Relation Between Stress Granules and Cytoplasmic Protein Aggregates Linked to Neurodegenerative Diseases

Aggregates associated with amyotrophic lateral sclerosis sequester the actin-binding protein profilin 2

Amyotrophic Lateral Sclerosis (ALS) is a devastating neurodegenerative disease characterized by the degeneration of upper and lower motoneurons. The four most frequently mutated genes causing familial ALS (fALS) are C9orf72, FUS, SOD1, and TARDBP. Some of the related wild-type proteins comprise intrinsically disordered regions (IDRs) which favor their assembly in liquid droplets-the biophysical mechanism behind the formation of physiological granules such as stress granules (SGs). SGs assemble and dissolve dependent on the cellular condition. However, it has been suggested that transition from reversible SGs to irreversible aggregates contributes to the toxic properties of ALS-related mutated proteins. Sequestration of additional proteins within these aggregates may then result in downstream toxicity. While the exact downstream mechanisms remain elusive, rare ALS-causing mutations in the actin binding protein profilin 1 suggest an involvement of the actin cytoskeleton. Here, we hypothesize that profilin isoforms become sequestered in aggregates of ALS-associated proteins which induce subsequent dysregulation of the actin cytoskeleton. Interestingly, localization of neuronal profilin 2 in SGs was more pronounced compared with the ubiquitously expressed profilin 1. Accordingly, FUS and C9orf72 aggregates prominently sequestered profilin 2 but not profilin 1. Moreover, we observed a distinct sequestration of profilin 2 and G-actin to C9orf72 aggregates in different cellular models. On the functional level, we identified dysregulated actin dynamics in cells with profilin 2-sequestering aggregates. In summary, our results suggest a more common involvement of profilins in ALS pathomechanisms than indicated from the rarely occurring profilin mutations.

Dysbiosis significantly elevates the probability of altered affective function in Alzheimer disease (AD)

Changes in the makeup of gut microbiota are linked to many neuropsychiatric diseases. Although the exact connection between gut dysbiosis and brain dysfunction is not yet fully understood, but recent data suggests that gut dysbiosis may contribute to the development of Alzheimer's disease (AD) by promoting neuroinflammation, insulin resistance, oxidative stress, and amyloid-beta (Aβ) aggregation. Gut dysbiosis in animal models is primarily characterized by an elevated ratio of Firmicutes/Bacteroidetes which may lead to the accumulation of amyloid precursor protein (APP) in the intestine, in the early stages of AD. Probiotics play a significant role in preventing against the symptoms of AD by restoring gut-brain homeostasis. This chapter provides an overview of the gut microbiota and its dysregulation in etiology of AD. Moreover, novel insights into alteration of the composition of gut microbiota as a preventive or therapeutic approach to AD are discussed.

TIA1-Mediated Stress Granules Promote the Neuroinflammation and Demyelination in Experimental Autoimmune Encephalomyelitis through Upregulating IL-31RA Signaling

The dysfunction of stress granules (SGs) plays a crucial role in the pathogenesis of various neurological disorders, with T cell intracellular antigen 1 (TIA1) being a key component of SGs. However, the role and mechanism of TIA1-mediated SGs in experimental autoimmune encephalomyelitis (EAE) remain unclear. In this study, upregulation of TIA1, its translocation from the nucleus to the cytoplasm, and co-localization with G3BP1 (a marker of SGs) are observed in the spinal cord neurons of EAE mice. Deletion of TIA1 in the CNS alleviates neuroinflammation, suppresses demyelination and axonal damage, and reduces neuronal loss in EAE mice. Furthermore, alleviation of autophagy dysfunction and reduction of chronic persistent SGs are observed in Tia1Nestin-CKO EAE mice. Mechanistically, IL-31RA levels are decreased in Tia1Nestin-CKO EAE mice, which inhibit the downstream PI3K/AKT signaling pathway associated with IL-31RA, thereby enhancing autophagy and suppressing the NF-κB signaling pathway, further alleviating EAE symptoms. Knockdown of TIA1 in primary neurons and N2a cells treated with sodium arsenite also reduces the formation of SGs. These findings reveal an unrecognized role of TIA1-mediated SGs in promoting neuroinflammation and demyelination, offering novel therapeutic targets for MS.

Amyloids, amorphous aggregates and assemblies of peptides - Assessing aggregation

Amyloid and amorphous aggregates represent the two major categories of aggregates associated with diseases, and although exhibiting distinct features, researchers often treat them as equivalent, which demonstrates the need for more thorough characterization. Here, we compare amyloid and amorphous aggregates based on their biochemical properties, kinetics, and morphological features. To further decipher this issue, we propose the use of peptide self-assemblies as minimalistic models for understanding the aggregation process. Peptide building blocks are significantly smaller than proteins that participate in aggregation, however, they make a plausible means to bridge the gap in discerning the aggregation process at the more complex, protein level. Additionally, we explore the potential use of peptide-inspired models to research the liquid-liquid phase separation as a feasible mechanism preceding amyloid formation. Connecting these concepts can help clarify our understanding of aggregation-related disorders and potentially provide novel drug targets to impede and reverse these serious illnesses.

