB10 directly targeted cytoplasmic bacteria through a mechanism requiring guanylate-binding proteins. Localization of IRGB10 to the bacterial cell membrane compromised bacterial structural integrity and mediated cytosolic release of ligands for recognition by inflammasome sensors. Overall, our results reveal IRGB10 as part of a conserved signaling hub at the interface between cell-autonomous immunity and innate immune sensing pathways.

SummaryThe physiological basis and mechanistic requirement for the high immunoreceptor tyrosine activation motifs (ITAM) multiplicity of the T cell receptor (TCR)-CD3 complex remains obscure. Here we show that while low TCR-CD3 ITAM multiplicity is sufficient to engage canonical TCR-induced signaling events that lead to cytokine secretion, high TCR-CD3 ITAM multiplicity is required for TCR-driven proliferation. This is dependent on compact immunological synapse formation, interaction of the adaptor Vav1 with phosphorylated CD3 ITAMs to mediate Notch1 recruitment and activation and ultimately c-Myc-induced proliferation. Analogous mechanistic events are also required to drive proliferation in response to weak peptide agonists. Thus, the TCR-driven pathways that initiate cytokine secretion and proliferation are separable and co-ordinated by the multiplicity of phosphorylated TCR-CD3 ITAMs.

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>).