Biology subjects

Mok, S.-A.

Publications and source records attributed to Mok, S.-A..

14 recordsLinked to original sources

bioRxiv2026

TRAPPC8 Is an Endogenous Brake on UFMylation That Suppresses Tauopathy

UFMylation, a ubiquitin-like protein modification, drives tau spread through the brain, but what keeps this process in check has remained unclear. We show that TRAPPC8 acts as a natural brake on UFMylation, binding directly to the E1 enzyme UBA5 to dampen pathway activity. In Alzheimer's disease brain tissue, TRAPPC8 is reduced while UFMylation is elevated, suggesting this brake fails as disease progresses. Sustaining UFMylation in human iPSC-derived neurons increased tau aggregation and spread while disrupting lysosomal function and lipid balance. Restoring TRAPPC8-UBA5 binding reversed these defects through the lysosomal protein CLN8, which restored lysosomal function and reduced tau pathology. Notably, expressing just the UBA5-binding region of TRAPPC8 was enough to suppress tau pathology in vivo, marking it as a promising therapeutic target.

neuroscience↗

bioRxiv2025

STING_IFN_CH25H lipid axis links innate immune activation to tau pathology

Genetic risk for Alzheimers disease is strongly enriched in pathways governing microglial activation and cholesterol metabolism, yet how these processes converge to drive neurodegeneration remains unclear. Here, we identify the oxysterol 25-hydroxycholesterol (25- HC) as a pathogenic lipid downstream of the cGAS-STING-IFN pathway. In models of tau pathology, interferon signaling induces Cholesterol 25-hydroxylase (CH25H) expression in microglia. Ch25h deletion in female P301S mice suppressed tau aggregation, preserved synapses, prevented brain atrophy, and rescued memory. Mechanistically, loss of CH25H disrupted STING trafficking, attenuated IFN activation, dampened self-perpetuating microglial inflammation. Strikingly, 25-HC directly accelerated tau propagation in human iPSC derived neurons. It also disrupted lysosomal and mitochondrial lipid composition, driving cholesteryl ester accumulation and promoting apoptosis under tau-induced stress. These findings define a STING-IFN-CH25H lipid axis that bridges innate immune activation to tau pathology and toxicity, offering a tractable therapeutic pathway for inflammation-driven neurodegenerative conditions.

neuroscience↗

bioRxiv2025

A metabolite extracted from E. coli suppresses tau aggregation

Tau aggregation is a key pathological feature of neurodegenerative diseases termed tauopathies. Identifying the various cellular factors that function to prevent tau aggregation in cells can generate key insights into how to mitigate diseases associated with protein misfolding. During an investigation into developing purification methods for the protein tau, we observed that isolates of E. coli lysate prevented human tau aggregation in vitro. Fractionation of the lysate was used to further isolate a small molecular weight (MW) inhibitory fraction containing multiple components, as determined by mass spectrometry and NMR. A putative inhibitory component, methylphosphonic acid (MePn), decreased tau amyloid formation when supplemented to in vitro aggregation assays. MePn also blocked the aggregation of expressed tau in live E. coli when supplemented to the culture media. Our findings can be directly applied to optimizing purification of recombinant tau protein and more broadly, highlight the potential of cellular metabolites to directly modulate tau amyloid formation.

biochemistry↗

bioRxiv2025

Deciphering the mechanism of protein aggregation and effects of inhibitors using single-molecule mass photometry

Aggregation of misfolded proteins is a prominent feature of many diseases and hence an attractive drug target. However, the small oligomers that are critical early species in the aggregation cascade are difficult to monitor directly owing to their heterogeneity and transience, complicating efforts to define aggregation mechanisms and target oligomers therapeutically. Here, we observe changes in oligomer populations directly using single-molecule mass photometry (SMMP). Studying the pathogenic P301L mutant of tau protein linked to frontotemporal dementia, we globally fit the growth/decay kinetics for every oligomer population observed by SMMP to microscopic models of aggregation. A simple extension to the best-fit model also accounts for amyloid fibril kinetics, as monitored by Thioflavin T fluorescence, providing the first quantitative model of aggregation kinetics across all stages of the cascade based on direct observation. Crucially, we find that models fitting amyloid kinetics alone fail to capture oligomer behavior, implying that--contrary to standard practice--amyloid kinetics cannot be relied on to deduce aggregation mechanisms. Furthermore, there is no single rate-limiting nucleation step preceding rapid growth, as generally assumed, suggesting that standard models of aggregation are overly simplistic. Repeating the analysis in the presence of aggregation inhibitors allows identification of the discrete steps in the cascade affected by the inhibitors. This work presents a powerful new approach for defining protein aggregation mechanisms and the mechanism of action of inhibitors, with applications to understanding many diseases and developing novel therapeutics.