α-Synuclein emulsifies TDP-43 prion-like domain-RNA liquid droplets to promote heterotypic amyloid fibrils

Many neurodegenerative diseases including frontotemporal lobar degeneration (FTLD), Lewy body disease (LBD), multiple system atrophy (MSA), etc., show colocalized deposits of TDP-43 and α-synuclein (αS) aggregates. To understand whether these colocalizations are driven by specific molecular interactions between the two proteins, we previously showed that the prion-like C-terminal domain of TDP-43 (TDP-43PrLD) and αS synergistically interact to form neurotoxic heterotypic amyloids in homogeneous buffer conditions. However, it remains unclear if αS can modulate TDP-43 present within liquid droplets and biomolecular condensates called stress granules (SGs). Here, using cell culture and in vitro TDP-43PrLD - RNA liquid droplets as models along with microscopy, nanoscale AFM-IR spectroscopy, and biophysical analyses, we uncover the interactions of αS with phase-separated droplets. We learn that αS acts as a Pickering agent by forming clusters on the surface of TDP-43PrLD - RNA droplets. The aggregates of αS on these clusters emulsify the droplets by nucleating the formation of heterotypic TDP-43PrLD amyloid fibrils, structures of which are distinct from those derived from homogenous solutions. Together, these results reveal an intriguing property of αS to act as a Pickering agent while interacting with SGs and unmask the hitherto unknown role of αS in modulating TDP-43 proteinopathies.

Post-translational modifications in stress granule and their implications in neurodegenerative diseases

Stress granules (SGs) arise as formations of mRNAs and proteins in response to translation initiation inhibition during stress. These dynamic compartments adopt a fluidic nature through liquid-liquid phase separation (LLPS), exhibiting a composition subject to constant change within cellular contexts. Research has unveiled an array of post-translational modifications (PTMs) occurring on SG proteins, intricately orchestrating SG dynamics. In the realm of neurodegenerative diseases, pathological mutant proteins congregate into insoluble aggregates alongside numerous SG proteins, manifesting resilience against disassembly. Specific PTMs conspicuously label these aggregates, designating them for subsequent degradation. The strategic manipulation of aberrant SGs via PTMs emerges as a promising avenue for therapeutic intervention. This review discerns recent strides in comprehending the impact of PTMs on LLPS behavior and the assembly/disassembly kinetics of SGs. By delving into the roles of PTMs in governing SG dynamics, we augment our cognizance of the molecular underpinnings of neurodegeneration. Furthermore, we offer invaluable insights into potential targets for therapeutic intervention in neurodegenerative afflictions, encompassing conditions like amyotrophic lateral sclerosis and frontotemporal dementia.

Stress granule homeostasis is modulated by TRIM21-mediated ubiquitination of G3BP1 and autophagy-dependent elimination of stress granules

Eukaryotic stress granules (SGs) are highly dynamic assemblies of untranslated mRNAs and proteins that form through liquid-liquid phase separation (LLPS) under cellular stress. SG formation and elimination process is a conserved cellular strategy to promote cell survival, although the precise regulation of this process is poorly understood. Here, we screened six E3 ubiquitin ligases present in SGs and identified TRIM21 (tripartite motif containing 21) as a central regulator of SG homeostasis that is highly enriched in SGs of cells under arsenite-induced oxidative stress. Knockdown of TRIM21 promotes SG formation whereas overexpression of TRIM21 inhibits the formation of physiological and pathological SGs associated with neurodegenerative diseases. TRIM21 catalyzes K63-linked ubiquitination of the SG core protein, G3BP1 (G3BP stress granule assembly factor 1), and G3BP1 ubiquitination can effectively inhibit LLPS, in vitro. Recent reports suggested the involvement of macroautophagy/autophagy, as a stress response pathway, in the regulation of SG homeostasis. We systematically investigated well-defined autophagy receptors and identified SQSTM1/p62 (sequestosome 1) and CALCOCO2/NDP52 (calcium binding and coiled-coil domain 2) as the primary receptors that directly interact with G3BP1 during arsenite-induced stress. Endogenous SQSTM1 and CALCOCO2 localize to the periphery of SGs under oxidative stress and mediate SG elimination, as single knockout of each receptor causes accumulation of physiological and pathological SGs. Collectively, our study broadens the understanding in the regulation of SG homeostasis by showing that TRIM21 and autophagy receptors modulate SG formation and elimination respectively, suggesting the possibility of clinical targeting of these molecules in therapeutic strategies for neurodegenerative diseases.Abbreviations: ACTB: actin beta; ALS: amyotrophic lateral sclerosis; BafA1: bafilomycin A1; BECN1: beclin 1; C9orf72: C9orf72-SMCR8 complex subunit; CALCOCO2/NDP52: calcium binding and coiled-coil domain 2; Co-IP: co-immunoprecipitation; DAPI: 4',6-diamidino-2-phenylindole; FTD: frontotemporal dementia; FUS: FUS RNA binding protein; G3BP1: G3BP stress granule assembly factor 1; GFP: green fluorescent protein; LLPS: liquid-liquid phase separation; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; NBR1: NBR1 autophagy cargo receptor; NES: nuclear export signal; OPTN: optineurin; RFP: red fluorescent protein; SQSTM1/p62: sequestosome 1; SG: stress granule; TAX1BP1: Tax1 binding protein 1; TOLLIP: toll interacting protein; TRIM21: tripartite motif containing 21; TRIM56: tripartite motif containing 56; UB: ubiquitin; ULK1: unc-51 like autophagy activating kinase 1; WT: wild-type.

Stress granule activation attenuates lipopolysaccharide-induced cardiomyocyte dysfunction

BACKGROUND

Sepsis is the leading cause of death in intensive care units. Sepsis-induced myocardial dysfunction, one of the most serious complications of sepsis, is associated with higher mortality rates. As the pathogenesis of sepsis-induced cardiomyopathy has not been fully elucidated, there is no specific therapeutic approach. Stress granules (SG) are cytoplasmic membrane-less compartments that form in response to cellular stress and play important roles in various cell signaling pathways. The role of SG in sepsis-induced myocardial dysfunction has not been determined. Therefore, this study aimed to determine the effects of SG activation in septic cardiomyocytes (CMs).

