Hsp70 exhibits a liquid-liquid phase separation ability and chaperones condensed FUS against amyloid aggregation

Stress granules: emerging players in neurodegenerative diseases

Stress granules (SGs) are membraneless organelles formed in the cellular cytoplasm under stressful conditions through liquid-liquid phase separation (LLPS). SG assembly can be both dependent and independent of the eIF2α pathway, whereas cellular protein quality control systems mediate SG disassembly. Chaperones and specific domains of RNA-binding proteins strongly contribute to the regulation SG dynamics. Chronic stress, arising in association with aging, may promote persistent SGs that are difficult to disassemble, thereby acting as a potential pathological nidus for protein aggregation in neurodegenerative diseases (NDDs). In this review, we discuss the dynamics of SGs and the factors involved with SG assembly and disassembly. We also highlight the relationship among LLPS, SGs, and the pathogenesis of different NDDs. More importantly, we summarize SG assembly-disassembly, which may be a double-edged sword in the pathophysiology of NDDs. This review aims to provide new insights into the biology and pathology of LLPS, SGs, and NDDs.

Liquid-liquid phase separation in microorganisms: Insights into existence, functions, and applications

Liquid-liquid phase separation (LLPS) is a universal mechanism essential for maintaining cellular integrity and function in microorganisms, facilitating the organization of biomolecules into dynamic compartments. Although extensively studied in mammalian cells, research on LLPS formation and regulation in microorganisms remains limited. This review integrates insights from diverse studies exploring LLPS across microorganisms. We discuss the role of intrinsic disorders in microbial proteins and their relationship with environmental adaptation. Additionally, we examine how microorganisms utilize LLPS to sense changes in environmental parameters, such as temperature, pH, and nutrient levels, enabling them to respond to stresses and regulate cellular processes, such as cell division, protein synthesis, and metabolic flux. We highlight that LLPS is a promising target for synthetic biology and therapeutic intervention against pathogenic microorganisms. We also explore the research landscape of LLPS in microorganisms and address challenges associated with the techniques used in LLPS research. Further research is needed to focus on the detailed molecular regulatory mechanisms of condensates, biotechnological and synthetic biology applications, facilitating improved manipulation of microorganisms, and the identification of novel therapeutic targets.

Small heat shock protein HSPB8 interacts with a pre-fibrillar TDP43 low complexity domain species to delay fibril formation

The loss of cellular proteostasis through aberrant stress granule formation is implicated in neurodegenerative diseases. Stress granules are formed by biomolecular condensation involving protein-protein and protein-RNA interactions. These assemblies are protective, but can rigidify, leading to amyloid-like fibril formation, a hallmark of the disease pathology. Key proteins dictating stress granule formation and disassembly, such as TDP43, contain low-complexity (LC) domains that drive fibril formation. HSPB8, a small heat shock protein, plays a critical role modulating stress granule fluidity, preventing aggregation and promoting degradation of misfolded proteins. We examined the interaction between HSPB8 and the TDP43 LC using thioflavin T (ThT) and fluorescence polarization (FP) aggregation assays, fluorescence microscopy and photobleaching experiments, and crosslinking mass spectrometry (XL-MS). Our results indicate that HSPB8 delays TDP43 LC aggregation through domain-specific interactions with fibril nucleating species, without affecting fibril elongation rates. These findings provide mechanistic insight into how ATP-independent chaperones mediate LC domain aggregation and provide a basis for investigating how the TDP43 LC subverts chaperone activity in neurodegenerative disease.