biophysics↗

bioRxiv2025

Gangliosides Modulate the Secretion of Extracellular Vesicles and Their Misfolded Protein Cargo

Gangliosides are glycosphingolipids that play an integral role in cell signaling and provide neuroprotection. While present on extracellular vesicles (EVs) - key mediators of intercellular communication - their role in EV biogenesis remains unclear. Here, we identify gangliosides, both endogenously synthesized and exogenously administered, as key modulators of EV biogenesis, with the specific composition of their glycan headgroup and the presence or absence of sialic acid and N-acetyl-D-galactosamine residues dictating whether they promote or inhibit EV biogenesis. We show that GM1 and other complex gangliosides enhance EV secretion, while disruption of ganglioside synthesis impairs it. GM1 supplementation restores EV secretion in Huntingtons disease (HD) fibroblasts and HD cell models that have lower than normal levels of gangliosides, and in cells with a genetic block of ganglioside synthesis that models rare early-onset neurodegenerative diseases. Notably, GM1 also enhances EV-mediated secretion of pathogenic misfolded proteins, including mutant huntingtin (mHTT), -synuclein and tau, reducing intracellular burden and providing mechanistic insight into the mHTT-lowering effects of GM1 treatments in HD models. Our findings shed light on the neuroprotective roles of gangliosides and highlight their potential for therapeutic exploitation in misfolded protein disorders.

cell biology↗

bioRxiv2024

APOE3-R136S mutation confers resilience against tau pathology via cGAS-STING-IFN inhibition

The Christchurch mutation (R136S) on the APOE3 (E3S/S) gene is associated with attenuation of tau load and cognitive decline despite the presence of a causal PSEN1 mutation and high levels of amyloid beta pathology in the carrier1. However, the specific molecular mechanisms enabling the E3S/S mutation to mitigate tau-induced neurodegeneration remain unclear. Here, we replaced mouse ApoE with wild-type human E3 or E3S/S on a tauopathy background. The R136S mutation markedly decreased tau load and protected against tau-induced synaptic loss, myelin loss, and reduction in theta and gamma powers. Additionally, the R136S mutation reduced interferon response to tau pathology in both mouse and human microglia, suppressing cGAS-STING activation. Treating tauopathy mice carrying wild-type E3 with a cGAS inhibitor protected against tau-induced synaptic loss and induced similar transcriptomic alterations to those induced by the R136S mutation across brain cell types. Thus, suppression of microglial cGAS-STING-IFN pathway plays a central role in mediating the protective effects of R136S against tauopathy. One-sentence summaryThe R136S mutation on APOE3 enhances resistance to tau-related pathology and toxicity by downregulating the cGAS-STING-IFN signaling pathway.

neuroscience↗

bioRxiv2023

DAP12 deficiency alters microglia-oligodendrocyte communication and enhances resilience against tau toxicity

Pathogenic tau accumulation fuels neurodegeneration in Alzheimers disease (AD). Enhancing aging brains resilience to tau pathology would lead to novel therapeutic strategies. DAP12 (DNAX-activation protein 12) is critically involved in microglial immune responses. Previous studies have showed that mice lacking DAP12 in tauopathy mice exhibit higher tau pathology but are protected from tau-induced cognitive deficits. However, the exact mechanism remains elusive. Our current study uncovers a novel resilience mechanism via microglial interaction with oligodendrocytes. Despite higher tau inclusions, Dap12 deletion curbs tau-induced brain inflammation and ameliorates myelin and synapse loss. Specifically, removal of Dap12 abolished tau-induced disease-associated clusters in microglia (MG) and intermediate oligodendrocytes (iOli), which are spatially correlated with tau pathology in AD brains. Our study highlights the critical role of interactions between microglia and oligodendrocytes in tau toxicity and DAP12 signaling as a promising target for enhancing resilience in AD.