METHODS

Neonatal CMs were treated with lipopolysaccharide (LPS). SG activation was visualized by immunofluorescence staining to detect the co-localization of GTPase-activating protein SH3 domain binding protein 1 (G3BP1) and T cell-restricted intracellular antigen 1 (TIA-1). Eukaryotic translation initiation factor alpha (eIF2α) phosphorylation, an indicator of SG formation, was assessed by western blotting. Tumor necrosis factor alpha (TNF-α) production was assessed by PCR and enzyme-linked immunosorbent assays. CMs function was evaluated by intracellular cyclic adenosine monophosphate (cAMP) levels in response to dobutamine. Pharmacological inhibition (ISRIB), a G3BP1 CRISPR activation plasmid, and a G3BP1 KO plasmid were employed to modulate SG activation. The fluorescence intensity of JC-1 was used to evaluate mitochondrial membrane potential.

RESULTS

LPS challenge in CMs induced SG activation and resulted in eIF2α phosphorylation, increased TNF-α production, and decreased intracellular cAMP in response to dobutamine. The pharmacological inhibition of SG (ISRIB) increased TNF-α expression and decreased intracellular cAMP levels in CMs treated with LPS. The overexpression of G3BP1 increased SG activation, attenuated the LPS-induced increase in TNF-α expression, and improved CMs contractility (as evidenced by increased intracellular cAMP). Furthermore, SG prevented LPS-induced mitochondrial membrane potential dissipation in CMs.

CONCLUSION

SG formation plays a protective role in CMs function in sepsis and is a candidate therapeutic target.

Stress granules: functions and mechanisms in cancer

Stress granules (SGs) are non-enveloped structures formed primarily via protein and RNA aggregation under various stress conditions, including hypoxia and viral infection, as well as oxidative, osmotic, and heat-shock stress. SGs assembly is a highly conserved cellular strategy to reduce stress-related damage and promote cell survival. At present, the composition and dynamics of SGs are well understood; however, data on the functions and related mechanisms of SGs are limited. In recent years, SGs have continued to attract attention as emerging players in cancer research. Intriguingly, SGs regulate the biological behavior of tumors by participating in various tumor-associated signaling pathways, including cell proliferation, apoptosis, invasion and metastasis, chemotherapy resistance, radiotherapy resistance, and immune escape. This review discusses the roles and mechanisms of SGs in tumors and suggests novel directions for cancer treatment.

PARP1 Activation Controls Stress Granule Assembly after Oxidative Stress and DNA Damage

DNA damage causes PARP1 activation in the nucleus to set up the machinery responsible for the DNA damage response. Here, we report that, in contrast to cytoplasmic PARPs, the synthesis of poly(ADP-ribose) by PARP1 opposes the formation of cytoplasmic mRNA-rich granules after arsenite exposure by reducing polysome dissociation. However, when mRNA-rich granules are pre-formed, whether in the cytoplasm or nucleus, PARP1 activation positively regulates their assembly, though without additional recruitment of poly(ADP-ribose) in stress granules. In addition, PARP1 promotes the formation of TDP-43- and FUS-rich granules in the cytoplasm, two RNA-binding proteins which form neuronal cytoplasmic inclusions observed in certain neurodegenerative diseases such as amyotrophic lateral sclerosis and frontotemporal lobar degeneration. Together, the results therefore reveal a dual role of PARP1 activation which, on the one hand, prevents the early stage of stress granule assembly and, on the other hand, enables the persistence of cytoplasmic mRNA-rich granules in cells which may be detrimental in aging neurons.

Neuronal RNA granules are ribosome complexes stalled at the pre-translocation state

The polarized cell morphology of neurons dictates many neuronal processes, including the axodendridic transport of specific mRNAs and subsequent translation. mRNAs together with ribosomes and RNA-binding proteins form RNA granules that are targeted to axodendrites for localized translation in neurons. It has been established that localized protein synthesis in neurons is essential for long-term memory formation, synaptic plasticity, and neurodegeneration. We have used proteomics and electron microscopy to characterize neuronal RNA granules (nRNAg) isolated from rat brain tissues or human neuroblastoma. We show that ribosome containing RNA granules are morula-like structures when visualized by electron microscopy. Crosslinking-coupled mass-spectrometry identified potential G3BP2 binding site on the ribosome near the eIF3d-binding site on the 40S ribosomal subunit. We used cryo-EM to resolve the structure of the ribosome-component of nRNAg. The cryo-EM reveals that predominant particles in nRNAg are 80S ribosomes, resembling the pre-translocation state where tRNA's are in the hybrid A/P and P/E site. We also describe a new kind of principal motion of the ribosome, which we call the rocking motion.

Stress granules in the spinal muscular atrophy and amyotrophic lateral sclerosis: The correlation and promising therapy

Increasing genetic and biochemical evidence has broadened our view of the pathomechanisms that lead to Spinal muscular atrophy (SMA) and Amyotrophic lateral sclerosis (ALS), two fatal neurodegenerative diseases with similar symptoms and causes. Stress granules are dynamic cytosolic storage hubs for mRNAs in response to stress exposures, that are evolutionarily conserved cytoplasmic RNA granules in somatic cells. A lot of previous studies have shown that the impaired stress granules are crucial events in SMA/ALS pathogenesis. In this review, we described the key stress granules related RNA binding proteins (SMN, TDP-43, and FUS) involved in SMA/ALS, summarized the reported mutations in these RNA binding proteins involved in SMA/ALS pathogenesis, and discussed the mechanisms through which stress granules dynamics participate in the diseases. Meanwhile, we described the applications and limitation of current therapies targeting SMA/ALS. We futher proposed the promising targets on stress granules in the future therapeutic interventions of SMA/ALS.

