Zinc promotes liquid-liquid phase separation of tau protein

Biomolecular condensates: phasing in regulated host-pathogen interactions

Biomolecular condensates are membraneless organelles formed through liquid-liquid phase separation. Innate immunity is essential to host defense against infections, but pathogens also harbor sophisticated mechanisms to evade host defense. The formation of biomolecular condensates emerges as a key biophysical mechanism in host-pathogen interactions, playing pivotal roles in regulating immune responses and pathogen life cycles within the host. In this review we summarize recent advances in our understanding of how biomolecular condensates remodel membrane-bound organelles, influence infection-induced cell death, and are hijacked by pathogens for survival, as well as how they modulate mammalian innate immunity. We discuss the implications of dysregulated formation of biomolecular condensates during host-pathogen interactions and infectious diseases and propose future directions for developing potential treatments against such infections.

Zinc as a Mediator Through the ROCK1 Pathway of Cognitive Impairment in Aluminum-Exposed Workers: A Clinical and Animal Study

Aluminum (Al) exposure was implicated in neurodegenerative diseases and cognitive impairment, yet the involvement of zinc (Zn) and its mechanism in Al-induced mild cognitive impairment (MCI) remains poorly understood. The objective is to explore the role of Zn in Al-induced cognitive impairment and its potential mechanisms. Montreal cognitive assessment (MoCA) test scores and serum Al, Zn from Al industry workers were collected. A mediation analysis was performed to evaluate the role of serum Zn among serum Al and MoCA test scores. Subsequently, an Al-exposure study was conducted on a rat model categorized into control, low-, medium-, and high-dose groups. After a Morris Water Maze test and detection of Al, Zn content in the hippocampus, integrated transcriptomic and proteomic analyses between the control group and the high-dose group were performed to identify the differentially expressed genes (DEPs), proteins (DEPs), and pathways. To corroborate these findings, quantitative real-time polymerase chain reaction (qRT-PCR) and western blotting (WB) were selected to identify the gene and protein results. Zn overall mediates the relationship between serum Al and cognitive function (mediation effect 17.82%, effect value =  - 0.0351). In the Al-exposed rat model, 734 DEGs, 18 miRNAs, 35 lncRNAs, 64 circRNAs, and 113 DEPs were identified between the high-dose group and the control group. Among them, ROCK1, DMD, and other four DEPs were identified as related to zinc finger proteins (ZNF). Co-enrichment analyses of the Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO) linked these changes to the RHOA/ROCK1 signaling axis. ZNF-related proteins Rock1, DMD, and DHX57 in the high-dose group were downregulated (p = 0.006, 0.003, 0.04), and the expression of Myl9, Rhoa, miR431, and miR182 was also downregulated (p = 0.003, 0.032, 0.032, and 0.046). These findings also show correlations between Al, Zn levels in the hippocampus, water maze performance, and expressions of Myl9, Rhoa, miR431, miR182, DMD, ROCK1, and DHX57, with both negative and positive associations. Based on the results, we determined that Zn was involved in Al-induced MCI in Al workers and Al-exposed rat models. Al exposure and interaction with Zn could trigger the downregulation of ZNF of ROCK1, DMD, and DHX57. miR431, miR182 regulate RHOA/ROCK1 was one of the Zn-involved pathways in Al-induced cognitive impairment.

Implications of liquid-liquid phase separation and ferroptosis in Alzheimer's disease

Neuronal cell demise represents a prevalent occurrence throughout the advancement of Alzheimer's disease (AD). However, the mechanism of triggering the death of neuronal cells remains unclear. Its potential mechanisms include aggregation of soluble amyloid-beta (Aβ) to form insoluble amyloid plaques, abnormal phosphorylation of tau protein and formation of intracellular neurofibrillary tangles (NFTs), neuroinflammation, ferroptosis, oxidative stress, liquid-liquid phase separation (LLPS) and metal ion disorders. Among them, ferroptosis is an iron-dependent lipid peroxidation-driven cell death and emerging evidences have demonstrated the involvement of ferroptosis in the pathological process of AD. The sensitivity to ferroptosis is tightly linked to numerous biological processes. Moreover, emerging evidences indicate that LLPS has great impacts on regulating human health and diseases, especially AD. Soluble Aβ can undergo LLPS to form liquid-like droplets, which can lead to the formation of insoluble amyloid plaques. Meanwhile, tau has a high propensity to condensate via the mechanism of LLPS, which can lead to the formation of NFTs. In this review, we summarize the most recent advancements pertaining to LLPS and ferroptosis in AD. Our primary focus is on expounding the influence of Aβ, tau protein, iron ions, and lipid oxidation on the intricate mechanisms underlying ferroptosis and LLPS within the domain of AD pathology. Additionally, we delve into the intricate cross-interactions that occur between LLPS and ferroptosis in the context of AD. Our findings are expected to serve as a theoretical and experimental foundation for clinical research and targeted therapy for AD.

RNA G-quadruplexes and calcium ions synergistically induce Tau phase transition in vitro

Tau aggregation is a defining feature of neurodegenerative tauopathies, including Alzheimer's disease, corticobasal degeneration, and frontotemporal dementia. This aggregation involves the liquid-liquid phase separation (LLPS) of Tau, followed by its sol-gel phase transition, representing a crucial step in aggregate formation both in vitro and in vivo. However, the precise cofactors influencing Tau phase transition and aggregation under physiological conditions (e.g., ion concentration and temperature) remain unclear. In this study, we unveil that nucleic acid secondary structures, specifically RNA G-quadruplexes (rG4s), and calcium ions (Ca2+) synergistically facilitated the sol-gel phase transition of human Tau under mimic intracellular ion conditions (140 mM KCl, 15 mM NaCl, and 10 mM MgCl2) at 37°C in vitro. In the presence of molecular crowding reagents, Tau formed stable liquid droplets through LLPS, maintaining fluidity for 24 h under physiological conditions. Notably, cell-derived RNA promoted Tau sol-gel phase transition, with rG4s emerging as a crucial factor. Surprisingly, polyanion heparin did not elicit a similar response, indicating a distinct mechanism not rooted in electrostatic interactions. Further exploration underscored the significance of Ca2+, which accumulate intracellularly during neurodegeneration, as additional cofactors in promoting Tau phase transition after 24 h. Importantly, our findings demonstrate that rG4s and Ca2+ synergistically enhance Tau phase transition within 1 h when introduced to Tau droplets. Moreover, rG4-Tau aggregates showed seeding ability in cells. In conclusion, our study illuminates the pivotal roles of rG4s and Ca2+ in promoting Tau aggregation under physiological conditions in vitro, offering insights into potential triggers for tauopathy.

