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Journal ArticleDOI

Pathophysiological implications of RNP granules in frontotemporal dementia and ALS.

01 Nov 2020-Neurochemistry International (Pergamon)-Vol. 140, pp 104819
TL;DR: A better understanding of RNA-binding proteins in R NP granule formation and the regulation and maintenance of RNP granule biophysical properties and dynamics may provide insights into mechanisms contributing to disrupted proteostasis in neurodegenerative pathology; and thus open up new avenues for therapeutic discovery and development.
About: This article is published in Neurochemistry International.The article was published on 2020-11-01 and is currently open access. It has received 5 citations till now. The article focuses on the topics: Stress granule & Proteostasis.

Summary (3 min read)

1. Introduction

  • Neurodegenerative disease encompasses a diverse range of chronic, progressive, agerelated neurological diseases with wide-ranging clinical symptoms (Erkkinen et al., 2018).
  • Not only are they a major health burden due to their increasing prevalence in the population, but there are also clear associated social and economic impacts.
  • Neurodegenerative diseases are multi-faceted with a highly complex aetiology.
  • Therefore, improving the efficiency of biochemical processes within cells and thus ultimately conferring a cytoprotective role (Boeynaems et al., 2018).
  • Dysregulation of the maintenance of RNP granules may provide a possible mechanism contributing to disrupted proteostasis in neurodegenerative disease.

2.1 Stress Granules

  • Cells are constantly exposed to various stresses, such as thermal, osmotic, oxidative or endoplasmic reticulum (ER) stress.
  • It is vital to effectively regulate and activate cellular response pathways to ensure that cell viability and functionality is maintained.
  • This core-shell structure of stress granules is summarised in Figure 1a.
  • These RNA-binding proteins are not necessarily required for stress granule formation, and appear to selectively promote stress granule assembly, perhaps through different protein-protein interactions (Kedersha et al., 1999; Tourrière et al., 2003; Protter and Parker, 2016).
  • Therefore, FUS and TDP-43 recruitment into stress granules and subsequent nuclear loss of the RNA-binding proteins, in addition to persistence of these RNP granules, may contribute to the progressive intraneuronal protein aggregation, neuronal degeneration and loss characteristic of frontotemporal dementia and ALS.

2.2 Paraspeckles

  • Paraspeckles are RNP granules located in the interchromatin space of mammalian cell nuclei.
  • Paraspeckles consist of RNA and RNA-binding protein components.
  • Nuclear Enriched Abundant Transcript 1 (NEAT1) is a long non-coding RNA that is ubiquitous, highly expressed and conserved across mammalian species (An et al., 2018).
  • Specifically, the longer NEAT1_2 transcript is required for paraspeckle assembly as it acts as a scaffold to which paraspeckle proteins can then bind, as opposed to NEAT1_1 which, in the presence of NEAT1_2, only enhances paraspeckle assembly (Chen and Carmichael, 2009; Sasaki et al., 2009; Naganuma et al., 2012).

3. Properties of RNA-Binding Proteins

  • RNA-binding proteins are a key component of RNP granules.
  • There are common features shared amongst this group of proteins.
  • They are intrinsically aggregation-prone proteins, and in general, structurally contain a nuclear localisation signal, nuclear export signal and prion-like low complexity domain which are all crucial to effectively carrying out their functions within cells (Dreyfuss et al., 2002; Lee et al., 2006; Protter and Parker, 2016; Uversky et al., 2017; Banani et al., 2017; Gehring et al., 2017).
  • Recent advances have enabled insights to be gained into the organisation of RNAbinding proteins and assembly into RNP granules.
  • The following subsections describe some interesting properties of RNA-binding proteins.

3.1 Liquid-Liquid Phase Separation

  • Multidisciplinary approaches have demonstrated that RNA-binding proteins mediate the assembly of membraneless organelles, such as stress granules and paraspeckles, via the process of liquid-liquid phase separation (Brangwynne et al., 2009; 2011; Li et al., 2012; Wright and Dyson, 2015; Conicella et al., 2016; Protter and Parker, 2016).
  • In a biological context, RNP granules themselves are considered to be the dense phase, whereas the dilute phase is the surrounding cytoplasm or nucleoplasm (Banani et al., 2017; Shin and Brangwynne, 2017).
  • In order to understand the properties and dynamics of these granule ‘droplets’ in real time, research has widely employed the use of techniques such as fluorescence recovery after photobleaching (FRAP) which provides measures on the mobility of droplets and fluorescence recovery rates.
  • These structures have been found to undergo a liquid-to-solid phase transition into stable, hence not easily reversible, insoluble aggregates (Patel et al., 2015; Murakami et al., 2015; Lim et al., 2016; Gasset-Rosa et al., 2019).
  • Therefore, a key determinant driving phase separation is a change in the concentration of the aggregation-prone RNA-binding proteins.

