Stress granules and neurodegeneration - PubMed (original) (raw)

Review

Stress granules and neurodegeneration

Benjamin Wolozin et al. Nat Rev Neurosci. 2019 Nov.

Abstract

Recent advances suggest that the response of RNA metabolism to stress has an important role in the pathophysiology of neurodegenerative diseases, particularly amyotrophic lateral sclerosis, frontotemporal dementias and Alzheimer disease. RNA-binding proteins (RBPs) control the utilization of mRNA during stress, in part through the formation of membraneless organelles termed stress granules (SGs). These structures form through a process of liquid-liquid phase separation. Multiple biochemical pathways regulate SG biology. The major signalling pathways regulating SG formation include the mammalian target of rapamycin (mTOR)-eukaryotic translation initiation factor 4F (eIF4F) and eIF2α pathways, whereas the pathways regulating SG dispersion and removal are mediated by valosin-containing protein and the autolysosomal cascade. Post-translational modifications of RBPs also strongly contribute to the regulation of SGs. Evidence indicates that SGs are supposed to be transient structures, but the chronic stresses associated with ageing lead to chronic, persistent SGs that appear to act as a nidus for the aggregation of disease-related proteins. We suggest a model describing how intrinsic vulnerabilities within the cellular RNA metabolism might lead to the pathological aggregation of RBPs when SGs become persistent. This process might accelerate the pathophysiology of many neurodegenerative diseases and myopathies, and it suggests new targets for disease intervention.

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Conflict of interest statement

Competing interests

B. W. is Co-founder and Chief Scientific Officer for Aquinnah Pharmaceuticals Inc. P. I. declares no competing interests.

Figures

Fig. 1 |. Types of membraneless organelles present in neurons.

a | Membraneless organelles exist throughout neurons. The membraneless organelles in the nucleus are termed ‘nuclear bodies’. The nuclear pore is another membraneless organelle. In the neuronal soma, membraneless organelles comprise stress granules and P-bodies, whereas in the axon and dendrites, such organelles consist of transport granules and storage granules; note that the abundance of RNA-binding proteins (RBPs) in RNA granules is much greater in the dendrite than the axon. The synapse contains activity-dependent granules, another type of membraneless organelle, which are required for synaptic plasticity. b | The boxes show that RBPs are commonly associated with different types of membraneless organelles. c | The boxes list examples of RBPs that shuttle between the nucleus and the cytoplasm, and those that are primarily cytoplasmic. Mutations in the genes encoding the highlighted RBPs have been linked to common neurodegenerative diseases.

Fig. 2 |. Formation of stress granules from the mRNA–ribosomal complex.

Multiple steps are involved in the transition from a translated mRNA to a primary nucleated stress granule. a | Initially, the mRNA associates with the 40S ribosomal particle to form the pre-initiation complex (PIC). b | The PIC complex then combines with the 60S ribosome particle to form the 80S ribosome, which actively translates mRNA to make proteins. The antibiotics cycloheximide and puromycin are commonly used to study stress granule (SG) biology and exert opposite actions; cycloheximide prevents SG formation whereas puromycin promotes SG formation. c | Cycloheximide acts by stalling translation elongation by inhibiting ribosome translocation, which ‘freezes’ the mRNA covered by ribosomes and hidden from RNA-binding proteins (RBPs) in the cytoplasm, whereas puromycin breaks up the translating ribosomes by becoming incorporated into the nascent polypeptide chain, and causing separation of the 60S ribosomal subunit from the 40S–mRNA complex. d | The free mRNA–40S complex associates with core nucleating RNA-binding proteins (RBPs), such as T-cell intracellular antigen 1 (TIA1), RAS GTP-activating protein-binding protein 1 (G3BP1), TIA1-related protein (TIAR), tristetraprolin (TTP) and cytoplasmic activation/proliferation-associated protein 1 (CAPRIN). e | Stimulated by the presence of mRNAs and RBPs (and likely other factors, such as post-translational modifications), mRNA–RBP complexes begin coalescing to become primary SGs through the process of liquid–liquid phase separation. Note that polysomes and SGs exist in an equilibrium, which is regulated by the cell state (not shown).