Biomolecular condensates and disease pathogenesis

Biomolecular condensates or membraneless organelles (MLOs) formed by liquid-liquid phase separation (LLPS) divide intracellular spaces into discrete compartments for specific functions. Dysregulation of LLPS or aberrant phase transition that disturbs the formation or material states of MLOs is closely correlated with neurodegeneration, tumorigenesis, and many other pathological processes. Herein, we summarize the recent progress in development of methods to monitor phase separation and we discuss the biogenesis and function of MLOs formed through phase separation. We then present emerging proof-of-concept examples regarding the disruption of phase separation homeostasis in a diverse array of clinical conditions including neurodegenerative disorders, hearing loss, cancers, and immunological diseases. Finally, we describe the emerging discovery of chemical modulators of phase separation.

Protein misfolding and amyloid nucleation through liquid-liquid phase separation

Liquid-liquid phase separation (LLPS) is an emerging phenomenon in cell physiology and diseases. The weak multivalent interaction prerequisite for LLPS is believed to be facilitated through intrinsically disordered regions, which are prevalent in neurodegenerative disease-associated proteins. These aggregation-prone proteins also exhibit an inherent property for phase separation, resulting in protein-rich liquid-like droplets. The very high local protein concentration in the water-deficient confined microenvironment not only drives the viscoelastic transition from the liquid to solid-like state but also most often nucleate amyloid fibril formation. Indeed, protein misfolding, oligomerization, and amyloid aggregation are observed to be initiated from the LLPS of various neurodegeneration-related proteins. Moreover, in these cases, neurodegeneration-promoting genetic and environmental factors play a direct role in amyloid aggregation preceded by the phase separation. These cumulative recent observations ignite the possibility of LLPS being a prominent nucleation mechanism associated with aberrant protein aggregation. The present review elaborates on the nucleation mechanism of the amyloid aggregation pathway and the possible early molecular events associated with amyloid-related protein phase separation. It also summarizes the recent advancement in understanding the aberrant phase transition of major proteins contributing to neurodegeneration focusing on the common disease-associated factors. Overall, this review proposes a generic LLPS-mediated multistep nucleation mechanism for amyloid aggregation and its implication in neurodegeneration.

Zinc utilization by microglia in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia defined by two key pathological characteristics in the brain, amyloid-β (Aβ) plaques and neurofibrillary tangles (NFTs) composed of hyperphosphorylated tau. Microglia, the primary innate immune cells of the central nervous system (CNS), provide neuroprotection through Aβ and tau clearance but may also be neurotoxic by promoting neuroinflammation to exacerbate Aβ and tau pathogenesis in AD. Recent studies have demonstrated the importance of microglial utilization of nutrients and trace metals in controlling their activation and effector functions. Trace metals, such as zinc, have essential roles in brain health and immunity, and zinc dyshomeostasis has been implicated in AD pathogenesis. As a result of these advances, the mechanisms by which zinc homeostasis influences microglial-mediated neuroinflammation in AD is a topic of continuing interest since new strategies to treat AD are needed. Here, we review the roles of zinc in AD, including zinc activation of microglia, the associated neuroinflammatory response, and the application of these findings in new therapeutic strategies.

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.

Biomolecular Condensates Decipher Molecular Codes of Cell Fate: From Biophysical Fundamentals to Therapeutic Practices

Cell fate is precisely modulated by complex but well-tuned molecular signaling networks, whose spatial and temporal dysregulation commonly leads to hazardous diseases. Biomolecular condensates (BCs), as a newly emerging type of biophysical assemblies, decipher the molecular codes bridging molecular behaviors, signaling axes, and clinical prognosis. Particularly, physical traits of BCs play an important role; however, a panoramic view from this perspective toward clinical practices remains lacking. In this review, we describe the most typical five physical traits of BCs, and comprehensively summarize their roles in molecular signaling axes and corresponding major determinants. Moreover, establishing the recent observed contribution of condensate physics on clinical therapeutics, we illustrate next-generation medical strategies by targeting condensate physics. Finally, the challenges and opportunities for future medical development along with the rapid scientific and technological advances are highlighted.

EB1 Increases the Dynamics of Tau Droplets and Inhibits Tau Aggregation: Implications in Tauopathies

EB1, a microtubule plus end-tracking protein (+TIP), regulates microtubule dynamics. Recent evidence indicates cross-talk between EB proteins and tau, a microtubule-associated neuronal protein that is important for the growth and stability of microtubules. We investigated the interaction between tau and EB1 and the effect of binding of EB1 on tau function and aggregation. EB1 colocalized with tau in SH-SY5Y cells and coimmunoprecipitated with tau. Further, purified EB1 impaired the ability of adult tau to induce tubulin polymerization in vitro. EB1 bound to tau with a dissociation constant of 2.5 ± 0.7 μM. EB1 reduced heparin-induced tau aggregation with a half-maximal inhibitory concentration of 4.3 ± 0.2 μM, and increased the dynamics of tau in phase-separated droplets. The fluorescence recovery rate in tau droplets increased from 0.02 ± 0.01 to 0.07 ± 0.03 s-1, while the half-time of recovery decreased from 44.5 ± 14 to 13.5 ± 6 s in the presence of 8 μM EB1, suggesting a delay in the transition of tau from the soluble to aggregated form in tau liquid-liquid phase separation. EB1 decreased the rate of aggregation and increased the critical concentration of tau aggregation. Dynamic light scattering, atomic force microscopy, dot blot assays, and SDS-PAGE analysis showed that EB1 inhibited the formation of oligomers and higher-order aggregates of tau. The data suggest a novel role for EB1 as a regulator of tau function and aggregation, and the findings indicated the role of the EB family proteins in neuronal function and neurodegeneration.

Oxidative Stress Induced by Arsenite is Involved in YTHDF2 Phase Separation

YTH N6-methyladenosine RNA binding protein 2 (YTHDF2) undergoes phase separation in response to the stimulation of high concentration of arsenite, suggesting that oxidative stress, the major mechanism of arsenite toxicity, may play a role in YTHDF2 phase separation. However, whether arsenite-induced oxidative stress is involved in phase separation of YTHDF2 has yet to be established. To explore the effect of arsenite-induced oxidative stress on YTHDF2 phase separation, the levels of oxidative stress, YTHDF2 phase separation, and N6-methyladenosine (m6A) in human keratinocytes were detected after exposure to various concentrations of sodium arsenite (0-500 µM; 1 h) and antioxidant N-acetylcysteine (0-10 mM; 2 h). We found that arsenite promoted oxidative stress and YTHDF2 phase separation in a concentration-dependent manner. In contrast, pretreatment with N-acetylcysteine significantly relieved arsenate-induced oxidative stress and inhibited YTHDF2 phase separation. As one of the key factors to YTHDF2 phase separation, N6-methyladenosine (m6A) levels in human keratinocytes were significantly increased after arsenite exposure, accompanied by upregulation of m6A methylesterase levels and downregulation of m6A demethylases levels. On the contrary, N-acetylcysteine mitigated the arsenite-induced increase of m6A and m6A methylesterase and the arsenite-induced decrease in m6A demethylase. Collectively, our study firstly revealed that oxidative stress induced by arsenite plays an important role in YTHDF2 phase separation driven by m6A modification, which provides new insights into the arsenite toxicity from the phase-separation perspective.