Neuromelanin granules of the substantia nigra: proteomic profile provides links to tyrosine hydroxylase, stress granules and lysosomes

Neuromelanin is a black-brownish pigment, present in so-called neuromelanin granules (NMGs) in the cell bodies of dopaminergic neurons in the substantia nigra (SN) pars compacta. These neurons are lost in neurodegenerative diseases, such as Parkinson’s disease and dementia with Lewy bodies. Although it is known that lipids, proteins, and environmental toxins accumulate in NMGs, the function of NMGs has not yet been finally clarified as well as their origin and the synthesis of neuromelanin. We, therefore, isolated NMGs and surrounding SN tissue from control patients by laser microdissection and analyzed the proteomic profile by tandem mass spectrometry. With our improved workflow, we were able to (1) strengthen the regularly reported link between NMGs and lysosomes, (2) detect tyrosine hydroxylase to be highly abundant in NMGs, which may be related to neuromelanin synthesis and (3) indicate a yet undescribed link between stress granules (SGs) and NMGs. Based on our findings, we cautiously hypothesize, that SGs may be the origin of NMGs or form in close proximity to them, potentially due to the oxidative stress caused by neuromelanin-bound metals.

The Role of Ubiquitin in Regulating Stress Granule Dynamics

Stress granules (SGs) are dynamic, reversible biomolecular condensates, which assemble in the cytoplasm of eukaryotic cells under various stress conditions. Formation of SGs typically occurs upon stress-induced translational arrest and polysome disassembly. The increase in cytoplasmic mRNAs triggers the formation of a protein-RNA network that undergoes liquid-liquid phase separation when a critical interaction threshold has been reached. This adaptive stress response allows a transient shutdown of several cellular processes until the stress is removed. During the recovery from stress, SGs disassemble to re-establish cellular activities. Persistent stress and disease-related mutations in SG components favor the formation of aberrant SGs that are impaired in disassembly and prone to aggregation. Recently, posttranslational modifications of SG components have been identified as major regulators of SG dynamics. Here, we summarize new insights into the role of ubiquitination in affecting SG dynamics and clearance and discuss implications for neurodegenerative diseases linked to aberrant SG formation.

Targeting DNA topoisomerases or checkpoint kinases results in an overload of chaperone systems, triggering aggregation of a metastable subproteome

A loss of the checkpoint kinase ataxia telangiectasia mutated (ATM) leads to impairments in the DNA damage response, and in humans causes cerebellar neurodegeneration, and an increased risk of cancer. A loss of ATM is also associated with increased protein aggregation. The relevance and characteristics of this aggregation are still incompletely understood. Moreover, it is unclear to what extent other genotoxic conditions can trigger protein aggregation as well. Here, we show that targeting ATM, but also ATR or DNA topoisomerases, results in the widespread aggregation of a metastable, disease-associated subfraction of the proteome. Aggregation-prone model substrates, including Huntingtin exon 1 containing an expanded polyglutamine repeat, aggregate faster under these conditions. This increased aggregation results from an overload of chaperone systems, which lowers the cell-intrinsic threshold for proteins to aggregate. In line with this, we find that inhibition of the HSP70 chaperone system further exacerbates the increased protein aggregation. Moreover, we identify the molecular chaperone HSPB5 as a cell-specific suppressor of it. Our findings reveal that various genotoxic conditions trigger widespread protein aggregation in a manner that is highly reminiscent of the aggregation occurring in situations of proteotoxic stress and in proteinopathies.

Melatonin: Regulation of Biomolecular Condensates in Neurodegenerative Disorders

Biomolecular condensates are membraneless organelles (MLOs) that form dynamic, chemically distinct subcellular compartments organizing macromolecules such as proteins, RNA, and DNA in unicellular prokaryotic bacteria and complex eukaryotic cells. Separated from surrounding environments, MLOs in the nucleoplasm, cytoplasm, and mitochondria assemble by liquid-liquid phase separation (LLPS) into transient, non-static, liquid-like droplets that regulate essential molecular functions. LLPS is primarily controlled by post-translational modifications (PTMs) that fine-tune the balance between attractive and repulsive charge states and/or binding motifs of proteins. Aberrant phase separation due to dysregulated membrane lipid rafts and/or PTMs, as well as the absence of adequate hydrotropic small molecules such as ATP, or the presence of specific RNA proteins can cause pathological protein aggregation in neurodegenerative disorders. Melatonin may exert a dominant influence over phase separation in biomolecular condensates by optimizing membrane and MLO interdependent reactions through stabilizing lipid raft domains, reducing line tension, and maintaining negative membrane curvature and fluidity. As a potent antioxidant, melatonin protects cardiolipin and other membrane lipids from peroxidation cascades, supporting protein trafficking, signaling, ion channel activities, and ATPase functionality during condensate coacervation or dissolution. Melatonin may even control condensate LLPS through PTM and balance mRNA- and RNA-binding protein composition by regulating N6-methyladenosine (m6A) modifications. There is currently a lack of pharmaceuticals targeting neurodegenerative disorders via the regulation of phase separation. The potential of melatonin in the modulation of biomolecular condensate in the attenuation of aberrant condensate aggregation in neurodegenerative disorders is discussed in this review.

