TDP-43 Oligomerization and Phase Separation Properties Are Necessary for Autoregulation

Mutations in human prion-like domains: pathogenic but not always amyloidogenic

Heterogeneous nuclear ribonucleoproteins (hnRNPs) are multifunctional proteins with integral roles in RNA metabolism and the regulation of alternative splicing. These proteins typically contain prion-like domains of low complexity (PrLDs or LCDs) that govern their assembly into either functional or pathological amyloid fibrils. To date, over 60 mutations targeting the LCDs of hnRNPs have been identified and associated with a spectrum of neurodegenerative diseases including amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and Alzheimer's disease (AD). The cryo-EM structures of pathological and functional fibrils formed by different hnRNPs have been recently elucidated, including those of hnRNPA1, hnRNPA2, hnRNPDL-2, TDP-43, and FUS. In this review, we discuss the structural features of these amyloid assemblies, placing particular emphasis on scrutinizing the impact of prevalent disease-associated mutations mapping within their LCDs. By performing systematic energy calculations, we reveal a prevailing trend of destabilizing effects induced by these mutations in the amyloid structure, challenging the traditionally assumed correlation between pathogenicity and amyloidogenic propensity. Understanding the molecular basis of this discrepancy might provide insights for developing targeted therapeutic strategies to combat hnRNP-associated diseases.

DNA-Mediated Formation of Phase-Separated Coacervates of the Nucleic Acid-Binding Domain of TAR DNA-Binding Protein (TDP-43) Prevents Its Amyloid-Like Misfolding

Sequestration of protein molecules and nucleic acids to stress granules is one of the most promising strategies that cells employ to protect themselves from stress. In vitro, studies suggest that the nucleic acid-binding domain of TDP-43 (TDP-43tRRM) undergoes amyloid-like aggregation to β-sheet-rich structures in low pH stress. In contrast, we observed that the TDP-43tRRM undergoes complex coacervation in the presence of ssDNA to a dense and light phase, preventing its amyloid-like aggregation. The soluble light phase consists of monomeric native-like TDP-43tRRM. The microscopic data suggest that the dense phase consists of spherical coacervates with limited internal dynamics. We performed multiparametric analysis by employing various biophysical techniques and found that complex coacervation depends on the concentration and ratio of the participating biomolecules and is driven by multivalent interactions. The modulation of these forces due to environmental conditions or disease mutations regulates the extent of coacervation, and the weakening of interactions between TDP-43tRRM and ssDNA leads to amyloid-like aggregation of TDP-43tRRM. Our results highlight a competition among the native state, amyloid-like aggregates, and complex coacervates tuned by various environmental factors. Together, our results illuminate an alternate function of TDP-43tRRM in response to pH stress in the presence of the ssDNA.

Decoding Missense Variants by Incorporating Phase Separation via Machine Learning

Computational models have made significant progress in predicting the effect of protein variants. However, deciphering numerous variants of uncertain significance (VUS) located within intrinsically disordered regions (IDRs) remains challenging. To address this issue, we introduce phase separation, which is tightly linked to IDRs, into the investigation of missense variants. Phase separation is vital for multiple physiological processes. By leveraging missense variants that alter phase separation propensity, we develop a machine learning approach named PSMutPred to predict the impact of missense mutations on phase separation. PSMutPred demonstrates robust performance in predicting missense variants that affect natural phase separation. In vitro experiments further underscore its validity. By applying PSMutPred on over 522,000 ClinVar missense variants, it significantly contributes to decoding the pathogenesis of disease variants, especially those in IDRs. Our work provides insights into the understanding of a vast number of VUSs in IDRs, expediting clinical interpretation and diagnosis.

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.

TDP-43 in nuclear condensates: where, how, and why

TDP-43 is an abundant and ubiquitously expressed nuclear protein that becomes dysfunctional in a spectrum of neurodegenerative diseases. TDP-43's ability to phase separate and form/enter biomolecular condensates of varying size and composition is critical for its functionality. Despite the high density of phase-separated assemblies in the nucleus and the nuclear abundance of TDP-43, our understanding of the condensate-TDP-43 relationship in this cellular compartment is only emerging. Recent studies have also suggested that misregulation of nuclear TDP-43 condensation is an early event in the neurodegenerative disease amyotrophic lateral sclerosis. This review aims to draw attention to the nuclear facet of functional and aberrant TDP-43 condensation. We will summarise the current knowledge on how TDP-43 containing nuclear condensates form and function and how their homeostasis is affected in disease.