Doxorubicin catalyses self-assembly of p53 by phase separation

Liquid-liquid phase separation plays a crucial role in cellular physiology, as it leads to the formation of membrane-less organelles in response to various internal stimuli, contributing to various cellular functions. However, the influence of exogenous stimuli on this process in the context of disease intervention remains unexplored. In this current investigation, we explore the impact of doxorubicin on the abnormal self-assembly of p53 using a combination of biophysical and imaging techniques. Additionally, we shed light on the potential mechanisms behind chemoresistance in cancer cells carrying mutant p53. Our findings reveal that doxorubicin co-localizes with both wild-type p53 (WTp53) and its mutant variants. Our in vitro experiments indicate that doxorubicin interacts with the N-terminal-deleted form of WTp53 (WTp53ΔNterm), inducing liquid-liquid phase separation, ultimately leading to protein aggregation. Notably, the p53 variants at the R273 position exhibit a propensity for phase separation even in the absence of doxorubicin, highlighting the destabilizing effects of point mutations at this position. The strong interaction between doxorubicin and p53 variants, along with its localization within the protein condensates, provides a potential explanation for the development of chemotherapy resistance. Collectively, our cellular and in vitro studies emphasize the role of exogenous agents in driving phase separation-mediated p53 aggregation.

Liquid-liquid phase separation in Alzheimer's disease

The pathological aggregation and misfolding of tau and amyloid-β play a key role in Alzheimer's disease (AD). However, the underlying pathological mechanisms remain unclear. Emerging evidences indicate that liquid-liquid phase separation (LLPS) has great impacts on regulating human health and diseases, especially neurodegenerative diseases. A series of studies have revealed the significance of LLPS in AD. In this review, we summarize the latest progress of LLPS in AD, focusing on the impact of metal ions, small-molecule inhibitors, and proteinaceous partners on tau LLPS and aggregation, as well as toxic oligomerization, the role of LLPS on amyloid-β (Aβ) aggregation, and the cross-interactions between amyloidogenic proteins in AD. Eventually, the fundamental methods and techniques used in LLPS study are introduced. We expect to present readers a deeper understanding of the relationship between LLPS and AD.

Molecular Mechanism of Tau Misfolding and Aggregation: Insights from Molecular Dynamics Simulation

Tau dysfunction has a close association with many neurodegenerative diseases, which are collectively referred to as tauopathies. Neurofibrillary tangles (NFTs) formed by misfolding and aggregation of tau are the main pathological process of tauopathy. Therefore, uncovering the misfolding and aggregation mechanism of tau protein will help to reveal the pathogenic mechanism of tauopathies. Molecular dynamics (MD) simulation is well suited for studying the dynamic process of protein structure changes. It provides detailed information on protein structure changes over time at the atomic resolution. At the same time, MD simulation can also simulate various conditions conveniently. Based on these advantages, MD simulations are widely used to study conformational transition problems such as protein misfolding and aggregation. Here, we summarized the structural features of tau, the factors affecting its misfolding and aggregation, and the applications of MD simulations in the study of tau misfolding and aggregation.

Modulation of biomolecular phase behavior by metal ions

Liquid-liquid phase separation (LLPS) appears to be a newly appreciated aspect of the cellular organization of biomolecules that leads to the formation of membraneless organelles (MLOs). MLOs generate distinct microenvironments where particular biomolecules are highly concentrated compared to those in the surrounding environment. Their thermodynamically driven formation is reversible, and their liquid nature allows them to fuse with each other. Dysfunctional biomolecular condensation is associated with human diseases. Pathological states of MLOs may originate from the mutation of proteins or may be induced by other factors. In most aberrant MLOs, transient interactions are replaced by stronger and more rigid interactions, preventing their dissolution, and causing their uncontrolled growth and dysfunction. For these reasons, there is great interest in identifying factors that modulate LLPS. In this review, we discuss an enigmatic and mostly unexplored aspect of this process, namely, the regulatory effects of metal ions on the phase behavior of biomolecules.

Ion binding with charge inversion combined with screening modulates DEAD box helicase phase transitions

Membraneless organelles, or biomolecular condensates, enable cells to compartmentalize material and processes into unique biochemical environments. While specific, attractive molecular interactions are known to stabilize biomolecular condensates, repulsive interactions, and the balance between these opposing forces, are largely unexplored. Here, we demonstrate that repulsive and attractive electrostatic interactions regulate condensate stability, internal mobility, interfaces, and selective partitioning of molecules both in vitro and in cells. We find that signaling ions, such as calcium, alter repulsions between model Ddx3 and Ddx4 condensate proteins by directly binding to negatively charged amino acid sidechains and effectively inverting their charge, in a manner fundamentally dissimilar to electrostatic screening. Using a polymerization model combined with generalized stickers and spacers, we accurately quantify and predict condensate stability over a wide range of pH, salt concentrations, and amino acid sequences. Our model provides a general quantitative treatment for understanding how charge and ions reversibly control condensate stability.

Toward a high-resolution mechanism of intrinsically disordered protein self-assembly

Membraneless organelles formed via the self-assembly of intrinsically disordered proteins (IDPs) play a crucial role in regulating various physiological functions. Elucidating the mechanisms behind IDP self-assembly is of great interest not only from a biological perspective but also for understanding how amino acid mutations in IDPs contribute to the development of neurodegenerative diseases and other disorders. Currently, two proposed mechanisms explain IDP self-assembly: (1) the sticker-and-spacer framework, which considers amino acid residues as beads to simulate the intermolecular interactions, and (2) the cross-β hypothesis, which focuses on the β-sheet interactions between the molecular surfaces constructed by multiple residues. This review explores the advancement of new models that provide higher resolution insights into the IDP self-assembly mechanism based on new findings obtained from structural studies of IDPs.

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.

Zinc Ions Modulate YY1 Activity: Relevance in Carcinogenesis

YY1 is widely recognized as an intrinsically disordered transcription factor that plays a role in development of many cancers. In most cases, its overexpression is correlated with tumor progression and unfavorable patient outcomes. Our latest research focusing on the role of zinc ions in modulating YY1's interaction with DNA demonstrated that zinc enhances the protein's multimeric state and affinity to its operator. In light of these findings, changes in protein concentration appear to be just one element relevant to modulating YY1-dependent processes. Thus, alterations in zinc ion concentration can directly and specifically impact the regulation of gene expression by YY1, in line with reports indicating a correlation between zinc ion levels and advancement of certain tumors. This review concentrates on other potential consequences of YY1 interaction with zinc ions that may act by altering charge distribution, conformational state distribution, or oligomerization to influence its interactions with molecular partners that can disrupt gene expression patterns.