The cooperative binding of TDP-43 to GU-rich RNA repeats antagonizes TDP-43 aggregation

TDP-43 is a nuclear RNA-binding protein that forms neuronal cytoplasmic inclusions in two major neurodegenerative diseases, ALS and FTLD. While the self-assembly of TDP-43 by its structured N-terminal and intrinsically disordered C-terminal domains has been widely studied, the mechanism by which mRNA preserves TDP-43 solubility in the nucleus has not been addressed. Here, we demonstrate that tandem RNA recognition motifs of TDP-43 bind to long GU-repeats in a cooperative manner through intermolecular interactions. Moreover, using mutants whose cooperativity is impaired, we found that the cooperative binding of TDP-43 to mRNA may be critical to maintain the solubility of TDP-43 in the nucleus and the miscibility of TDP-43 in cytoplasmic stress granules. We anticipate that the knowledge of a higher order assembly of TDP-43 on mRNA may clarify its role in intron processing and provide a means of interfering with the cytoplasmic aggregation of TDP-43.

Huntington's disease mice and human brain tissue exhibit increased G3BP1 granules and TDP43 mislocalization

Chronic cellular stress associated with neurodegenerative disease can result in the persistence of stress granule (SG) structures, membraneless organelles that form in response to cellular stress. In Huntington's disease (HD), chronic expression of mutant huntingtin generates various forms of cellular stress, including activation of the unfolded protein response and oxidative stress. However, it has yet to be determined whether SGs are a feature of HD neuropathology. We examined the miRNA composition of extracellular vesicles (EVs) present in the cerebrospinal fluid (CSF) of patients with HD and show that a subset of their target mRNAs were differentially expressed in the prefrontal cortex. Of these targets, SG components were enriched, including the SG-nucleating Ras GTPase-activating protein-binding protein 1 (G3BP1). We investigated localization and levels of G3BP1 and found a significant increase in the density of G3BP1-positive granules in the cortex and hippocampus of R6/2 transgenic mice and in the superior frontal cortex of the brains of patients with HD. Intriguingly, we also observed that the SG-associated TAR DNA-binding protein 43 (TDP43), a nuclear RNA/DNA binding protein, was mislocalized to the cytoplasm of G3BP1 granule-positive HD cortical neurons. These findings suggest that G3BP1 SG dynamics may play a role in the pathophysiology of HD.

Dr. Jekyll and Mr. Hyde? Physiology and Pathology of Neuronal Stress Granules

Stress granules (SGs) are membraneless cytosolic granules containing dense aggregations of RNA-binding proteins and RNAs. They appear in the cytosol under stress conditions and inhibit the initiation of mRNA translation. SGs are dynamically assembled under stressful conditions and rapidly disassembled after stress removal. They are heterogeneous in their RNA and protein content and are cell type- and stress-specific. In post-mitotic neurons, which do not divide, the dynamics of neuronal SGs are tightly regulated, implying that their dysregulation leads to neurodegeneration. Mutations in RNA-binding proteins are associated with SGs. SG components accumulate in cytosolic inclusions in many neurodegenerative diseases, such as frontotemporal dementia and amyotrophic lateral sclerosis. Although SGs primarily mediate a pro-survival adaptive response to cellular stress, abnormal persistent SGs might develop into aggregates and link to the pathogenesis of diseases. In this review, we present recent advances in the study of neuronal SGs in physiology and pathology, and discuss potential therapeutic approaches to remove abnormal, persistent SGs associated with neurodegeneration.

Recent Developments in the Field of Intrinsically Disordered Proteins: Intrinsic Disorder-Based Emergence in Cellular Biology in Light of the Physiological and Pathological Liquid-Liquid Phase Transitions

This review deals with two important concepts-protein intrinsic disorder and proteinaceous membrane-less organelles (PMLOs). The past 20 years have seen an upsurge of scientific interest in these phenomena. However, neither are new discoveries made in this century, but instead are timely reincarnations of old ideas that were mostly ignored by the scientific community for a long time. Merging these concepts in the form of the intrinsic disorder-based biological liquid-liquid phase separation provides a basis for understanding the molecular mechanisms of PMLO biogenesis.

'Trojan-Horse' stress-granule formation mediated by manganese oxide nanoparticles

Exposure to nanomaterials is considered as one of the risk factors for neurodegenerative pathology. In vitro inorganic nanoparticles (NPs) absorb intrinsically disordered proteins, many of which are the constituents of stress-granules (SGs). SGs normally form in response to cellular stress and, here, we addressed whether selected inorganic NPs could trigger SGs formation in cells. To this end, we have tested a series of inorganic NPs for their ability to induce SGs formation in human glioblastoma and fibroblast cell lines. Among tested NPs, only Mn₃O₄ NPs triggered SGs formation in cell-type-specific and metabolic-dependent manner. In human glioblastoma U87 MG cell line, Mn₃O₄ NPs entered cells within minutes and resided inside intracellular vesicles for at least 48 h. Mn₃O₄ NPs induced a strong reduction in oxidative phosphorylation rate, but not glycolysis. We showed that Mn₃O₄ NPs slowly dissolve producing a local net of Mn²⁺ cations, which are known to inhibit oxidative phosphorylation. Indeed, direct incubation of cells with equimolar amounts of Mn²⁺ cations triggered SGs formation and reduced cellular respiration rate. However, while SGs formed in response to Mn₃O₄ NPs persisted for hours, SGs formation by Mn²⁺ peaked and dropped within minutes. Finally, Mn₃O₄ NPs mediated SGs formation via the phosphorylation of eIF2α. Thus, we conclude that exposure of U87 MG cells to Mn₃O₄ NPs caused a 'Trojan-horse' prolonged SGs response.

