Protein phase separation drives the assembly of membraneless organelles, but little is known about how these membraneless organelles are maintained in a metastable liquid- or gel-like phase rather ...than proceeding to solid aggregation. Here, we find that human small heat-shock protein 27 (Hsp27), a canonical chaperone that localizes to stress granules (SGs), prevents FUS from undergoing liquid-liquid phase separation (LLPS) via weak interactions with the FUS low complexity (LC) domain. Remarkably, stress-induced phosphorylation of Hsp27 alters its activity, leading Hsp27 to partition with FUS LC to preserve the liquid phase against amyloid fibril formation. NMR spectroscopy demonstrates that Hsp27 uses distinct structural mechanisms for both functions. Our work reveals a fine-tuned regulation of Hsp27 for chaperoning FUS into either a polydispersed state or a LLPS state and suggests an essential role for Hsp27 in stabilizing the dynamic phase of stress granules.
Subcellular membrane-less organelles consist of proteins with low complexity domains. Many of them, such as hnRNPA1, can assemble into both a polydisperse liquid phase and an ordered solid phase of ...amyloid fibril. The former mirrors biological granule assembly, while the latter is usually associated with neurodegenerative disease. Here, we observe a reversible amyloid formation of hnRNPA1 that synchronizes with liquid-liquid phase separation, regulates the fluidity and mobility of the liquid-like droplets, and facilitates the recruitment of hnRNPA1 into stress granules. We identify the reversible amyloid-forming cores of hnRNPA1 (named hnRACs). The atomic structures of hnRACs reveal a distinct feature of stacking Asp residues, which contributes to fibril reversibility and explains the irreversible pathological fibril formation caused by the Asp mutations identified in familial ALS. Our work characterizes the structural diversity and heterogeneity of reversible amyloid fibrils and illuminates the biological function of reversible amyloid formation in protein phase separation.
Mutations in RNA-binding proteins (RBPs) localized in ribonucleoprotein (RNP) granules, such as hnRNP A1 and TDP-43, promote aberrant protein aggregation, which is a pathological hallmark of various ...neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Protein posttranslational modifications (PTMs) are known to regulate RNP granules. In this study, we investigate the function of poly(ADP-ribosyl)ation (PARylation), an important PTM involved in DNA damage repair and cell death, in RNP granule-related neurodegeneration. We reveal that PARylation levels are a major regulator of the assembly-disassembly dynamics of RNP granules containing disease-related RBPs, hnRNP A1 and TDP-43. We find that hnRNP A1 can both be PARylated and bind to PARylated proteins or poly(ADP-ribose) (PAR). We further uncover that PARylation of hnRNP A1 at K298 controls its nucleocytoplasmic transport, whereas PAR-binding via the PAR-binding motif (PBM) of hnRNP A1 regulates its association with stress granules. Moreover, we reveal that PAR not only dramatically enhances the liquid-liquid phase separation of hnRNP A1, but also promotes the co-phase separation of hnRNP A1 and TDP-43 in vitro and their interaction in vivo. Finally, both genetic and pharmacological inhibition of PARP mitigates hnRNP A1- and TDP-43-mediated neurotoxicity in cell and Drosophila models of ALS. Together, our findings suggest a novel and crucial role for PARylation in regulating the dynamics of RNP granules, and that dysregulation in PARylation and PAR levels may contribute to ALS disease pathogenesis by promoting protein aggregation.
Thermostable cross-β structures are characteristic of pathological amyloid fibrils, but these structures cannot explain the reversible nature of fibrils formed by RNA-binding proteins such as fused ...in sarcoma (FUS), involved in RNA granule assembly. Here, we find that two tandem (S/G)Y(S/G) motifs of the human FUS low-complexity domain (FUS LC) form reversible fibrils in a temperature- and phosphorylation-dependent manner. We named these motifs reversible amyloid cores, or RAC1 and RAC2, and determined their atomic structures in fibrillar forms, using microelectron and X-ray diffraction techniques. The RAC1 structure features an ordered-coil fibril spine rather than the extended β-strand typical of amyloids. Ser42, a phosphorylation site of FUS, is critical in the maintenance of the ordered-coil structure, which explains how phosphorylation controls fibril formation. The RAC2 structure shows a labile fibril spine with a wet interface. These structures illuminate the mechanism of reversible fibril formation and dynamic assembly of RNA granules.