DpdtpA, A Multi-metal Ion Chelator, Attenuates Tau Phosphorylation and Microglial Inflammatory Response via Regulating the PI3K/AKT/GSK-3β Signal Pathways

Tau protein hyperphosphorylation and formation of intracellular neurofibrillary tangles (NFTs) are one of the histopathological hallmarks of Alzheimer's disease (AD) and positively correlated with the severity of AD symptoms. NFTs contain a large number of metal ions that play an important role in regulating tau protein phosphorylation and AD progression. Extracellular tau induces primary phagocytosis of stressed neurons and neuronal loss by activating microglia. Here, we studied the effects of a multi-metal ion chelator, DpdtpA, on tau-induced microglial activation and inflammatory responses and the underlying mechanisms. Treatment with DpdtpA attenuated the increase in the expression of NF-κB and production of inflammatory cytokines, IL-1β, IL-6 and IL-10, in rat microglial cells induced by expression of human tau40 proteins. Treatment with DpdtpA also suppressed tau protein expression and phosphorylation. Moreover, treatment with DpdtpA prevented tau-induced activation of glycogen synthase kinase-3β (GSK-3β) and inhibition of phosphatidylinositol-3-hydroxy kinase (PI3K)/AKT. Collectively, these results show that DpdtpA can attenuate tau phosphorylation and inflammatory responses of microglia by regulating the PI3K/AKT/GSK-3β signal pathways, providing a new option to alleviate neuroinflammation for the treatment of AD.

Impact of hierarchical water dipole orderings on the dynamics of aqueous salt solutions

Ions exhibit highly ion-specific complex behaviours when solvated in water, which remains a mystery despite the fundamental importance of ion solvation in nature, science, and technology. Here we explain these ion-specific properties by the ion-induced hierarchical dipolar, translational, and bond-orientational orderings of ion hydration shell under the competition between ion-water electrostatic interactions and inter-water hydrogen bonding. We first characterise this competition by a new length λHB(q), explaining the ion-specific effects on solution dynamics. Then, by continuously tuning ion size and charge, we find that the bond-orientational order of the ion hydration shell highly develops for specific ion size and charge combinations. This ordering drastically stabilises the hydration shell; its degree changes the water residence time around ions by 11 orders of magnitude for main-group ions. These findings are fundamental to ionic processes in aqueous solutions, providing a physical principle for electrolyte design and application.

Zn2+ decoration of microtubules arrests axonal transport and displaces tau, doublecortin, and MAP2C

Intracellular Zn2+ concentrations increase via depolarization-mediated influx or intracellular release, but the immediate effects of Zn2+ signals on neuron function are not fully understood. By simultaneous recording of cytosolic Zn2+ and organelle motility, we find that elevated Zn2+ (IC50 ≈ 5-10 nM) reduces both lysosomal and mitochondrial motility in primary rat hippocampal neurons and HeLa cells. Using live-cell confocal microscopy and in vitro single-molecule TIRF imaging, we reveal that Zn2+ inhibits activity of motor proteins (kinesin and dynein) without disrupting their microtubule binding. Instead, Zn2+ directly binds to microtubules and selectively promotes detachment of tau, DCX, and MAP2C, but not MAP1B, MAP4, MAP7, MAP9, or p150glued. Bioinformatic predictions and structural modeling show that the Zn2+ binding sites on microtubules partially overlap with the microtubule binding sites of tau, DCX, dynein, and kinesin. Our results reveal that intraneuronal Zn2+ regulates axonal transport and microtubule-based processes by interacting with microtubules.

Sequence variations of phase-separating proteins and resources for studying biomolecular condensates

Phase separation (PS) is an important mechanism underlying the formation of biomolecular condensates. Physiological condensates are associated with numerous biological processes, such as transcription, immunity, signaling, and synaptic transmission. Changes in particular amino acids or segments can disturb the protein's phase behavior and interactions with other biomolecules in condensates. It is thus presumed that variations in the phase-separating-prone domains can significantly impact the properties and functions of condensates. The dysfunction of condensates contributes to a number of pathological processes. Pharmacological perturbation of these condensates is proposed as a promising way to restore physiological states. In this review, we characterize the variations observed in PS proteins that lead to aberrant biomolecular compartmentalization. We also showcase recent advancements in bioinformatics of membraneless organelles (MLOs), focusing on available databases useful for screening PS proteins and describing endogenous condensates, guiding researchers to seek the underlying pathogenic mechanisms of biomolecular condensates.

Tau liquid-liquid phase separation is modulated by the Ca2+ -switched chaperone activity of the S100B protein

Aggregation of the microtubule-associated protein tau is implicated in several neurodegenerative tauopathies including Alzheimer's disease (AD). Recent studies evidenced tau liquid-liquid phase separation (LLPS) into droplets as an early event in tau pathogenesis with the potential to enhance aggregation. Tauopathies like AD are accompanied by sustained neuroinflammation and the release of alarmins at early stages of inflammatory responses encompass protective functions. The Ca2+ -binding S100B protein is an alarmin augmented in AD that was recently implicated as a proteostasis regulator acting as a chaperone-type protein, inhibiting aggregation and toxicity through interactions of amyloidogenic clients with a regulatory surface exposed upon Ca2+ -binding. Here we expand the regulatory functions of S100B over protein condensation phenomena by reporting its Ca2+ -dependent activity as a modulator of tau LLPS induced by crowding agents (PEG) and metal ions (Zn2+ ). We observe that apo S100B has a negligible effect on PEG-induced tau demixing but that Ca2+ -bound S100B prevents demixing, resulting in a shift of the phase diagram boundary to higher crowding concentrations. Also, while incubation with apo S100B does not compromise tau LLPS, addition of Ca2+ results in a sharp decrease in turbidity, indicating that interactions with S100B-Ca2+ promote transition of tau to the mixed phase. Further, electrophoretic analysis and FLIM-FRET studies revealed that S100B incorporates into tau liquid droplets, suggesting an important stabilizing and chaperoning role contributing to minimize toxic tau aggregates. Resorting to Alexa488-labeled tau we observed that S100B-Ca2+ reduces the formation of tau fluorescent droplets, without compromising liquid-like behavior and droplet fusion events. The Zn2+ -binding properties of S100B also contribute to regulate Zn2+ -promoted tau LLPS as droplets are decreased by Zn2+ buffering by S100B, in addition to the Ca2+ -triggered interactions with tau. Altogether this work uncovers the versatility of S100B as a proteostasis regulator acting on protein condensation phenomena of relevance across the neurodegeneration continuum.