Targeting stress granules: A novel therapeutic strategy for human diseases

Stress granules (SGs) are assemblies of mRNA and proteins that form from mRNAs stalled in translation initiation in response to stress. Chronic stress might even induce formation of cytotoxic pathological SGs. SGs participate in various biological functions including response to apoptosis, inflammation, immune modulation, and signalling pathways; moreover, SGs are involved in pathogenesis of neurodegenerative diseases, viral infection, aging, cancers and many other diseases. Emerging evidence has shown that small molecules can affect SG dynamics, including assembly, disassembly, maintenance and clearance. Thus, targeting SGs is a potential therapeutic strategy for the treatment of human diseases and the promotion of health. The established methods for detecting SGs provided ready tools for large-scale screening of agents that alter the dynamics of SGs. Here, we describe the effects of small molecules on SG assembly, disassembly, and their roles in the disease. Moreover, we provide perspective for the possible application of small molecules targeting SGs in the treatment of human diseases.

Gut Microbiota and Dysbiosis in Alzheimer's Disease: Implications for Pathogenesis and Treatment

Understanding how gut flora influences gut-brain communications has been the subject of significant research over the past decade. The broadening of the term “microbiota-gut-brain axis” from “gut-brain axis” underscores a bidirectional communication system between the gut and the brain. The microbiota-gut-brain axis involves metabolic, endocrine, neural, and immune pathways which are crucial for the maintenance of brain homeostasis. Alterations in the composition of gut microbiota are associated with multiple neuropsychiatric disorders. Although a causal relationship between gut dysbiosis and neural dysfunction remains elusive, emerging evidence indicates that gut dysbiosis may promote amyloid-beta aggregation, neuroinflammation, oxidative stress, and insulin resistance in the pathogenesis of Alzheimer’s disease (AD). Illustration of the mechanisms underlying the regulation by gut microbiota may pave the way for developing novel therapeutic strategies for AD. In this narrative review, we provide an overview of gut microbiota and their dysregulation in the pathogenesis of AD. Novel insights into the modification of gut microbiota composition as a preventive or therapeutic approach for AD are highlighted.

Dietary Gluten and Neurodegeneration: A Case for Preclinical Studies

Although celiac disease (CD) is an autoimmune disease that primarily involves the intestinal tract, mounting evidence suggests that a sizeable number of patients exhibit neurological deficits. About 40% of the celiac patients with neurological manifestations have circulating antibodies against neural tissue transglutaminase-6 (tTG6). While early diagnosis and strict adherence to a gluten-free diet (GFD) have been recommended to prevent neurological dysfunction, better therapeutic strategies are needed to improve the overall quality of life. Dysregulation of the microbiota-gut-brain axis, presence of anti-tTG6 antibodies, and epigenetic mechanisms have been implicated in the pathogenesis. It is also possible that circulating or gut-derived extracellular structures and including biomolecular condensates and extracellular vesicles contribute to disease pathogenesis. There are several avenues for shaping the dysregulated gut homeostasis in individuals with CD, non-celiac gluten sensitivity (NCGS) and/or neurodegeneration. In addition to GFD and probiotics, nutraceuticals, such as phyto and synthetic cannabinoids, represent a new approach that could shape the host microbiome towards better prognostic outcomes. Finally, we provide a data-driven rationale for potential future pre-clinical research involving non-human primates (NHPs) to investigate the effect of nutraceuticals, such as phyto and synthetic cannabinoids, either alone or in combination with GFD to prevent/mitigate dietary gluten-induced neurodegeneration.

The Impact of ALS-Associated Genes hnRNPA1, MATR3, VCP and UBQLN2 on the Severity of TDP-43 Aggregation

Amyotrophic lateral sclerosis is a progressive neurodegenerative disorder, characterized by cytoplasmic inclusions of RNA-binding protein TDP-43. Despite decades of research and identification of more than 50 genes associated with amyotrophic lateral sclerosis (ALS), the cause of TDP-43 translocation from the nucleus and its aggregation in the cytoplasm still remains unknown. Our study addressed the impact of selected ALS-associated genes on TDP-43 aggregation behavior in wild-type and aggregation prone TDP-43 in vitro cell models. These were developed by deleting TDP-43 nuclear localization signal and stepwise shortening its low-complexity region. The SH-SY5Y cells were co-transfected with the constructs of aggregation-prone TDP-43 and wild-type or mutant ALS-associated genes hnRNPA1, MATR3, VCP or UBQLN2. The investigated genes displayed a unique impact on TDP-43 aggregation, generating distinct types of cytoplasmic inclusions, similar to those already described as resembling prion strains, which could represent the basis for neurodegenerative disease heterogeneity.

Translational Repression of G3BP in Cancer and Germ Cells Suppresses Stress Granules and Enhances Stress Tolerance

Stress granules (SGs) are membrane-less ribonucleoprotein condensates that form in response to various stress stimuli via phase separation. SGs act as a protective mechanism to cope with acute stress, but persistent SGs have cytotoxic effects that are associated with several age-related diseases. Here, we demonstrate that the testis-specific protein, MAGE-B2, increases cellular stress tolerance by suppressing SG formation through translational inhibition of the key SG nucleator G3BP. MAGE-B2 reduces G3BP protein levels below the critical concentration for phase separation and suppresses SG initiation. Knockout of the MAGE-B2 mouse ortholog or overexpression of G3BP1 confers hypersensitivity of the male germline to heat stress in vivo. Thus, MAGE-B2 provides cytoprotection to maintain mammalian spermatogenesis, a highly thermosensitive process that must be preserved throughout reproductive life. These results demonstrate a mechanism that allows for tissue-specific resistance against stress and could aid in the development of male fertility therapies.

Hitchhiking on vesicles: a way to harness age-related proteopathies?