neuroscience↗

bioRxiv2023

Human iPSC 4R tauopathy model uncovers modifiers of tau propagation

Tauopathies are age-associated neurodegenerative diseases whose mechanistic underpinnings remain elusive, partially due to lack of appropriate human models. Current human induced pluripotent stem cell (hiPSC)-derived neurons express very low levels of 4-repeat (4R)-tau isoforms that are normally expressed in adult brain. Here, we engineered new iPSC lines to express 4R-tau and 4R-tau carrying the P301S MAPT mutation when differentiated into neurons. 4R-P301S neurons display progressive Tau inclusions upon seeding with Tau fibrils and recapitulate features of tauopathy phenotypes, including shared transcriptomic signatures, autophagic body accumulation, and impaired neuronal activity. A CRISPRi screen of genes associated with Tau pathobiology identified over 500 genetic modifiers of Tau-seeding-induced Tau propagation, including retromer VPS29 and the UFMylation cascade as top modifiers. In AD brains, the UFMylation cascade is altered in neurofibrillary-tangle-bearing neurons. Inhibiting the UFMylation cascade suppressed seeding-induced Tau propagation. This model provides a powerful platform to identify novel therapeutic strategies for 4R tauopathy.

neuroscience↗

bioRxiv2023

Disease associated mutations in tau encode for changes in aggregate structure conformation

The accumulation of tau aggregates is associated with neurodegenerative diseases collectively known as tauopathies. Tau aggregates isolated from different tauopathies such as Alzheimers disease, corticobasal degeneration and progressive supranuclear palsy have distinct cryo-electron microscopy structures with respect to their packed fibril cores. To understand the mechanisms by which tau can be sensitized to form distinct aggregate conformations, we created a panel of tau variants encoding for individual disease-associated missense mutations in full-length 0N4R tau (wild-type and 36 mutants). We developed a high-throughput protein purification platform for direct comparison of tau variants in biochemical assays. Structural analysis of the protease-resistant core of tau aggregates formed in vitro reveals that mutations can promote aggregate core packing distinct from that produced by WT tau. Comparing aggregate structure changes with aggregation kinetic parameters for tau mutants revealed no clear linkage between these two aggregation properties. We also found that tau mutation-dependent alterations of tau aggregate structure are not readily explained by current tau fibril structure data. This is the first study to show the broad potential of tau mutations to alter the packed core structures contained within aggregated tau and sheds new insights into the molecular mechanisms underlying the formation of tau aggregate structures that may drive their associated pathology in disease.

biochemistry↗

bioRxiv2021

Tau interactome mapping reveals dynamic processes in synapses and mitochondria associated with neurodegenerative disease

Tau (MAPT) drives neuronal dysfunction in Alzheimers disease (AD) and other tauopathies. To dissect the underlying mechanisms, we combined an engineered ascorbic acid peroxidase (APEX) approach with quantitative affinity purification mass spectrometry (AP-MS) followed by proximity ligation assay (PLA) to characterize Tau interactomes modified by neuronal activity and mutations that cause frontotemporal dementia (FTD) in human induced pluripotent stem cell (iPSC)-derived neurons. We established activity-dependent interactions of Tau with presynaptic vesicle proteins during Tau secretion and mapped the exact APEX-tau-induced biotinylated tyrosines to the cytosolic domains of the interacting vesicular proteins. We showed that FTD mutations impair bioenergetics and markedly diminished Taus interaction with mitochondria proteins, which were downregulated in AD brains of multiple cohorts and correlated with disease severity. These multi-modal and dynamic Tau interactomes with unprecedented spatiotemporal resolution shed novel insights into Taus role in neuronal function and disease-related processes with potential therapeutic targets to block Tau-mediated pathogenesis.