Regulation of physiological and pathological condensates by molecular chaperones

Biomolecular condensates are dynamic membraneless compartments that regulate a myriad of cellular functions. A particular type of physiological condensate called stress granules (SGs) has gained increasing interest due to its role in the cellular stress response and various diseases. SGs, composed of several hundred RNA-binding proteins, form transiently in response to stress to protect mRNAs from translation and disassemble when the stress subsides. Interestingly, SGs contain several aggregation-prone proteins, such as TDP-43, FUS, hnRNPA1, and others, which are typically found in pathological inclusions seen in autopsy tissues from amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) patients. Moreover, mutations in these genes lead to the familial form of ALS and FTD. This has led researchers to propose that pathological aggregation is seeded by aberrant SGs: SGs that fail to properly disassemble, lose their dynamic properties, and become pathological condensates which finally 'mature' into aggregates. Here, we discuss the evidence supporting this model for various ALS/FTD-associated proteins. We further continue to focus on molecular chaperone-mediated regulation of ALS/FTD-associated physiological condensates on one hand, and pathological condensates on the other. In addition to SGs, we review ALS/FTD-relevant nuclear condensates, namely paraspeckles, anisosomes, and nucleolar amyloid bodies, and discuss their emerging regulation by chaperones. As the majority of chaperoning mechanisms regulate physiological condensate disassembly, we highlight parallel themes of physiological and pathological condensation regulation across different chaperone families, underscoring the potential for early disease intervention.

Making Proteins with Electricity

Ribosomes use multiple electrical forces to regulate new protein construction, to ensure efficient protein cotranslation, chaperoning, and folding. When these electrical regulatory forces are disrupted as in point charge mutations, specific disease occurs from aberrantly folded proteins. α1 antitrypsin deficiency is perhaps the best-known misfolded protein disease and is covered in some detail.

The role of biomolecular condensates in protein aggregation

There is an increasing amount of evidence that biomolecular condensates are linked to neurodegenerative diseases associated with protein aggregation, such as Alzheimer's disease and amyotrophic lateral sclerosis, although the mechanisms underlying this link remain elusive. In this Review, we summarize the possible connections between condensates and protein aggregation. We consider both liquid-to-solid transitions of phase-separated proteins and the partitioning of proteins into host condensates. We distinguish five key factors by which the physical and chemical environment of a condensate can influence protein aggregation, and we discuss their relevance in studies of protein aggregation in the presence of biomolecular condensates: increasing the local concentration of proteins, providing a distinct chemical microenvironment, introducing an interface wherein proteins can localize, changing the energy landscape of aggregation pathways, and the presence of chaperones in condensates. Analysing the role of biomolecular condensates in protein aggregation may be essential for a full understanding of amyloid formation and offers a new perspective that can help in developing new therapeutic strategies for the prevention and treatment of neurodegenerative diseases.

Hsp90α forms condensate engaging client proteins with RG motif repeats

Hsp90α, a pivotal canonical chaperone, is renowned for its broad interaction with numerous protein clients to maintain protein homeostasis, chromatin remodeling, and cell growth. Recent studies indicate its role in modifying various components of membraneless organelles (MLOs) such as stress granules and processing bodies, suggesting its participation in the regulation of protein condensates. In this study, we found that Hsp90α possesses an inherent ability to form dynamic condensates in vitro. Utilizing LC-MS/MS, we further pinpointed proteins in cell lysates that preferentially integrate into Hsp90α condensates. Significantly, we observed a prevalence of RG motif repeats in client proteins of Hsp90α condensates, many of which are linked to various MLOs. Moreover, each of the three domains of Hsp90α was found to undergo phase separation, with numerous solvent-exposed negatively charged residues on these domains being crucial for driving Hsp90α condensation through multivalent weak electrostatic interactions. Additionally, various clients like TDP-43 and hnRNPA1, along with poly-GR and PR dipeptide repeats, exhibit varied impacts on the dynamic behavior of Hsp90α condensates. Our study spotlights various client proteins associated with Hsp90α condensates, illustrating its intricate adaptive nature in interacting with diverse clients and its functional adaptability across multiple MLOs.

Engineered droplet-forming peptide as photocontrollable phase modulator for fused in sarcoma protein

The assembly and disassembly of biomolecular condensates are crucial for the subcellular compartmentalization of biomolecules in the control of cellular reactions. Recently, a correlation has been discovered between the phase transition of condensates and their maturation (aggregation) process in diseases. Therefore, modulating the phase of condensates to unravel the roles of condensation has become a matter of interest. Here, we create a peptide-based phase modulator, JSF1, which forms droplets in the dark and transforms into amyloid-like fibrils upon photoinitiation, as evidenced by their distinctive nanomechanical and dynamic properties. JSF1 is found to effectively enhance the condensation of purified fused in sarcoma (FUS) protein and, upon light exposure, induce its fibrilization. We also use JSF1 to modulate the biophysical states of FUS condensates in live cells and elucidate the relationship between FUS phase transition and FUS proteinopathy, thereby shedding light on the effect of protein phase transition on cellular function and malfunction.