Central to proteopathies and leading to most age-related neurodegenerative disorders is a failure in protein quality control (PQC). To harness the toxicity of misfolded and damaged disease proteins, such proteins are either refolded, degraded by temporal PQC, or sequestered by spatial PQC into specific, organelle-associated, compartments within the cell. Here, we discuss the impact of vesicle trafficking pathways in general, and syntaxin 5 in particular, as key players in spatial PQC directing misfolded proteins to the surface of vacuole and mitochondria, which facilitates their clearance and detoxification. Since boosting vesicle trafficking genetically can positively impact on spatial PQC and make cells less sensitive to misfolded disease proteins, we speculate that regulators of such trafficking might serve as therapeutic targets for age-related neurological disorders.

Liquid-Liquid Phase Separation and Its Mechanistic Role in Pathological Protein Aggregation

Liquid-liquid phase separation (LLPS) of proteins underlies the formation of membrane-less organelles. While it has been recognized for some time that these organelles are of key importance for normal cellular functions, a growing number of recent observations indicate that LLPS may also play a role in disease. In particular, numerous proteins that form toxic aggregates in neurodegenerative diseases, such as amyotrophic lateral sclerosis, frontotemporal lobar degeneration, and Alzheimer's disease, were found to be highly prone to phase separation, suggesting that there might be a strong link between LLPS and the pathogenic process in these disorders. This review aims to assess the molecular basis of this link through exploration of the intermolecular interactions that underlie LLPS and aggregation and the underlying mechanisms facilitating maturation of liquid droplets into more stable assemblies, including so-called labile fibrils, hydrogels, and pathological amyloids. Recent insights into the structural basis of labile fibrils and potential mechanisms by which these relatively unstable structures could transition into more stable pathogenic amyloids are also discussed. Finally, this review explores how the environment of liquid droplets could modulate protein aggregation by altering kinetics of protein self-association, affecting folding of protein monomers, or changing aggregation pathways.

Biomolecular condensates in neurodegeneration and cancer

The intracellular environment is partitioned into functionally distinct compartments containing specific sets of molecules and reactions. Biomolecular condensates, also referred to as membrane-less organelles, are diverse and abundant cellular compartments that lack membranous enclosures. Molecules assemble into condensates by phase separation; multivalent weak interactions drive molecules to separate from their surroundings and concentrate in discrete locations. Biomolecular condensates exist in all eukaryotes and in some prokaryotes, and participate in various essential house-keeping, stress-response and cell type-specific processes. An increasing number of recent studies link abnormal condensate formation, composition and material properties to a number of disease states. In this review, we discuss current knowledge and models describing the regulation of condensates and how they become dysregulated in neurodegeneration and cancer. Further research on the regulation of biomolecular phase separation will help us to better understand their role in cell physiology and disease.

Tau oligomers mediate aggregation of RNA-binding proteins Musashi1 and Musashi2 inducing Lamin alteration

The exact mechanisms leading to neurodegeneration in Alzheimer's disease (AD) and other tauopathies are not yet entirely understood. However, it is known that several RNA-binding proteins (RBPs) form toxic aggregates and also interact with tau in such granules in tauopathies, including AD. The Musashi (MSI) family of RBPs, consisting of two homologues: Musashi1 and Musashi2, have not been extensively investigated in neurodegenerative diseases. Here, using a tau inducible HEK (iHEK) model we investigate whether MSI proteins contribute to the aggregation of toxic tau oligomers (TauO). Wild-type and mutant P301L tau iHEK cells are used to study the effect of different tau variants on the cellular localization of MSI proteins. Interestingly, we observe that tau co-localizes with MSI in the cytoplasm and nuclei, altering the nuclear transport of MSI. Furthermore, incremental changes in the size and density of nuclear MSI/tau foci are observed. We also report here that TauO interact with MSI to cause the formation of distinct nuclear aggregates. Moreover, tau/MSI aggregates induce structural changes to LaminB1, leading to nuclear instability. These results illustrate a possible mechanism of neurodegeneration mediated by the aggregation of MSI proteins and TauO, suggesting that MSI plays a critical role in cellular dysfunction.

Strategies for Success. Viral Infections and Membraneless Organelles

Regulation of RNA homeostasis or “RNAstasis” is a central step in eukaryotic gene expression. From transcription to decay, cellular messenger RNAs (mRNAs) associate with specific proteins in order to regulate their entire cycle, including mRNA localization, translation and degradation, among others. The best characterized of such RNA-protein complexes, today named membraneless organelles, are Stress Granules (SGs) and Processing Bodies (PBs) which are involved in RNA storage and RNA decay/storage, respectively. Given that SGs and PBs are generally associated with repression of gene expression, viruses have evolved different mechanisms to counteract their assembly or to use them in their favor to successfully replicate within the host environment. In this review we summarize the current knowledge about the viral regulation of SGs and PBs, which could be a potential novel target for the development of broad-spectrum antiviral therapies.

Influence of selenium on the emergence of neuro tubule defects in a neuron-like cell line and its implications for amyotrophic lateral sclerosis

Impairment of the axonal transport system mediated by intracellular microtubules (MTs) is known to be a major drawback in neurodegenerative processes. Due to a growing interest on the neurotoxic effects of selenium in environmental health, our study aimed to assess the relationship between selenium and MTs perturbation, that may favour disease onset over a genetic predisposition to amyotrophic lateral sclerosis. We treated a neuron-like cell line with sodium selenite, sodium selenate and seleno-methionine and observed that the whole cytoskeleton was affected. We then investigated the protein interactome of cells overexpressing αTubulin-4A (TUBA4A) and found that selenium increases the interaction of TUBA4A with DNA- and RNA-binding proteins. TUBA4A ubiquitination and glutathionylation were also observed, possibly due to a selenium-dependent increase of ROS, leading to perturbation and degradation of MTs. Remarkably, the TUBA4A mutants R320C and A383 T, previously described in ALS patients, showed the same post-translational modifications to a similar extent. In conclusion this study gives insights into a specific mechanism characterizing selenium neurotoxicity.