3.2 Low Complexity Domains

  • The RNA-binding protein low complexity domains are a critical structural component for phase separation (Molliex at al., 2015; Harrison and Shorter, 2017).
  • These domains are intrinsically disordered, enriched in polar amino acids, such as asparagine, glutamine, tyrosine and serine, that often occur in short repetitive sequences (Michelitsch et al., 2000; Alberti et al., 2009).
  • The importance of the prion-like low complexity domains has been elucidated in mutational studies investigating key RNA-binding proteins and RNP granule properties.
  • Mackenzie et al. (2017) investigated the effect of frontotemporal dementia- and ALSassociated mutations in the low complexity domain of TIA 1 on stress granule properties and dynamics.
  • More recently, ALS-associated mutations in the TDP-43 low complexity domain were also found to result in disrupted phase separation of TDP-43 droplets by altering interactions via specific helix-helix contacts in the protein low complexity domain (Conicella et al., 2016).

3.3 Post-Translational Modifications

  • RNA-binding proteins are rich in amino acid residues that are common targets for posttranslational modifications, and such molecular changes have been found to affect the propensity of these proteins to phase separate (Tourrière et al., 2003; Ohn et al., 2008; Rhoads et al., 2018).
  • These changes alter the intermolecular interactions between RNA-binding proteins and therefore regulate their ability to undergo phase separation, but the direction of which can be dependent on the protein being modified as well as the modification itself (Kwon et al., 2013; Lin et al., 2017; Gomes and Shorter, 2019).
  • Together, this suggests that phosphorylation of FUS disrupts its phase separation, thereby reducing its aggregation and subsequent toxic effects.
  • These findings are aligned with the occurrence of hyperphosphorylated tau present in protein aggregates in the brains of individuals with tauopathies such as frontotemporal dementia (Wang et al., 2016).
  • The effect of arginine methylation of G3BP1 on stress granule formation was investigated by Tsai et al. (2016).

3.4 Neurodegenerative Disease-Associated Mutations

  • Mutational studies have enabled great progress to be made towards understanding phase separation of RNA-binding proteins and RNP granule dynamics.
  • This may be through affecting either the localisation of the proteins and/or their phase separation behaviour.
  • Under normal conditions, FUS is predominantly a nuclear protein, however causative mutations in neurodegenerative disease are known to affect its localisation as well as its propensity to aggregate (Svetoni et al., 2016).
  • Moreover, there was an enhanced formation of paraspeckles through NEAT1 accumulation in cells.
  • Shelkovnikova et al. (2018) identified enhanced paraspeckle formation in ALS and investigated the role of TDP-43 dysfunction, as a known paraspeckle protein, in driving this effect.

4. Conclusion and Therapeutic Implications

  • RNP granules such as stress granules and paraspeckles are essential for biochemical processes to occur efficiently within cells and importantly to maintain function and cell viability under conditions of stress.
  • Many studies investigating the effect of post-translational modifications in RNAbinding proteins have largely studied single modifications in isolation, whereas a more physiologically relevant situation would involve an interaction of many factors which could perhaps include multiple post-translational modifications.
  • There are still shared pathological features at the molecular and cellular level between sporadic and familial forms of these neurodegenerative diseases.
  • Based on their current understanding, promising therapeutic strategies may involve modulating the biophysical properties of RNP granules, stimulating disassembly of RNP granules or upregulating degradation pathways (Shorter, 2016; 2017; Schoch and Miller, 2017).
  • Ultimately, this will be crucial in order to identify novel targets and discover new avenues for developing effective disease-modifying therapeutics and fulfil the pressing need to combat these devastating diseases.

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Citations
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Journal ArticleDOI
TL;DR: In this paper, the authors summarize the current knowledge regarding the function of disease-associated RNA-binding proteins and their role in the dysfunction of membrane-less organelles, which can help elucidate the complex pathophysiology of ALS.
Abstract: Genetic analyses of patients with amyotrophic lateral sclerosis (ALS) have identified disease-causing mutations and accelerated the unveiling of complex molecular pathogenic mechanisms, which may be important for understanding the disease and developing therapeutic strategies. Many disease-related genes encode RNA-binding proteins, and most of the disease-causing RNA or proteins encoded by these genes form aggregates and disrupt cellular function related to RNA metabolism. Disease-related RNA or proteins interact or sequester other RNA-binding proteins. Eventually, many disease-causing mutations lead to the dysregulation of nucleocytoplasmic shuttling, the dysfunction of stress granules, and the altered dynamic function of the nucleolus as well as other membrane-less organelles. As RNA-binding proteins are usually components of several RNA-binding protein complexes that have other roles, the dysregulation of RNA-binding proteins tends to cause diverse forms of cellular dysfunction. Therefore, understanding the role of RNA-binding proteins will help elucidate the complex pathophysiology of ALS. Here, we summarize the current knowledge regarding the function of disease-associated RNA-binding proteins and their role in the dysfunction of membrane-less organelles.