Human heterogeneous nuclear ribonucleoprotein A1 (hnRNPA1) serves as a key regulating protein in RNA metabolism. Malfunction of hnRNPA1 in nucleo-cytoplasmic transport or dynamic phase separation ...leads to abnormal amyloid aggregation and neurodegeneration. The low complexity (LC) domain of hnRNPA1 drives both dynamic phase separation and amyloid aggregation. Here, we use cryo-electron microscopy to determine the amyloid fibril structure formed by hnRNPA1 LC domain. Remarkably, the structure reveals that the nuclear localization sequence of hnRNPA1 (termed PY-NLS), which is initially known to mediate the nucleo-cytoplamic transport of hnRNPA1 through binding with karyopherin-β2 (Kapβ2), represents the major component of the fibril core. The residues that contribute to the binding of PY-NLS with Kapβ2 also exert key molecular interactions to stabilize the fibril structure. Notably, hnRNPA1 mutations found in familial amyotrophic lateral sclerosis (ALS) and multisystem proteinopathoy (MSP) are all involved in the fibril core and contribute to fibril stability. Our work illuminates structural understandings of the pathological amyloid aggregation of hnRNPA1 and the amyloid disaggregase activity of Kapβ2, and highlights the multiple roles of PY-NLS in hnRNPA1 homeostasis.
Despite the prominent role of TDP-43 in neurodegeneration, its physiological and pathological functions are not fully understood. Here, we report an unexpected role of TDP-43 in the formation of ...dynamic, reversible, liquid droplet-like nuclear bodies (NBs) in response to stress. Formation of NBs alleviates TDP-43-mediated cytotoxicity in mammalian cells and fly neurons. Super-resolution microscopy reveals distinct functions of the two RRMs in TDP-43 NB formation. TDP-43 NBs are partially colocalized with nuclear paraspeckles, whose scaffolding lncRNA NEAT1 is dramatically upregulated in stressed neurons. Moreover, increase of NEAT1 promotes TDP-43 liquid-liquid phase separation (LLPS) in vitro. Finally, we discover that the ALS-associated mutation D169G impairs the NEAT1-mediated TDP-43 LLPS and NB assembly, causing excessive cytoplasmic translocation of TDP-43 to form stress granules, which become phosphorylated TDP-43 cytoplasmic foci upon prolonged stress. Together, our findings suggest a stress-mitigating role and mechanism of TDP-43 NBs, whose dysfunction may be involved in ALS pathogenesis.
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•Stress induces phase-separated TDP-43 NBs to alleviate cytotoxicity•The two RRMs respond to different RNAs and act distinctly in the assembly of TDP-43 NBs•LncRNA NEAT1 promotes TDP-43 LLPS and is upregulated in stressed neurons•The ALS-causing D169G mutation is NB-defective and forms pTDP-43 cytoplasmic foci
The RNA-binding protein TDP-43 is associated with neurodegenerative diseases, such as ALS. Wang et al. report that TDP-43 forms protective nuclear bodies in response to stress, which is mediated by lncRNA NEAT1-promoted liquid-liquid phase separation. Defects in the assembly of stress-mitigating TDP-43 nuclear bodies may contribute to ALS pathogenesis.