Cellular uptake of nickel by NikR is regulated by phase separation

Bacterial cells were long thought to be "bags of enzymes" with minimal internal structures. In recent years, membrane-less organelles formed by liquid-liquid phase separation (LLPS) of proteins or nucleic acids have been found to be involved in many important biological processes, although most of them were studied on eukaryotic cells. Here, we report that NikR, a bacterial nickel-responsive regulatory protein, exhibits LLPS both in solution and inside cells. Analyses of cellular nickel uptake and cell growth of E. coli confirm that LLPS enhances the regulatory function of NikR, while disruption of LLPS in cells promotes the expression of nickel transporter (nik) genes, which are negatively regulated by NikR. Mechanistic study shows that Ni(II) ions induces the accumulation of nik promoter DNA into the condensates formed by NikR. This result suggests that the formation of membrane-less compartments can be a regulatory mechanism of metal transporter proteins in bacterial cells.

Small molecules in regulating protein phase separation

Biomolecular condensates formed by phase separation are involved in many cellular processes. Dysfunctional or abnormal condensates are closely associated with neurodegenerative diseases, cancer and other diseases. Small molecules can effectively regulate protein phase separation by modulating the formation, dissociation, size and material properties of condensates. Discovery of small molecules to regulate protein phase separation provides chemical probes for deciphering the underlying mechanism and potential novel treatments for condensate-related diseases. Here we review the advances of small molecule regulation of phase separation. The discovery, chemical structures of recently found small molecule phase separation regulators and how they modulate biological condensates are summarized and discussed. Possible strategies to accelerate the discovery of more liquid-liquid phase separation (LLPS)-regulating small molecules are proposed.

Study liquid-liquid phase separation with optical microscopy: A methodology review

Intracellular liquid-liquid phase separation (LLPS) is a critical process involving the dynamic association of biomolecules and the formation of non-membrane compartments, playing a vital role in regulating biomolecular interactions and organelle functions. A comprehensive understanding of cellular LLPS mechanisms at the molecular level is crucial, as many diseases are linked to LLPS, and insights gained can inform drug/gene delivery processes and aid in the diagnosis and treatment of associated diseases. Over the past few decades, numerous techniques have been employed to investigate the LLPS process. In this review, we concentrate on optical imaging methods applied to LLPS studies. We begin by introducing LLPS and its molecular mechanism, followed by a review of the optical imaging methods and fluorescent probes employed in LLPS research. Furthermore, we discuss potential future imaging tools applicable to the LLPS studies. This review aims to provide a reference for selecting appropriate optical imaging methods for LLPS investigations.

Liquid - liquid phase separation of tau: Driving forces, regulation, and biological implications

The past 15 years have witnessed an explosion in the studies of biomolecular condensates that are implicated in numerous biological processes and play vital roles in human health and diseases. Recent findings demonstrate that the microtubule-associated protein tau forms liquid condensates through liquid-liquid phase separation (LLPS) in in vitro experiments using purified recombinant proteins and cell-based experiments. Although in vivo studies are lacking, liquid condensates have emerged as an important assembly state of physiological and pathological tau and LLPS can regulate the function of microtubules, mediate stress granule formation, and accelerate tau amyloid aggregation. In this review, we summarize recent advances in tau LLPS, aiming to unveiling the delicate interactions driving tau LLPS. We further discuss the association of tau LLPS with physiology and disease in the context of the sophisticated regulation of tau LLPS. Deciphering the mechanisms underlying tau LLPS and the liquid-to-solid transition enables rational design of molecules that inhibit or delay the formation of tau solid species, thus providing novel targeted therapeutic strategies for tauopathies.

Metals and inorganic molecules in regulating protein and nucleic acid phase separation

The realization that liquid-liquid phase separation (LLPS) underlies the formation of membraneless compartments in cells has motivated efforts to modulate the condensation process of biomolecules. Increasing evidence shows that metals and inorganic molecules abundantly distributed in cells play important roles in the regulation of biomolecular condensation. Herein, we briefly reviewed the background of biomacromolecular phase separation and summarized the recent research progress on the roles of metals and inorganic molecules in regulating protein and nucleic acid phase separation in vitro and in cells.

Biomolecular Liquid-Liquid Phase Separation for Biotechnology

The liquid-liquid phase separation (LLPS) of biomolecules induces condensed assemblies called liquid droplets or membrane-less organelles. In contrast to organelles with lipid membrane barriers, the liquid droplets induced by LLPS do not have distinct barriers (lipid bilayer). Biomolecular LLPS in cells has attracted considerable attention in broad research fields from cellular biology to soft matter physics. The physical and chemical properties of LLPS exert a variety of functions in living cells: activating and deactivating biomolecules involving enzymes; controlling the localization, condensation, and concentration of biomolecules; the filtration and purification of biomolecules; and sensing environmental factors for fast, adaptive, and reversible responses. The versatility of LLPS plays an essential role in various biological processes, such as controlling the central dogma and the onset mechanism of pathological diseases. Moreover, biomolecular LLPS could be critical for developing new biotechnologies such as the condensation, purification, and activation of a series of biomolecules. In this review article, we introduce some fundamental aspects and recent progress of biomolecular LLPS in living cells and test tubes. Then, we discuss applications of biomolecular LLPS toward biotechnologies.

14-3-3ζ Participates in the Phase Separation of Phosphorylated and Glycated Tau and Modulates the Physiological and Pathological Functions of Tau

Tau plays a major role in Alzheimer's disease (AD) and several other neurodegenerative diseases. Tau undergoing liquid-liquid phase separation (LLPS) performs specific physiological functions, induces pathological processes, and contributes to neurodegeneration. Regulating Tau phase separation helps maintain physiological functions of Tau and inhibits pathological aggregation. Here, we show that the 14-3-3 zeta isoform (14-3-3ζ) participates in Tau LLPS. 14-3-3ζ can undergo co-phase separation with WT Tau, participate in and stabilize Tau droplets, and inhibit Tau droplet-driven tubulin assembly. On the other hand, 14-3-3ζ disrupts the LLPS of phosphorylated and glycated Tau, thereby inhibiting the amyloid aggregation initiated by LLPS.

Liquid-liquid phase separation of protein tau: An emerging process in Alzheimer's disease pathogenesis

Metabolic reactions within cells occur in various isolated compartments with or without borders, the latter being known as membrane-less organelles (MLOs). The MLOs show liquid-like properties and are formed by a process known as liquid-liquid phase separation (LLPS). MLOs contribute to different molecules interactions such as protein-protein, protein-RNA, and RNA-RNA driven by various factors, such as multivalency of intrinsic disorders. MLOs are involved in several cell signaling pathways such as transcription, immune response, and cellular organization. However, disruption of these processes has been found in different pathologies. Recently, it has been demonstrated that protein aggregates, a characteristic of some neurodegenerative diseases, undergo similar phase separation. Tau protein is known as a major neurofibrillary tangles component in Alzheimer's disease (AD). This protein can undergo phase separation to form a MLO known as tau droplet in vitro and in vivo, and this process can be facilitated by several factors, including crowding agents, RNA, and phosphorylation. Tau droplet has been shown to mature into insoluble aggregates suggesting that this process may precede and induce neurodegeneration in AD. Here we review major factors involved in liquid droplet formation within a cell. Additionally, we highlight recent findings concerning tau aggregation following phase separation in AD, along with the potential therapeutic strategies that could be explored in this process against the progression of this pathology.