The Role of Liquid-Liquid Phase Separation in the Accumulation of Pathological Proteins: New Perspectives on the Mechanism of Neurodegenerative Diseases

It is widely accepted that living organisms form highly dynamic membrane-less organelles (MLOS) with various functions through phase separation, and the indispensable role that phase separation plays in the mechanisms of normal physiological functions and pathogenesis is gradually becoming clearer. Pathological aggregates, regarded as hallmarks of neurodegenerative diseases, have been revealed to be closely related to aberrant phase separation. Specific proteins are assembled into condensates and transform into insoluble inclusions through aberrant phase separation, contributing to the development of diseases. In this review, we present an overview of the progress of phase separation research, involving its biological mechanisms and the status of research in neurodegenerative diseases, focusing on five main disease-specific proteins, tau, TDP-43, FUS, α-Syn and HTT, and how exactly these proteins reside within dynamic liquid-like compartments and thus turn into solid deposits. Further studies will yield new perspectives for understanding the aggregation mechanisms and potential therapeutic strategies, and future research directions are anticipated.

A chaperone-like function of FUS ensures TAZ condensate dynamics and transcriptional activation

The Hippo pathway has important roles in organ development, tissue homeostasis and tumour growth. Its downstream effector TAZ is a transcriptional coactivator that promotes target gene expression through the formation of biomolecular condensates. However, the mechanisms that regulate the biophysical properties of TAZ condensates to enable Hippo signalling are not well understood. Here using chemical crosslinking combined with an unbiased proteomics approach, we show that FUS associates with TAZ condensates and exerts a chaperone-like effect to maintain their proper liquidity and robust transcriptional activity. Mechanistically, the low complexity sequence domain of FUS targets the coiled-coil domain of TAZ in a phosphorylation-regulated manner, which ensures the liquidity and dynamicity of TAZ condensates. In cells lacking FUS, TAZ condensates transition into gel-like or solid-like assembles with immobilized TAZ, which leads to reduced expression of target genes and inhibition of pro-tumorigenic activity. Thus, our findings identify a chaperone-like function of FUS in Hippo regulation and demonstrate that appropriate biophysical properties of transcriptional condensates are essential for gene activation.

Elucidation of Site-Specific Ubiquitination on Chaperones in Response to Mutant Huntingtin

Huntington's disease (HD) is one of the prominent neurodegenerative diseases, characterized by the progressive decline of neuronal function, due to the accumulation and aggregation of misfolded proteins. Pathological progression of HD is hallmarked by the aberrant aggregation of the huntingtin protein (HTT) and subsequent neurotoxicity. Molecular chaperones (heat shock proteins, HSPs) play a pivotal role in maintaining proteostasis by facilitating protein refolding, degradation, or sequestration to limit the accumulation of misfolded proteins during neurotoxicity. However, the role of post-translational modifications such as ubiquitination among HSPs during HD is less known. In this study, we aimed to elucidate HSPs ubiquitin code in the context of HD pathogenesis. In a comprehensive proteomic analysis, we identified site-specific ubiquitination events in HSPs associated with HTT in HD-affected brain regions. To assess the impact of ubiquitination on HSPs during HD, we quantified the abundance of ubiquitinated lysine sites in both the rat cortex/striatum and in the mouse primary cortical neurons. Strikingly, we observed highly tissue-specific alterations in the relative ubiquitination levels of HSPs under HD conditions, emphasizing the importance of spatial perturbed post-translational modifications (PTMs) in shaping disease pathology. These ubiquitination events, combined with other PTMs on HSPs, are likely to influence the phase transitions of HTT. In conclusion, our study uncovered differential site-specific ubiquitination of molecular chaperones and offers a comprehensive view of the intricate relationship between protein aggregation, and PTMs in the context of Huntington's disease.