Membraneless organelles contain a wide spectrum of molecular chaperones, indicating their important roles in modulating the metastable conformation and biological function of membraneless organelles. ...Here we report that class I and II Hsp40 (DNAJ) proteins possess a high ability of phase separation rendered by the flexible G/F-rich region. Different Hsp40 proteins localize in different membraneless organelles. Specifically, human Hdj1 (DNAJB1), a class II Hsp40 protein, condenses in ubiquitin (Ub)-rich nuclear bodies, while Hdj2 (DNAJA1), a class I Hsp40 protein, condenses in nucleoli. Upon stress, both Hsp40 proteins incorporate into stress granules (SGs). Mutations of the G/F-rich region not only markedly impaired Hdj1 phase separation and SG involvement and disrupted the synergistic phase separation and colocalization of Hdj1 and fused in sarcoma (FUS) in cells. Being cophase separated with FUS, Hdj1 stabilized the liquid phase of FUS against proceeding into amyloid aggregation in vitro and alleviated abnormal FUS aggregation in cells. Moreover, Hdj1 uses different domains to chaperone FUS phase separation and amyloid aggregation. This paper suggests that phase separation is an intrinsic property of Hsp40 proteins, which enables efficient incorporation and function of Hsp40 in membraneless organelles and may further mediate the buildup of chaperone network in membraneless organelles.
While acetylated, RNA-binding-deficient TDP-43 reversibly phase separates within nuclei into complex droplets (anisosomes) comprised of TDP-43-containing liquid outer shells and liquid centres of ...HSP70-family chaperones, cytoplasmic aggregates of TDP-43 are hallmarks of multiple neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS). Here we show that transient oxidative stress, proteasome inhibition or inhibition of the ATP-dependent chaperone activity of HSP70 provokes reversible cytoplasmic TDP-43 de-mixing and transition from liquid to gel/solid, independently of RNA binding or stress granules. Isotope labelling mass spectrometry was used to identify that phase-separated cytoplasmic TDP-43 is bound by the small heat-shock protein HSPB1. Binding is direct, mediated through TDP-43's RNA binding and low-complexity domains. HSPB1 partitions into TDP-43 droplets, inhibits TDP-43 assembly into fibrils, and is essential for disassembly of stress-induced TDP-43 droplets. A decrease in HSPB1 promotes cytoplasmic TDP-43 de-mixing and mislocalization. HSPB1 depletion was identified in spinal motor neurons of patients with ALS containing aggregated TDP-43. These findings identify HSPB1 to be a regulator of cytoplasmic TDP-43 phase separation and aggregation.
Emerging evidence has shown that exposure to ambient fine particulate matter (PM2.5) is associated with hepatic lipid accumulation. However, the underlying mechanism is not fully characterized yet. ...Autonomous circadian clock in the liver plays a fundamental role in maintaining lipid metabolism homeostasis. In this study, we evaluated the effects of ambient PM2.5 exposure on the expression of hepatic circadian clock genes and expression rhythm of genes associated with lipid metabolism in mice liver. Male C57BL/6 mice were randomly assigned to ambient PM2.5 or filtered air for 10 weeks via a whole body exposure system. We found that the liver mass was reduced significantly at zeitgeber time (ZT) 8 in mice exposed to PM2.5 but not levels or circadian rhythm of hepatic triglycerides or free fatty acid (FFA). In addition, exposure to PM2.5 led to enhanced expression of bmal1 at ZT0/24, cry1 at ZT16 and rev-erbα at ZT4 and ZT8. Furthermore, the expression of pparα was enhanced in mice liver at ZT4 and ZT8 after PM2.5 exposure, with upregulation of pparα-mediated genes responsible for fatty acid transport and oxidation. Finally, the expression of rate-limiting enzymes for lipid synthesis was all significantly increased in the liver of PM2.5 exposed mice at ZT12. Therefore, the present study provides new perspectives for revealing the etiology of hepatic lipid metabolism abnormality from PM2.5-induced circadian rhythm disorder.
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•PM2.5 exposure altered the expression pattern or level of core circadian clock genes in mouse liver.•PM2.5 exposure disrupted the rhythms of key transcription factors in hepatic lipid metabolism.•PM2.5 exposure disrupted the rhythms of the key enzymes involved in hepatic lipid metabolism.
Perturbing hepatic circadian oscillations and lipid metabolism with ambient fine particulate matter exposure.
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