(-)-Epigallocatechin-3-gallate, a Polyphenol from Green Tea, Regulates the Liquid-Liquid Phase Separation of Alzheimer's-Related Protein Tau

The microtubule-associated protein tau is involved in Alzheimer's disease and other tauopathies. Recently, tau has been shown to undergo liquid-liquid phase separation (LLPS), which is implicated in the physiological function and pathological aggregation of tau. In this report, we demonstrate that the green tea polyphenol (-)-epigallocatechin-3-gallate (EGCG) promotes the formation of liquid tau droplets at neutral pH by creating a network of hydrophobic interactions and hydrogen bonds, mainly with the proline-rich domain of tau. We further show that EGCG oxidation, tau phosphorylation, and the chemical structure of the polyphenol influence the efficacy of EGCG in facilitating tau LLPS. Complementary to the inhibitory activity of EGCG in tau fibrillization, our findings provide novel insights into the biological activity of EGCG and offer new clues for future studies on the molecular mechanism by which EGCG alleviates neurodegenerative diseases.

A Zn-dependent structural transition of SOD1 modulates its ability to undergo phase separation

The misfolding and mutation of Cu/Zn superoxide dismutase (SOD1) is commonly associated with amyotrophic lateral sclerosis (ALS). SOD1 can accumulate within stress granules (SGs), a type of membraneless organelle, which is believed to form via liquid-liquid phase separation (LLPS). Using wild-type, metal-deficient, and different ALS disease mutants of SOD1 and computer simulations, we report here that the absence of Zn leads to structural disorder within two loop regions of SOD1, triggering SOD1 LLPS and amyloid formation. The addition of exogenous Zn to either metal-free SOD1 or to the severe ALS mutation I113T leads to the stabilization of the loops and impairs SOD1 LLPS and aggregation. Moreover, partial Zn-mediated inhibition of LLPS was observed for another severe ALS mutant, G85R, which shows perturbed Zn-binding. By contrast, the ALS mutant G37R, which shows reduced Cu-binding, does not undergo LLPS. In addition, SOD1 condensates induced by Zn-depletion exhibit greater cellular toxicity than aggregates formed by prolonged incubation under aggregating conditions. Overall, our work establishes a role for Zn-dependent modulation of SOD1 conformation and LLPS properties that may contribute to amyloid formation.

Fused in sarcoma undergoes cold denaturation: Implications for phase separation

The mediation of liquid-liquid phase separation (LLPS) for fused in sarcoma (FUS) protein is generally attributed to the low-complexity, disordered domains and is enhanced at low temperature. The role of FUS folded domains on the LLPS process remains relatively unknown since most studies are mainly based on fragmented FUS domains. Here, we investigate the effect of metabolites on full-length (FL) FUS LLPS using turbidity assays and differential interference contrast (DIC) microscopy, and explore the behavior of the folded domains by nuclear magnetic resonance (NMR) spectroscopy. FL FUS LLPS is maximal at low concentrations of glucose and glutamate, moderate concentrations of NaCl, Zn²⁺ , and Ca²⁺ and at the isoelectric pH. The FUS RNA recognition motif (RRM) and zinc-finger (ZnF) domains are found to undergo cold denaturation above 0°C at a temperature that is determined by the conformational stability of the ZnF domain. Cold unfolding exposes buried nonpolar residues that can participate in LLPS-promoting hydrophobic interactions. Therefore, these findings constitute the first evidence that FUS globular domains may have an active role in LLPS under cold stress conditions and in the assembly of stress granules, providing further insight into the environmental regulation of LLPS.

Study of Tau Liquid-Liquid Phase Separation In Vitro

Aggregation of the microtubule-associated protein tau is one of the major pathogenic events in Alzheimer's disease and several other neurodegenerative disorders. Recent reports have demonstrated that purified tau can undergo liquid-liquid phase separation in vitro, forming liquid droplets. The protein within these droplets was also found to undergo accelerated transition to fibrillar aggregates, suggesting that LLPS may play an important role in pathological aggregation of tau in neurodegenerative disorders. Here, we describe several protocols for studying LLPS behavior of the recombinant full-length tau by turbidimetric and light microscopy-based methods.

Binding of two zinc ions promotes liquid-liquid phase separation of Tau

Tau is a naturally disordered microtubule associated protein which forms intraneuronal aggregates in several neurodegenerative diseases including Alzheimer's disease (AD). It was reported that zinc interaction with tau protein can trigger its aggregation. Recently we identified three zinc binding sites located in the N-terminal part, repeat region and the C-terminal part of tau. Here we characterized zinc binding to each of the three sites using isothermal titration calorimetry (ITC) and determined the impact of each site on aggregation using dynamic light scattering (DLS) assays. First, we confirmed the presence of three zinc binding sites on tau and determined the thermodynamic parameters of binding of zinc to these sites. We found a high-affinity zinc binding site located in the repeat region of tau and two N- and C-terminus binding sites with a lower binding constant for zinc. Second, we showed that tau aggregation necessitates zinc binding to the high affinity site in the R2R3 region, while LLPS necessitates zinc binding to any two binding sites. With regard to the role of zinc ions in the aggregation of proteins in neurodegenerative diseases, these findings bring new insights to the understanding of the aggregation mechanism of tau protein induced by zinc.

Rapid Reversible Osmoregulation of Cytoplasmic Biomolecular Condensates of Human Interferon-α-Induced Antiviral MxA GTPase

We previously discovered that exogenously expressed GFP-tagged cytoplasmic human myxovirus resistance protein (MxA), a major antiviral effector of Type I and III interferons (IFNs) against several RNA- and DNA-containing viruses, existed in the cytoplasm in phase-separated membraneless biomolecular condensates of varying sizes and shapes with osmotically regulated disassembly and reassembly. In this study we investigated whether cytoplasmic IFN-α-induced endogenous human MxA structures were also biomolecular condensates, displayed hypotonic osmoregulation and the mechanisms involved. Both IFN-α-induced endogenous MxA and exogenously expressed GFP-MxA formed cytoplasmic condensates in A549 lung and Huh7 hepatoma cells which rapidly disassembled within 1-2 min when cells were exposed to 1,6-hexanediol or to hypotonic buffer (~40-50 mOsm). Both reassembled into new structures within 1-2 min of shifting cells to isotonic culture medium (~330 mOsm). Strikingly, MxA condensates in cells continuously exposed to culture medium of moderate hypotonicity (in the range one-fourth, one-third or one-half isotonicity; range 90-175 mOsm) first rapidly disassembled within 1-3 min, and then, in most cells, spontaneously reassembled 7-15 min later into new structures. This spontaneous reassembly was inhibited by 2-deoxyglucose (thus, was ATP-dependent) and by dynasore (thus, required membrane internalization). Indeed, condensate reassembly was preceded by crowding of the cytosolic space by large vacuole-like dilations (VLDs) derived from internalized plasma membrane. Remarkably, the antiviral activity of GFP-MxA against vesicular stomatitis virus survived hypoosmolar disassembly and subsequent reassembly. The data highlight the exquisite osmosensitivity of MxA condensates, and the preservation of antiviral activity in the face of hypotonic stress.