Structural details of helix-mediated TDP-43 C-terminal domain multimerization

The primarily disordered C-terminal domain (CTD) of TAR DNA binding protein-43 (TDP-43), a key nuclear protein in RNA metabolism, forms neuronal inclusions in several neurodegenerative diseases. A conserved region (CR, spanning residues 319-341) in CTD forms transient helix-helix contacts important for its higher-order oligomerization and function that are disrupted by ALS-associated mutations. However, the structural details of CR assembly and the explanation for several ALS-associated variants' impact on phase separation and function remain unclear due to challenges in analyzing the dynamic association of TDP-43 CTD using traditional structural biology approaches. By employing an integrative approach, combining biophysical experiments, biochemical assays, AlphaFold2-Multimer (AF2-Multimer), and atomistic simulations, we generated structural models of helical oligomerization of TDP-43 CR. Using NMR, we first established that the native state of TDP-43 CR under physiological conditions is α-helical. Next, alanine scanning mutagenesis revealed that while hydrophobic residues in the CR are important for CR assembly, phase separation and TDP-43 nuclear retention function, polar residues down regulate these processes. Finally, pairing AF2-Multimer modeling with AAMD simulations indicated that dynamic, oligomeric assemblies of TDP-43 that are stabilized by a methionine-rich core with specific contributions from a tryptophan/leucine pair. In conclusion, our results advance the structural understanding of the mechanisms driving TDP-43 function and provide a window into the initial stages of its conversion into pathogenic aggregates.

TDP43 autoregulation gives rise to shortened isoforms that are tightly controlled by both transcriptional and post-translational mechanisms

The nuclear RNA-binding protein TDP43 is integrally involved in the pathogenesis of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). Previous studies uncovered N-terminal TDP43 isoforms that are predominantly cytosolic in localization, highly prone to aggregation, and enriched in susceptible spinal motor neurons. In healthy cells, however, these shortened (s)TDP43 isoforms are difficult to detect in comparison to full-length (fl)TDP43, raising questions regarding their origin and selective regulation. Here, we show that sTDP43 is created as a byproduct of TDP43 autoregulation and cleared by nonsense mediated RNA decay (NMD). The sTDP43-encoding transcripts that escape NMD can lead to toxicity but are rapidly degraded post-translationally. Circumventing these regulatory mechanisms by overexpressing sTDP43 results in neurodegeneration in vitro and in vivo via N-terminal oligomerization and impairment of flTDP43 splicing activity, in addition to RNA binding-dependent gain-of-function toxicity. Collectively, these studies highlight endogenous mechanisms that tightly regulate sTDP43 expression and provide insight into the consequences of aberrant sTDP43 accumulation in disease.

Multivalent GU-rich oligonucleotides sequester TDP-43 in the nucleus by inducing high molecular weight RNP complexes

TDP-43 nuclear clearance and cytoplasmic aggregation are hallmarks of TDP-43 proteinopathies. We recently demonstrated that binding to endogenous nuclear GU-rich RNAs sequesters TDP-43 in the nucleus by restricting its passive nuclear export. Here, we tested the feasibility of synthetic RNA oligonucleotide-mediated augmentation of TDP-43 nuclear localization. Using biochemical assays, we compared the ability of GU-rich oligonucleotides to engage in multivalent, RRM-dependent binding with TDP-43. When transfected into cells, (GU)16 attenuated TDP-43 mislocalization induced by transcriptional blockade or RanGAP1 ablation. Clip34nt and (GU)16 accelerated TDP-43 nuclear re-import after cytoplasmic mislocalization. RNA pulldowns confirmed that multivalent GU-oligonucleotides induced high molecular weight RNP complexes, incorporating TDP-43 and possibly other GU-binding proteins. Transfected GU-repeat oligos disrupted TDP-43 cryptic exon repression, likely by diverting TDP-43 from endogenous RNAs, except for Clip34nt that contains interspersed A and C. Thus, exogenous multivalent GU-RNAs can promote TDP-43 nuclear localization, though pure GU-repeat motifs impair TDP-43 function.

Intra-condensate demixing of TDP-43 inside stress granules generates pathological aggregates

Cytosolic aggregation of the nuclear protein TDP-43 is associated with many neurodegenerative diseases, but the triggers for TDP-43 aggregation are still debated. Here, we demonstrate that TDP-43 aggregation requires a double event. One is up-concentration in stress granules beyond a threshold, and the other is oxidative stress. These two events collectively induce intra-condensate demixing, giving rise to a dynamic TDP-43 enriched phase within stress granules, which subsequently transitions into pathological aggregates. Mechanistically, intra-condensate demixing is triggered by local unfolding of the RRM1 domain for intermolecular disulfide bond formation and by increased hydrophobic patch interactions in the C-terminal domain. By engineering TDP-43 variants resistant to intra-condensate demixing, we successfully eliminate pathological TDP-43 aggregates in cells. We conclude that up-concentration inside condensates and simultaneous exposure to environmental stress could be a general pathway for protein aggregation, with intra-condensate demixing constituting a key intermediate step.

RNA-binding properties orchestrate TDP-43 homeostasis through condensate formation in vivo

Insoluble cytoplasmic aggregate formation of the RNA-binding protein TDP-43 is a major hallmark of neurodegenerative diseases including Amyotrophic Lateral Sclerosis. TDP-43 localizes predominantly in the nucleus, arranging itself into dynamic condensates through liquid-liquid phase separation (LLPS). Mutations and post-translational modifications can alter the condensation properties of TDP-43, contributing to the transition of liquid-like biomolecular condensates into solid-like aggregates. However, to date it has been a challenge to study the dynamics of this process in vivo. We demonstrate through live imaging that human TDP-43 undergoes nuclear condensation in spinal motor neurons in a living animal. RNA-binding deficiencies as well as post-translational modifications can lead to aberrant condensation and altered TDP-43 compartmentalization. Single-molecule tracking revealed an altered mobility profile for RNA-binding deficient TDP-43. Overall, these results provide a critically needed in vivo characterization of TDP-43 condensation, demonstrate phase separation as an important regulatory mechanism of TDP-43 accessibility, and identify a molecular mechanism of how functional TDP-43 can be regulated.