10 citations

Journal ArticleDOI
TL;DR: In this paper , the authors summarized the regulation of stress granules and their relationship with Tau protein and autophagy, as well as the pathological mechanisms of AD such as RNA splicing, microglial cell proliferation and phagocytosis.

5 citations

Journal ArticleDOI
TL;DR: In this article , the authors explore the benefits of gene therapy as an approach to treating ALS, specifically focusing on the use of adeno-associated virus (AAV) as a vector for gene delivery to the CNS.
Abstract: Despite the devastating clinical outcome of the neurodegenerative disease, amyotrophic lateral sclerosis (ALS), its etiology remains mysterious. Approximately 90% of ALS is characterized as sporadic, signifying that the patient has no family history of the disease. The development of an impactful disease modifying therapy across the ALS spectrum has remained out of grasp, largely due to the poorly understood mechanisms of disease onset and progression. Currently, ALS is invariably fatal and rapidly progressive. It is hypothesized that multiple factors can lead to the development of ALS, however, treatments are often focused on targeting specific familial forms of the disease (10% of total cases). There is a strong need to develop disease modifying treatments for ALS that can be effective across the full ALS spectrum of familial and sporadic cases. Although the onset of disease varies significantly between patients, there are general disease mechanisms and progressions that can be seen broadly across ALS patients. Therefore, this review explores the targeting of these widespread disease mechanisms as possible areas for therapeutic intervention to treat ALS broadly. In particular, this review will focus on targeting mechanisms of defective protein homeostasis and RNA processing, which are both increasingly recognized as design principles of ALS pathogenesis. Additionally, this review will explore the benefits of gene therapy as an approach to treating ALS, specifically focusing on the use of adeno‐associated virus (AAV) as a vector for gene delivery to the CNS and recent advances in the field.

1 citations

Journal ArticleDOI
01 Apr 2023-Gene
TL;DR: In this article , a new hypothesis that neuronal senescence may contribute to the mechanism of stress granule seeding in ND by altering SG dynamics in aged cells, thereby providing additional aggregation opportunities within aged neurons.

1 citations

Journal ArticleDOI
TL;DR: In this paper , the age-related changes of the transcriptome and translatome in the female mouse hippocampus were profiled by RNA sequencing of total RNA and polysome preparations at four ages (3, 6, 12, 20-month old).
Abstract: Aging is associated with substantial physiological changes and constitutes a major risk factor for neurological disorders including dementia. Alterations in gene expression upon aging have been extensively studied; however, an in-depth characterization of post-transcriptional regulatory events remains elusive. Here, we profiled the age-related changes of the transcriptome and translatome in the female mouse hippocampus by RNA sequencing of total RNA and polysome preparations at four ages (3-, 6-, 12-, 20-month-old); and we implemented a variety of bioinformatics approaches to unravel alterations in transcript abundance, alternative splicing, and polyadenylation site selection. We observed mostly well-coordinated transcriptome and translatome expression signatures across age including upregulation of transcripts related to immune system processes and neuroinflammation, though transcripts encoding ribonucleoproteins or associated with mitochondrial functions, calcium signaling and the cell-cycle displayed substantial discordant profiles, suggesting translational control associated with age-related deficits in hippocampal-dependent behavior. By contrast, alternative splicing was less preserved, increased with age and was associated with distinct functionally-related transcripts encoding proteins acting at synapses/dendrites, RNA-binding proteins; thereby predicting regulatory roles for RBM3 and CIRBP. Only minor changes in polyadenylation site selection were identified, indicating pivotal 3′-end selection in young adults compared to older groups. Overall, our study provides a comprehensive resource of age-associated post-transcriptional regulatory events in the mouse hippocampus, enabling further examination of the molecular features underlying age-associated neurological diseases.
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2,134 citations

Frequently Asked Questions (2)
Q1. What contributions have the authors mentioned in the paper "Pathophysiological implications of rnp granules in frontotemporal dementia and als" ?

In this paper, Bandopadhyay et al. focused on two RNP granules, namely, stress granules and paraspeckles, and explored their potential pathological role in neurodegeneration using frontotemporal dementia and ALS as disease examples. 

Overall, although there is a growing body of evidence suggesting a link between biophysical properties and dynamics of RNP granules, protein aggregation, and neurodegenerative disease pathology, future work will need to uncover the underlying molecular mechanisms regulating this process. Ultimately, this will be crucial in order to identify novel targets and discover new avenues for developing effective disease-modifying therapeutics and fulfil the pressing need to combat these devastating diseases.