neuroscience↗

bioRxiv2021

Microglial NF-κB drives tau spreading and toxicity in a mouse model of tauopathy

Activation of microglia, the brains innate immune cells, is a prominent pathological feature in tauopathies, including Alzheimers disease. How microglia activation contributes to tau toxicity remains largely unknown. Here we show that nuclear factor kappa-light-chain-enhancer of activated B cells (NF-{kappa}B) signaling, activated by tau, drives microglial-mediated tau propagation and toxicity. Constitutive activation of microglial NF-{kappa}B exacerbated, while inactivation diminished, tau seeding and spreading in PS19 mice, consistent with the observation that NF-{kappa}B activation accelerates processing of internalized tau fibrils in primary microglia. Remarkably, inhibition of microglial NF-{kappa}B specifically also rescued tau-mediated learning and memory deficits, and restored overall transcriptomic changes while increasing tau inclusions. On a single cell level, we discovered that tau-associated disease states in microglia were diminished by NF-{kappa}B inactivation and further transformed by constitutive NF-{kappa}B activation. Our study establishes a central role for microglial NF-{kappa}B signaling in mediating tau toxicity in tauopathy.

neuroscience↗

bioRxiv2020

Tau conformers in FTLD-MAPT undergo liquid-liquid phase separation and perturb the nuclear envelope

Recent studies show that a single MAPT gene mutation can promote alternative tau misfolding pathways engendering divergent forms of frontotemporal dementia and that under conditions of molecular crowding, the repertoire of tau forms can include liquid-liquid phase separation (LLPS). We show here that following pathogenic seeding, tau condenses on the nuclear envelope (NE) and disrupts nuclear-cytoplasmic transport (NCT). Interestingly, NE fluorescent tau signals and small fluorescent inclusions behaved as demixed liquid droplets in living cells. Thioflavin S-positive intracellular aggregates were prevalent in tau-derived inclusions with a size bigger than 3 m2, indicating that a threshold of critical mass in the liquid state condensation may drive liquid-solid phase transitions. Our findings indicate that tau undergoing LLPS is more toxic amongst a spectrum of alternative conformers; LLPS droplets on the NE that disrupt NCT serve to trigger cell death and can act as nurseries for fibrillar structures abundantly detected in end-stage disease.

neuroscience↗

bioRxiv2019

Compromised function of an ESCRT complex promotes endolysosomal escape of tau seeds and propagation of tau aggregation

Intercellular propagation of protein aggregation is emerging as a key mechanism in the progression of several neurodegenerative diseases, including Alzheimers Disease and frontotemporal dementia. However, we lack a systematic understanding of the cellular path-ways controlling prion-like propagation. To uncover such pathways, we performed CRISPR interference (CRISPRi) screens in a human cell-based model of propagation of tau aggregation. Our screens uncovered that knockdown of several components of the ESCRT machinery, including CHMP6, or CHMP2A in combination with CHMP2B (a gene linked to familial fronto-temporal dementia), promote propagation of tau aggregation. We found that knockdown of these genes caused damage to endolysosomal membranes, consistent with a role for the ESCRT pathway in endolysosomal membrane repair. Leakiness of the endolysosomal compartment significantly enhanced prion-like propagation of tau aggregation, likely by making tau seeds more available to pools of cytoplasmic tau. Together, these findings suggest that endolysosomal escape is a critical step in tau propagation.

cell biology↗

bioRxiv2018

Tau Secretion and Propagation Is Regulated by p300/CBP via Autophagy-Lysosomal Pathway in Tauopathy

The trans-neuronal propagation of tau has been implicated in the progression of tau-mediated neurodegeneration. Tau secretion from neurons is the first step in tau transmission, but little is known about the cellular mechanism. Here, we report that p300/CBP, the lysine acetyltransferase that acetylates tau and regulates its homeostasis and toxicity, serves as a key regulator of tau secretion by inhibiting the autophagy-lysosomal pathway (ALP). Increased p300/CBP activity was associated with impaired function of this pathway in a tau transgenic mouse model. p300/CBP hyperactivation increased tau secretion by blocking autophagic flux. Conversely, inhibiting p300/CBP genetically or pharmacologically promoted autophagic flux, and reduced tau accumulation, tau secretion, and tau propagation in fibril-induced tau spreading models in vitro and in vivo. Our findings show that p300/CBP-induced impairment in the ALP underlies excessive unconventional secretion and pathogenic spread of tau.

neuroscience↗