Phase separation of the microtubule-associated protein tau

The aggregation and misfolding of the neuronal microtubule-associated protein tau is closely linked to the pathology of Alzheimer's disease and several other neurodegenerative diseases. Recent evidence suggest that tau undergoes liquid-liquid phase separation in vitro and forms or associates with membrane-less organelles in cells. Biomolecular condensation driven by phase separation can influence the biological activities of tau including its ability to polymerize tubulin into microtubules. In addition, the high concentrations that tau can reach in biomolecular condensates provide a mechanism to promote its aggregation and the formation of amyloid fibrils potentially contributing to the pathology of different tauopathies. Here, the authors discuss the role of tau phase separation in physiology and disease.

Modulating Nanodroplet Formation En Route to Fibrillization of Amyloid Peptides with Designed Flanking Sequences

Soluble oligomers populating early amyloid aggregation can be regarded as nanodroplets of liquid-liquid phase separation (LLPS). Amyloid peptides typically contain hydrophobic aggregation-prone regions connected by hydrophilic linkers and flanking sequences, and such a sequence hydropathy pattern drives the formation of supramolecular structures in the nanodroplets and modulates subsequent fibrillization. Here, we studied LLPS and fibrillization of coarse-grained amyloid peptides with increasing flanking sequences. Nanodroplets assumed lamellar, cylindrical micellar, and spherical micellar structures with increasing peptide hydrophilic/hydrophobic ratios, and such morphologies governed subsequent fibrillization processes. Adding glycine-serine repeats as flanking sequences to Aβ16-22, the amyloidogenic core of amyloid-β, our computational predictions of morphological transitions were corroborated experimentally. The uncovered inter-relationships between the peptide sequence pattern, oligomer/nanodroplet morphology, and fibrillization pathway, kinetics, and structure may contribute to our understanding of pathogenic amyloidosis in aging, facilitate future efforts ameliorating amyloidosis through peptide engineering, and aid in the design of novel amyloid-based functional nanobiomaterials and nanocomposites.

The Role of Intrinsically Disordered Proteins in Liquid–Liquid Phase Separation during Calcium Carbonate Biomineralization

Some animal organs contain mineralized tissues. These so-called hard tissues are mostly deposits of calcium salts, usually in the form of calcium phosphate or calcium carbonate. Examples of this include fish otoliths and mammalian otoconia, which are found in the inner ear, and they are an essential part of the sensory system that maintains body balance. The composition of ear stones is quite well known, but the role of individual components in the nucleation and growth of these biominerals is enigmatic. It is sure that intrinsically disordered proteins (IDPs) play an important role in this aspect. They have an impact on the shape and size of otoliths. It seems probable that IDPs, with their inherent ability to phase separate, also play a role in nucleation processes. This review discusses the major theories on the mechanisms of biomineral nucleation with a focus on the importance of protein-driven liquid–liquid phase separation (LLPS). It also presents the current understanding of the role of IDPs in the formation of calcium carbonate biominerals and predicts their potential ability to drive LLPS.

When liquid-liquid phase separation meets viral infections

Eukaryotic cells have both membranous and membraneless organelles. While the formation mechanism of membranous organelles is well understood, the formation mechanism of membraneless organelles remains unknown. Many biomolecules in the cytoplasm transition from the liquid phase to the agglutinated phase are known as liquid-liquid phase separation (LLPS). The biomolecular agglomerates’ physical properties enable them to function as dynamic compartments that respond to external pressures and stimuli. Scientists have gradually recognized the importance of phase separation during viral infections. LLPS provides a powerful new framework for understanding the viral life cycle from viral replication to evasion of host immune surveillance. As a result, this review focuses on the progress of LLPS research in viral infection and immune regulation to provide clues for antiviral therapeutic strategies.

Role of micronutrients in Alzheimer's disease: Review of available evidence

Alzheimer's disease (AD) is one of the most common age-related neurodegenerative disorders that have been studied for more than 100 years. Although an increased level of amyloid precursor protein is considered a key contributor to the development of AD, the exact pathogenic mechanism remains known. Multiple factors are related to AD, such as genetic factors, aging, lifestyle, and nutrients. Both epidemiological and clinical evidence has shown that the levels of micronutrients, such as copper, zinc, and iron, are closely related to the development of AD. In this review, we summarize the roles of eight micronutrients, including copper, zinc, iron, selenium, silicon, manganese, arsenic, and vitamin D in AD based on recently published studies.

Regulation of TIA-1 Condensates: Zn2+ and RGG Motifs Promote Nucleic Acid Driven LLPS and Inhibit Irreversible Aggregation

Stress granules are non-membrane bound RNA-protein granules essential for survival during acute cellular stress. TIA-1 is a key protein in the formation of stress granules that undergoes liquid-liquid phase separation by association with specific RNAs and protein-protein interactions. However, the fundamental properties of the TIA-1 protein that enable phase-separation also render TIA-1 susceptible to the formation of irreversible fibrillar aggregates. Despite this, within physiological stress granules, TIA-1 is not present as fibrils, pointing to additional factors within the cell that prevent TIA-1 aggregation. Here we show that heterotypic interactions with stress granule co-factors Zn2+ and RGG-rich regions from FUS each act together with nucleic acid to induce the liquid-liquid phase separation of TIA-1. In contrast, these co-factors do not enhance nucleic acid induced fibril formation of TIA-1, but rather robustly inhibit the process. NMR titration experiments revealed specific interactions between Zn2+ and H94 and H96 in RRM2 of TIA-1. Strikingly, this interaction promotes multimerization of TIA-1 independently of the prion-like domain. Thus, through different molecular mechanisms, these stress granule co-factors promote TIA-1 liquid-liquid phase separation and suppress fibrillar aggregates, potentially contributing to the dynamic nature of stress granules and the cellular protection that they provide.