Nucleocapsid proteins from human coronaviruses possess phase separation capabilities and promote FUS pathological aggregation

The nucleocapsid (N) protein is an essential structural component necessary for genomic packaging and replication in various human coronaviruses (HCoVs), such as SARS-CoV-2 and MERS-CoV. Recent studies have revealed that the SARS-CoV-2 N protein exhibits a high capacity for liquid-liquid phase separation (LLPS), which plays multiple roles in viral infection and replication. In this study, we systematically investigate the LLPS capabilities of seven homologous N proteins from different HCoVs using a high-throughput protein phase separation assay. We found that LLPS is a shared intrinsic property among these N proteins. However, the phase separation profiles of the various N protein homologs differ, and they undergo phase separation under distinct in vitro conditions. Moreover, we demonstrate that N protein homologs can co-phase separate with FUS, a SG-containing protein, and accelerate its liquid-to-solid phase transition and amyloid aggregation, which is closely related to amyotrophic lateral sclerosis. Further study shows that N protein homologs can directly bind to the low complexity domain of FUS. Together, our work demonstrates that N proteins of different HCoVs possess phase separation capabilities, which may contribute to promoting pathological aggregation of host proteins and disrupting SG homeostasis during the infection and replication of various HCoVs.

Methylene blue accelerates liquid-to-gel transition of tau condensates impacting tau function and pathology

Preventing tau aggregation is a potential therapeutic strategy in Alzheimer's disease and other tauopathies. Recently, liquid-liquid phase separation has been found to facilitate the formation of pathogenic tau conformations and fibrillar aggregates, although many aspects of the conformational transitions of tau during the phase transition process remain unknown. Here, we demonstrate that the tau aggregation inhibitor methylene blue promotes tau liquid-liquid phase separation and accelerates the liquid-to-gel transition of tau droplets independent of the redox activity of methylene blue. We further show that methylene blue inhibits the conversion of tau droplets into fibrils and reduces the cytotoxicity of tau aggregates. Although gelation slows down the mobility of tau and tubulin, it does not impair microtubule assembly within tau droplets. These findings suggest that methylene blue inhibits tau amyloid fibrillization and accelerates tau droplet gelation via distinct mechanisms, thus providing insights into the activity of tau aggregation inhibitors in the context of phase transition.

Targeting Biomolecular Condensation and Protein Aggregation against Cancer

Biomolecular condensates, membrane-less entities arising from liquid-liquid phase separation, hold dichotomous roles in health and disease. Alongside their physiological functions, these condensates can transition to a solid phase, producing amyloid-like structures implicated in degenerative diseases and cancer. This review thoroughly examines the dual nature of biomolecular condensates, spotlighting their role in cancer, particularly concerning the p53 tumor suppressor. Given that over half of the malignant tumors possess mutations in the TP53 gene, this topic carries profound implications for future cancer treatment strategies. Notably, p53 not only misfolds but also forms biomolecular condensates and aggregates analogous to other protein-based amyloids, thus significantly influencing cancer progression through loss-of-function, negative dominance, and gain-of-function pathways. The exact molecular mechanisms underpinning the gain-of-function in mutant p53 remain elusive. However, cofactors like nucleic acids and glycosaminoglycans are known to be critical players in this intersection between diseases. Importantly, we reveal that molecules capable of inhibiting mutant p53 aggregation can curtail tumor proliferation and migration. Hence, targeting phase transitions to solid-like amorphous and amyloid-like states of mutant p53 offers a promising direction for innovative cancer diagnostics and therapeutics.

Molecular Chaperones' Potential against Defective Proteostasis of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a neuronal degenerative condition identified via a build-up of mutant aberrantly folded proteins. The native folding of polypeptides is mediated by molecular chaperones, preventing their pathogenic aggregation. The mutant protein expression in ALS is linked with the entrapment and depletion of chaperone capacity. The lack of a thorough understanding of chaperones' involvement in ALS pathogenesis presents a significant challenge in its treatment. Here, we review how the accumulation of the ALS-linked mutant FUS, TDP-43, SOD1, and C9orf72 proteins damage cellular homeostasis mechanisms leading to neuronal loss. Further, we discuss how the HSP70 and DNAJ family co-chaperones can act as potential targets for reducing misfolded protein accumulation in ALS. Moreover, small HSPB1 and HSPB8 chaperones can facilitate neuroprotection and prevent stress-associated misfolded protein apoptosis. Designing therapeutic strategies by pharmacologically enhancing cellular chaperone capacity to reduce mutant protein proteotoxic effects on ALS pathomechanisms can be a considerable advancement. Chaperones, apart from directly interacting with misfolded proteins for protein quality control, can also filter their toxicity by initiating strong stress-response pathways, modulating transcriptional expression profiles, and promoting anti-apoptotic functions. Overall, these properties of chaperones make them an attractive target for gaining fundamental insights into misfolded protein disorders and designing more effective therapies against ALS.