Fig. 3 |. Regulation of stress granule assembly.

a | The pre-initiation complex (PIC) has a key role in RNA translation and forms through a multi-step process. The elongation initiation factor 4F (eIF4F) complex recognizes the 5’ cap structures on mRNAs (1). Meanwhile, eIF2 combines with an initiator tRNA (tRNAiMet) to form a ternary complex (2), which then combines with the eIF3–40S ribosome to form the 43S PIC (3). This complex associates with the eIF4F–mRNA complex to form the 48S PIC (4), which then links up with the 60S complex to initiate mRNA translation (5). b | Each of the three major signalling cascades regulating stress granule (SG) formation causes displacement of a key element of the PIC, allowing RNA-binding proteins (RBPs) such as T-cell intracellular antigen 1 (TIA1) to bind and nucleate SGs. Mammalian target of rapamycin (mTOR) inhibition reduces phosphorylation of eIF4E-binding protein (4E-BP), which binds eIF4E and displaces eIF4G–eIF4A from the cap structures of an mRNA (1). Phosphorylation of eIF2α prevents it from forming the ternary complex (2). Drugs (for example, pateamine A and silvestrol) interfere with eIF4F complex assembly on the mRNA cap structures by targeting the helicase eIF4A, whereas tRNA-derived stress-induced RNAs (tiRNAs) displace eIF4F complexes from mRNA (3). In each case, the incomplete pre-initiation complex (PIC) allows RBPs such as TIA1 or RAS GTP-activating protein-binding protein 1 (G3BP1) to bind to mRNA and nucleate SG formation. The nucleated SG then matures over time, as additional types of RBPs bind, each attaching to existing mRNA as well as bringing in new mRNAs via their individual RNA recognition motifs. PABP, poly(A)-binding protein; TIAR, TIA1-related protein.

Fig. 4 |. Phases in the stress granule cycle.

a | The physiological stress granule (SG) cycle comprises several phases. Phase 1 represents basal conditions, in which nuclear RNA-binding proteins (RBPs; T-cell intracellular antigen 1 (TIA1), FUS, TAR DNA-binding protein 43 (TDP43), heterogeneous nuclear ribonucleoprotein A0 (hnRNPA0), hnRNPA1, hnRNPA2B1, EWS RNA binding protein 1 (EWSR1) and ataxin 2 (ATXN2)) perform their classical functions in the nucleus, such as participating in RNA splicing, while cytoplasmic RBPs (RAS GTP-activating protein-binding protein 1 (G3BP1) and G3BP2, polyA-bnding protein cytoplasmic 1 (PABPC1), eukaryotic initiation factor 2 (eIF2), eIF3, eIF4 and fragile X mental retardation protein (FMRP)) spread diffusely throughout the neuronal soma. In phase 2, nuclear RBPs translocate to the cytoplasm where they spread diffusely as either monomers or small complexes; neurons appear to have a strong capacity to maintain diffuse distributions of cytoplasmic RBPs without the induction of membraneless organelles. In phase 3, core nucleating RBPs (for example, TIA1, TIAR, G3BP1, and FMRP) begin to coalesce into SGs, which also include mRNA and 40S ribosomal subunits. In phase 4, the SGs mature, bringing in secondary RBPs, which include proteins such as hnRNPA0, hnRNPA1, hnRNPA2B1, EWSR1 and ATXN2. In phase 5, SG resolution begins with disaggregases (such as valosin-containing protein (VCP) and transportin), dispersing the RBPs that make up SGs. Soluble nuclear RBPs shuttle back to the nucleus, whereas RBPs that have formed insoluble amyloids are ubiquitinated and shunted to the autophagosome for disposal. b | The formation of pathological granules seems to differ from the cycle described above. As described above, most RBPs are in the nucleus under basal conditions. With acute stress, RBPs translocate to the cytoplasm and mostly associate with SGs. Note that in neurodegenerative diseases, seeding with extracellular fibrils can also induce cytoplasmic granules; the granules induced by seeding contain RBPs or nuclear pore proteins. With chronic stress, the phase-separated proteins mature to become gel-like and aggregated. The aggregated TDP43 migrates to the periphery of the SGs, where it becomes phosphorylated; note that pathological tissue also exhibits aggregated TDP43 that is not associated with SGs but is also phosphorylated. Upon resolution, such as might theoretically occur with treatment, much of the pathology might disappear. Reversible components of SGs might resolve with mobile RBPs possibly returning to the nucleus. However, the most insoluble elements of the pathological granules could remain aggregated as potentially inert pathological remnants. Part b is modified with permission from Ref. .