Myricetin Inhibits α-Synuclein Amyloid Aggregation by Delaying the Liquid-to-Solid Phase Transition

The aggregation of α-synuclein (α-Syn) is a critical pathological hallmark of Parkinson's disease (PD). Prevention of α-Syn aggregation has become a key strategy for treating PD. Recent studies have suggested that α-Syn undergoes liquid-liquid phase separation (LLPS) to facilitate nucleation and amyloid formation. Here, we examined the modulation of α-Syn aggregation by myricetin, a polyhydroxyflavonol compound, under the conditions of LLPS. Unexpectedly, neither the initial morphology nor the phase-separated fraction of α-Syn was altered by myricetin. However, the dynamics of α-Syn condensates decreased upon myricetin binding. Further studies showed that myricetin dose-dependently inhibits amyloid aggregation in the condensates by delaying the liquid-to-solid phase transition. In addition, myricetin could disassemble the preformed α-Syn amyloid aggregates matured from the condensates. Together, our study shows that myricetin inhibits α-Syn amyloid aggregation in the condensates by retarding the liquid-to-solid phase transition and reveals that α-Syn phase transition can be targeted to inhibit amyloid aggregation.

In vitro characterization and molecular dynamics simulation reveal mechanism of 14-3-3ζ regulated phase separation of the tau protein

As a major microtubule-associated protein, tau is involved in the assembly of microtubules in the central nervous system. However, under pathological conditions tau assembles into amyloid filaments. Liquid droplets formed by liquid-liquid phase separation (LLPS) are a recently identified assembly state of tau and may have a major effect on the physiological function of tau and the formation of tau aggregates. 14-3-3 proteins are ubiquitously expressed in various tissues and regulate a wide variety of biological processes. In this work, we demonstrate that 14-3-3ζ is recruited into tau droplets and regulates tau LLPS by in vitro assays. While the mobility of tau molecules inside the droplets is not affected in the presence of 14-3-3ζ, the amount and size of droplets can vary significantly. Mechanistic studies reveal that 14-3-3ζ regulates tau LLPS by electrostatic interactions and hydrophobic interactions with the proline-rich domain and the microtubule-binding domain of tau. Surprisingly, the disordered C-terminal tail rather than the amphipathic binding groove of 14-3-3ζ plays a key role. Our findings not only provide a novel dimension to understand the interactions between 14-3-3 proteins and tau, but also suggest that 14-3-3 proteins may play an important role in regulating the LLPS of their binding partners.

Phase separation and zinc-induced transition modulate synaptic distribution and association of autism-linked CTTNBP2 and SHANK3

Many synaptic proteins form biological condensates via liquid-liquid phase separation (LLPS). Synaptopathy, a key feature of autism spectrum disorders (ASD), is likely relevant to the impaired phase separation and/or transition of ASD-linked synaptic proteins. Here, we report that LLPS and zinc-induced liquid-to-gel phase transition regulate the synaptic distribution and protein-protein interaction of cortactin-binding protein 2 (CTTNBP2), an ASD-linked protein. CTTNBP2 forms self-assembled condensates through its C-terminal intrinsically disordered region and facilitates SHANK3 co-condensation at dendritic spines. Zinc binds the N-terminal coiled-coil region of CTTNBP2, promoting higher-order assemblies. Consequently, it leads to reduce CTTNBP2 mobility and enhance the stability and synaptic retention of CTTNBP2 condensates. Moreover, ASD-linked mutations alter condensate formation and synaptic retention of CTTNBP2 and impair mouse social behaviors, which are all ameliorated by zinc supplementation. Our study suggests the relevance of condensate formation and zinc-induced phase transition to the synaptic distribution and function of ASD-linked proteins.

Autism impacts synapses. This study reports that autism-linked mutations of CTTNBP2 regulate phase separation to control synaptic enrichment of that protein. A zinc-induced liquid-to-gel transition improves synaptic retention of CTTNBP2 and SHANK3.

Deciphering the Structure and Formation of Amyloids in Neurodegenerative Diseases With Chemical Biology Tools

Protein aggregation into highly ordered, regularly repeated cross-β sheet structures called amyloid fibrils is closely associated to human disorders such as neurodegenerative diseases including Alzheimer's and Parkinson's diseases, or systemic diseases like type II diabetes. Yet, in some cases, such as the HET-s prion, amyloids have biological functions. High-resolution structures of amyloids fibrils from cryo-electron microscopy have very recently highlighted their ultrastructural organization and polymorphisms. However, the molecular mechanisms and the role of co-factors (posttranslational modifications, non-proteinaceous components and other proteins) acting on the fibril formation are still poorly understood. Whether amyloid fibrils play a toxic or protective role in the pathogenesis of neurodegenerative diseases remains to be elucidated. Furthermore, such aberrant protein-protein interactions challenge the search of small-molecule drugs or immunotherapy approaches targeting amyloid formation. In this review, we describe how chemical biology tools contribute to new insights on the mode of action of amyloidogenic proteins and peptides, defining their structural signature and aggregation pathways by capturing their molecular details and conformational heterogeneity. Challenging the imagination of scientists, this constantly expanding field provides crucial tools to unravel mechanistic detail of amyloid formation such as semisynthetic proteins and small-molecule sensors of conformational changes and/or aggregation. Protein engineering methods and bioorthogonal chemistry for the introduction of protein chemical modifications are additional fruitful strategies to tackle the challenge of understanding amyloid formation.

α-Synuclein phase separation and amyloid aggregation are modulated by C-terminal truncations

The aggregation of α-synuclein (α-Syn) is a key pathological hallmark of Parkinson's disease (PD). α-Syn undergoes liquid-liquid phase separation (LLPS) to drive amyloid aggregation. How the LLPS of α-Syn is regulated remains largely unknown. Here, we discovered that the C-terminal region modulates α-Syn phase separation through electrostatic interactions. The wild-type (WT) and PD disease-related truncated α-Syn can co-exist in the condensates. The truncated α-Syn could dramatically promote WT α-Syn phase separation. Further studies demonstrated that the truncated α-Syn accelerated WT α-Syn turning to amyloid aggregates by modulation of phase separation. Together, our findings disclose the role of the C-terminal domain in the LLPS of α-Syn and pave the path for understanding the mechanism of truncated α-Syn in PD pathology.

Identification of the three zinc-binding sites on tau protein

Tau protein has been extensively studied due to its key roles in microtubular cytoskeleton regulation and in the formation of aggregates found in some neurodegenerative diseases. Recently it has been shown that zinc is able to induce tau aggregation by interacting with several binding sites. However, the precise location of these sites and the molecular mechanism of zinc-induced aggregation remain unknown. Here we used Nuclear Magnetic Resonance (NMR) to identify zinc binding sites on tau. These experiments revealed three distinct zinc binding sites on tau, located in the N-terminal part, the repeat region and the C-terminal part. Further analysis enabled us to show that the N-terminal and the C-terminal sites are independent of each other. Using molecular simulations, we proposed a model of each site in a complex with zinc. Given the clinical importance of zinc in tau aggregation, our findings pave the way for designing potential therapies for tauopathies.