Concomitant protein pathogenesis in Parkinson's disease and perspective mechanisms

Comorbidity is a common phenotype in Parkinson's disease (PD). Patients with PD not only have motor deficit symptoms, but also have heterogeneous non-motor symptoms, including cognitive impairment and emotional changes, which are the featured symptoms observed in patients with Alzheimer's disease (AD), frontotemporal dementia (FTD) and cerebrovascular disease. Moreover, autopsy studies have also confirmed the concomitant protein pathogenesis, such as the co-existences of α-synuclein, amyloid-β and tau pathologies in PD and AD patients' brains. Here, we briefly summarize the recent reports regarding the comorbidity issues in PD from both clinical observations and neuropathological evidences. Furthermore, we provide some discussion about the perspective potential mechanisms underlying such comorbidity phenomenon, with a focus on PD and related neurodegenerative diseases.

Role of Proteostasis Regulation in the Turnover of Stress Granules

RNA-binding proteins (RBPs) and RNAs can form dynamic, liquid droplet-like cytoplasmic condensates, known as stress granules (SGs), in response to a variety of cellular stresses. This process is driven by liquid-liquid phase separation, mediated by multivalent interactions between RBPs and RNAs. The formation of SGs allows a temporary suspension of certain cellular activities such as translation of unnecessary proteins. Meanwhile, non-translating mRNAs may also be sequestered and stalled. Upon stress removal, SGs are disassembled to resume the suspended biological processes and restore the normal cell functions. Prolonged stress and disease-causal mutations in SG-associated RBPs can cause the formation of aberrant SGs and/or impair SG disassembly, consequently raising the risk of pathological protein aggregation. The machinery maintaining protein homeostasis (proteostasis) includes molecular chaperones and co-chaperones, the ubiquitin-proteasome system, autophagy, and other components, and participates in the regulation of SG metabolism. Recently, proteostasis has been identified as a major regulator of SG turnover. Here, we summarize new findings on the specific functions of the proteostasis machinery in regulating SG disassembly and clearance, discuss the pathological and clinical implications of SG turnover in neurodegenerative disorders, and point to the unresolved issues that warrant future exploration.

A comparative meta-analysis of membraneless organelle-associated proteins with age related proteome of C. elegans

Proteome imbalance can lead to protein misfolding and aggregation which is associated with pathologies. Protein aggregation can also be an active, organized process and can be exploited by cells as a survival strategy. In adverse conditions, it is beneficial to deposit the proteins in a condensate rather degrading and resynthesizing. Membraneless organelles (MLOs) are biological condensates formed through liquid-liquid phase separation (LLPS), involving cellular components such as nucleic acids and proteins. LLPS is a regulated process, which when perturbed, can undergo a transition from a physiological liquid condensate to pathological solid-like protein aggregates. To understand how the MLO-associated proteins (MLO-APs) behave during aging, we performed a comparative meta-analysis with age-related proteome of C. elegans. We found that the MLO-APs are highly abundant throughout the lifespan in wild-type and long-lived daf-2 mutant animals. Interestingly, they are aggregating more in long-lived mutant animals compared to the age matched wild-type and short-lived daf-16 and hsf-1 mutant animals. GO term analysis revealed that the cell cycle and embryonic development are among the top enriched processes in addition to RNP components in aggregated proteome. Considering antagonistic pleotropic nature of these developmental genes and post mitotic status of C. elegans, we assume that these proteins phase transit during post development. As the organism ages, these MLO-APs either mature to become more insoluble or dissolve in uncontrolled manner. However, in the long-lived daf-2 mutant animals, the MLOs may attain protective states due to extended availability and association of molecular chaperones.