Stress granules, RNA-binding proteins and polyglutamine diseases: too much aggregation?

Abstract

Stress granules (SGs) are membraneless cell compartments formed in response to different stress stimuli, wherein translation factors, mRNAs, RNA-binding proteins (RBPs) and other proteins coalesce together. SGs assembly is crucial for cell survival, since SGs are implicated in the regulation of translation, mRNA storage and stabilization and cell signalling, during stress. One defining feature of SGs is their dynamism, as they are quickly assembled upon stress and then rapidly dispersed after the stress source is no longer present. Recently, SGs dynamics, their components and their functions have begun to be studied in the context of human diseases. Interestingly, the regulated protein self-assembly that mediates SG formation contrasts with the pathological protein aggregation that is a feature of several neurodegenerative diseases. In particular, aberrant protein coalescence is a key feature of polyglutamine (PolyQ) diseases, a group of nine disorders that are caused by an abnormal expansion of PolyQ tract-bearing proteins, which increases the propensity of those proteins to aggregate. Available data concerning the abnormal properties of the mutant PolyQ disease-causing proteins and their involvement in stress response dysregulation strongly suggests an important role for SGs in the pathogenesis of PolyQ disorders. This review aims at discussing the evidence supporting the existence of a link between SGs functionality and PolyQ disorders, by focusing on the biology of SGs and on the way it can be altered in a PolyQ disease context.

Fig. 1. Stress granule components.

Stress granules are multimolecular cytoplasmic foci that assemble as part of the cellular response to stress. They largely derive from stalled pre-initiation translation complexes and are mainly comprised of poly(A)⁺ mRNA molecules, 40S ribosomal subunits and a vast array of proteins (more than 450). The majority of these are RNA-binding proteins (RBPs) that bind to each other and to the other SG components. They include eukaryotic translation initiation factor 2 subunit alpha (eIF2α), ras GTPase-activating protein-binding protein 1 (G3BP1), T-cell intracellular antigen-1 (TIA-1), polyadenylate-binding protein 1 (PABP1), ataxin-2, fragile X mental retardation protein (FMRP) and tristetraprolin (TTP). The unspecified shapes coloured in grayscale represent the remaining proteins counted among the numerous SG protein components described so far.

Fig. 2. The canonical stress granule assembly pathway.

(1) Formation of stress granules (SGs) can be triggered by diverse cell damaging conditions, including viral infection, oxidative stress, heat shock, nutrient deprivation, ultraviolet radiation or proteotoxic stress. Particular stress conditions are detected by specific kinases—protein kinase R (PKR), haem-regulated inhibitor (HRI), general control non-derepressible-2 (GCN2) and pancreatic eIF2α kinase (PEF)—that then become activated and (2) phosphorylate eukaryotic translation initiation factor 2 subunit alpha (eIF2α). (3) eIF2α is involved in the formation of translation initiation complexes and, when phosphorylated, leads to dissociation of these complexes and to translational arrest. (4) mRNAs, 40S ribosomal subunits, and proteins involved in translation start to accumulate and to assemble together, along with other proteins that are recruited to the forming SGs. This primary aggregation process produces a stable SG core. RNA-binding proteins (RBPs) that constitute SG cores include ras GTPase-activating protein-binding protein 1 (G3BP1), T-cell intracellular antigen-1 (TIA-1), tristetraprolin (TTP) and fragile X mental retardation protein (FMRP). (5) A secondary aggregation step resulting from additional, albeit weaker, intermolecular interactions originate the shell of the SGs. RBPs recruited in this step include heterogenous nuclear ribonucleoprotein A0 (hnRNPA0), hnRNPA1, hnRNPA2B1 and RNA-binding protein EWS (EWSR1). (6) When stress conditions abate, SGs are either disassembled by molecular chaperons or (7) are cleared by autophagy. (8) Disassembly allows for a rapid recovery of protein synthesis.

Fig. 3. Stress granules functions.

Stress granules (SGs) participate in the cellular response to stress through a set of different actions that are interconnected and derive from the individual activities of SG components and from the assembly/disassembly of these foci. A SG formation involves the disassembly of translation initiation complexes and the coalescence of mRNAs molecules, ribosomal subunits and many proteins involved in translation, resulting in translational arrest and protein synthesis suppression. Additionally, several SG components are known to act as translational repressors. B SGs function as stores of RNAs and proteins in cells under stress, but allow rapid mobilization of these molecules when the damaging conditions subside. C SGs may modulate the expression of specific proteins during stress, by directing particular mRNA species to different possible fates. This action appears to involve an exchange of mRNAs between SGs and processing bodies, upon docking of these two types of RNA granules. Translation of some proteins involved in stress responses may be prioritized, some mRNAs may be kept stored in SGs or elsewhere, while others, such as those codifying proteins prone to misfolding, may be targeted for degradation by the processing bodies. D By intercepting particular signalling molecules, SGs may trigger signalling cascades that regulate or modify cell growth, survival or metabolism, or which promote apoptosis. E SGs play a role in the cellular response against viral infection, by sequestering the endogenous translational machinery necessary for viral protein expression and by activating proteins involved in antiviral response.

Fig. 4. The putative involvement of stress granules in the molecular pathophysiology of polyglutamine diseases.

Proteins bearing an expanded polyglutamine (PolyQ) tract display a tendency to aggregate and to engage in aberrant intermolecular interactions. Association of PolyQ-expanded proteins with stress granules (SGs) components, in particular RNA-binding proteins (RBPs) that are often prone to aggregate, may alter the dynamics of SGs assembly and disassembly. This can compromise SGs functionality, contributing to the globally deficient cell stress response that has been described to be a component of the molecular pathophysiology of PolyQ diseases. Oxidative stress (associated with an increase in reactive oxidative species - ROS - levels) and proteotoxic stress, which are known to result from expanded PolyQ protein expression, may culminate in a state of chronic cell stress, which may add to the abnormal SG assembly/disassembly dynamics and lead to the persistence of SGs. SGs formed under chronic stress are known to acquire abnormal properties and to seed toxic aggregation. Defects in autophagy caused by PolyQ protein expression may also contribute to the persistence of SGs and to their altered dynamics. Additionally, both the sequestration of RBPs to PolyQ aggregates and the toxic aggregation triggered by SGs may alter RNA metabolism and its subcellular localization, which in turn may lead to transcriptional aberrations. Chronic stress, a reduced ability to cope with cell stress, transcriptional alterations and a pernicious cascade of protein aggregation involving both the PolyQ-expanded proteins and the SGs may combine to produce the cytotoxic profile with is at the basis of cell dysfunction and loss, in PolyQ diseases.