Fig. 4 |. Phases in the stress granule cycle.

Fig. 5 |. The RBP cascade hypothesis.

The RNA binding protein (RBP) cascade hypothesis of neurodegeneration proposes that disease mechanisms feed into a central biochemical pathway that has three levels of core components. The top level of the central cascade comprises the extracellular factors that cause neuronal stress, including increased levels of oligomeric amyloid-β (Aβ) or decreased levels of progranulin (PRGN). The middle level of the central cascade contains tau (abbreviated to pTau to reflect hyperphosphorylation) and TAR DNA-binding protein 43 (TDP43), which are proteins that mediate the effects of the extracellular stresses described above. The bottom level of the central cascade comprises the RBPs that mediate the translational stress response and form stress granules (SGs). Because this entire cascade feeds forward, the pathology characterizing the top and middle levels includes the RBPs from the lower level. At the last stage, maturation of the stress response leads to involvement of many RBPs in the stress cascade. Mutations in RBP genes associated with the SG response feed directly into this level by increasing the tendency of these proteins to aggregate, producing amyotrophic lateral sclerosis (ALS; and myopathies). The chronic nature of neurodegenerative disease causes the SGs to persist. The prolonged stress response provides time for unstable proteins associated with SGs, such as tau, TDP43 and other RBPs, to evolve into highly stable amyloid conformations, which produce the classic pathological inclusions that are associated with disease (bottom row). Genetics and environmental factors feed into each stage (left). Cardiovascular factors appear to feed in at the top of the cascade, brain trauma feeds in at the middle and lower levels, and viruses might feed into the lower level because they modulate protein synthesis by co-opting the biology of RBPs. Processes impacting after the pathological aggregates form are shown on the right column as modifying factors. These factors include mutations in genes encoding proteins that regulate proteostasis and the removal of pathological aggregates; these proteins generally function as part of the autolysosomal system. Inflammatory reactions to pathological aggregates and cellular damage play an important role at every level of the cascade. The coloured boxes at the bottom of the cascade depict the types of neuropathology resulting from the accumulation of pathological proteins and resulting neurodegeneration. The colour of each disease box (bottom row) is coordinated to reflect the respective genetic (left column), environmental (left column) and biochemical factors (central column) that ultimately leads to each particular disease. AD, Alzheimer disease; FTD-tau, frontotemporal dementia with tau pathology; FTD-TDP, frontotemporal dementia with TDP43 pathology.

References

1. Nott TJ et al. Phase transition of a disordered nuage protein generates environmentally responsive membraneless organelles. Mol Cell 57, 936–947, doi:10.1016/j.molcel.2015.01.013 (2015). - DOI - PMC - PubMed
1. Boeynaems S et al. Protein Phase Separation: A New Phase in Cell Biology. Trends Cell Biol 28, 420–435, doi:10.1016/j.tcb.2018.02.004 (2018). - DOI - PMC - PubMed
1. Taylor JP, Brown RH Jr. & Cleveland DW Decoding ALS: from genes to mechanism. Nature 539, 197–206, doi:10.1038/nature20413 (2016). - DOI - PMC - PubMed
1. Irwin DJ et al. Frontotemporal lobar degeneration: defining phenotypic diversity through personalized medicine. Acta Neuropathol 129, 469–491, doi:10.1007/s00401-014-1380-1 (2015). - DOI - PMC - PubMed
1. Kim HJ et al. Therapeutic modulation of eIF2alpha phosphorylation rescues TDP-43 toxicity in amyotrophic lateral sclerosis disease models. Nat Genet 46, 152–160, doi:10.1038/ng.2853 (2014). - DOI - PMC - PubMed

Stress granules and neurodegeneration - PubMed (original) (raw)