Supramolecular Fuzziness of Intracellular Liquid Droplets: Liquid-Liquid Phase Transitions, Membrane-Less Organelles, and Intrinsic Disorder

Cells are inhomogeneously crowded, possessing a wide range of intracellular liquid droplets abundantly present in the cytoplasm of eukaryotic and bacterial cells, in the mitochondrial matrix and nucleoplasm of eukaryotes, and in the chloroplast’s stroma of plant cells. These proteinaceous membrane-less organelles (PMLOs) not only represent a natural method of intracellular compartmentalization, which is crucial for successful execution of various biological functions, but also serve as important means for the processing of local information and rapid response to the fluctuations in environmental conditions. Since PMLOs, being complex macromolecular assemblages, possess many characteristic features of liquids, they represent highly dynamic (or fuzzy) protein–protein and/or protein–nucleic acid complexes. The biogenesis of PMLOs is controlled by specific intrinsically disordered proteins (IDPs) and hybrid proteins with ordered domains and intrinsically disordered protein regions (IDPRs), which, due to their highly dynamic structures and ability to facilitate multivalent interactions, serve as indispensable drivers of the biological liquid–liquid phase transitions (LLPTs) giving rise to PMLOs. In this article, the importance of the disorder-based supramolecular fuzziness for LLPTs and PMLO biogenesis is discussed.

New pathologic mechanisms in nucleotide repeat expansion disorders

Tandem microsatellite repeats are common throughout the human genome and intrinsically unstable, exhibiting expansions and contractions both somatically and across generations. Instability in a small subset of these repeats are currently linked to human disease, although recent findings suggest more disease-causing repeats await discovery. These nucleotide repeat expansion disorders (NREDs) primarily affect the nervous system and commonly lead to neurodegeneration through toxic protein gain-of-function, protein loss-of-function, and toxic RNA gain-of-function mechanisms. However, the lines between these categories have blurred with recent findings of unconventional Repeat Associated Non-AUG (RAN) translation from putatively non-coding regions of the genome. Here we review two emerging topics in NREDs: 1) The mechanisms by which RAN translation occurs and its role in disease pathogenesis and 2) How nucleotide repeats as RNA and translated proteins influence liquid-liquid phase separation, membraneless organelle dynamics, and nucleocytoplasmic transport. We examine these topics with a particular eye on two repeats: the CGG repeat expansion responsible for Fragile X syndrome and Fragile X-associated Tremor Ataxia Syndrome (FXTAS) and the intronic GGGGCC repeat expansion in C9orf72, the most common inherited cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Our thesis is that these emerging disease mechanisms can inform a broader understanding of the native roles of microsatellites in cellular function and that aberrations in these native processes provide clues to novel therapeutic strategies for these currently untreatable disorders.

Intrinsic Disorder-Based Emergence in Cellular Biology: Physiological and Pathological Liquid-Liquid Phase Transitions in Cells

The visible outcome of liquid-liquid phase transitions (LLPTs) in cells is the formation and disintegration of various proteinaceous membrane-less organelles (PMLOs). Although LLPTs and related PMLOs have been observed in living cells for over 200 years, the physiological functions of these transitions (also known as liquid-liquid phase separation, LLPS) are just starting to be understood. While unveiling the functionality of these transitions is important, they have come into light more recently due to the association of abnormal LLPTs with various pathological conditions. In fact, several maladies, such as various cancers, different neurodegenerative diseases, and cardiovascular diseases, are known to be associated with either aberrant LLPTs or some pathological transformations within the resultant PMLOs. Here, we will highlight both the physiological functions of cellular liquid-liquid phase transitions as well as the pathological consequences produced through both dysregulated biogenesis of PMLOs and the loss of their dynamics. We will also discuss the potential downstream toxic effects of proteins that are involved in pathological formations.

PARPs and PAR as novel pharmacological targets for the treatment of stress granule-associated disorders

Among the post-translational modifications, ADP-ribosylation has been for long time the least integrated in the scheme of the structural protein modifications affecting physiological functions. In spite of the original findings on bacterial-dependent ADP-ribosylation catalysed by toxins such as cholera and pertussis toxin, only with the discovery of the poly-ADP-ribosyl polymerase (PARP) family the field has finally expanded and the role of ADP-ribosylation has been recognised in both physiological and pathological processes, including cancer, infectious and neurodegenerative diseases. This is now a rapidly expanding field of investigation, centred on the role of the different PARPs and their substrates in various diseases, and on the potential of PARP inhibitors as novel pharmacological tools to be employed in relevant pathological context. In this review we analyse the role that members of the PARP family and poly-ADP-ribose (PAR; the product of PARP1 and PARP5a activity) play in the processes following the exposure of cells to different stresses. The cell response that arises following conditions such as heat, osmotic, oxidative stresses or viral infection relies on the formation of stress granules, which are transient cytoplasmic membrane-less structures, that include untranslated mRNA, specific proteins and PAR, this last one serving as the "collector" of all components (that bind to it in a non-covalent manner). The resulting phenotypes are cells in which translation, intracellular transport or pro-apoptotic pathways are reversibly inhibited, for the time the given stress holds. Interestingly, the formation of defective stress granules has been detected in diverse pathological conditions including neurological disorders and cancer. Analysing the molecular details of stress granule formation under these conditions offers a novel view on the pathogenesis of these diseases and, as a consequence, the possibility of identifying novel drug targets for their treatment.