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.

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.

Depletion of Mettl3 in cholinergic neurons causes adult-onset neuromuscular degeneration

Motor neuron (MN) demise is a hallmark of several neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS). Post-transcriptional gene regulation can control RNA's fate, and defects in RNA processing are critical determinants of MN degeneration. N6-methyladenosine (m6A) is a post-transcriptional RNA modification that controls diverse aspects of RNA metabolism. To assess the m6A requirement in MNs, we depleted the m6A methyltransferase-like 3 (METTL3) in cells and mice. METTL3 depletion in embryonic stem cell-derived MNs has profound and selective effects on survival and neurite outgrowth. Mice with cholinergic neuron-specific METTL3 depletion display a progressive decline in motor behavior, accompanied by MN loss and muscle denervation, culminating in paralysis and death. Reader proteins convey m6A effects, and their silencing phenocopies METTL3 depletion. Among the m6A targets, we identified transactive response DNA-binding protein 43 (TDP-43) and discovered that its expression is under epitranscriptomic control. Thus, impaired m6A signaling disrupts MN homeostasis and triggers neurodegeneration conceivably through TDP-43 deregulation.

Multivalent GU-rich oligonucleotides sequester TDP-43 in the nucleus by inducing high molecular weight RNP complexes

TDP-43 nuclear clearance and cytoplasmic aggregation are hallmarks of TDP-43 proteinopathies. We recently demonstrated that binding to endogenous nuclear GU-rich RNAs sequesters TDP-43 in the nucleus by restricting its passive nuclear export. Here, we tested the feasibility of synthetic RNA oligonucleotide-mediated augmentation of TDP-43 nuclear localization. Using biochemical assays, we compared the ability of GU-rich oligonucleotides to engage in multivalent, RRM-dependent binding with TDP-43. When transfected into cells, (GU)16 attenuated TDP-43 mislocalization induced by transcriptional blockade or RanGAP1 ablation. Clip34nt and (GU)16 accelerated TDP-43 nuclear re-import after cytoplasmic mislocalization. RNA pulldowns confirmed that multivalent GU-oligonucleotides induced high molecular weight RNP complexes, incorporating TDP-43 and possibly other GU-binding proteins. Transfected GU-repeat oligos disrupted TDP-43 cryptic exon repression, likely by diverting TDP-43 from endogenous RNAs, except for Clip34nt which contains interspersed A and C. Thus, exogenous multivalent GU-RNAs can promote TDP-43 nuclear localization, though pure GU-repeat motifs impair TDP-43 function.

The ALS/FTD-related C9orf72 hexanucleotide repeat expansion forms RNA condensates through multimolecular G-quadruplexes

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are neurodegenerative diseases that exist on a clinico-pathogenetic spectrum, designated ALS/FTD. The most common genetic cause of ALS/FTD is expansion of the intronic hexanucleotide repeat (GGGGCC)n in C9orf72. Here, we investigate the formation of nucleic acid secondary structures in these expansion repeats, and their role in generating condensates characteristic of ALS/FTD. We observe significant aggregation of the hexanucleotide sequence (GGGGCC)n, which we associate to the formation of multimolecular G-quadruplexes (mG4s) by using a range of biophysical techniques. Exposing the condensates to G4-unfolding conditions leads to prompt disassembly, highlighting the key role of mG4-formation in the condensation process. We further validate the biological relevance of our findings by detecting an increased prevalence of G4-structures in C9orf72 mutant human motor neurons when compared to healthy motor neurons by staining with a G4-selective fluorescent probe, revealing signal in putative condensates. Our findings strongly suggest that RNA G-rich repetitive sequences can form protein-free condensates sustained by multimolecular G-quadruplexes, highlighting their potential relevance as therapeutic targets for C9orf72 mutation-related ALS/FTD.

Proteomics elucidating physiological and pathological functions of TDP-43

Trans-activation response DNA binding protein of 43 kDa (TDP-43) regulates a great variety of cellular processes in the nucleus and cytosol. In addition, a defined subset of neurodegenerative diseases is characterized by nuclear depletion of TDP-43 as well as cytosolic mislocalization and aggregation. To perform its diverse functions TDP-43 can associate with different ribonucleoprotein complexes. Combined with transcriptomics, MS interactome studies have unveiled associations between TDP-43 and the spliceosome machinery, polysomes and RNA granules. Moreover, the highly dynamic, low-valency interactions regulated by its low-complexity domain calls for innovative proximity labeling methodologies. In addition to protein partners, the analysis of post-translational modifications showed that they may play a role in the nucleocytoplasmic shuttling, RNA binding, liquid-liquid phase separation and protein aggregation of TDP-43. Here we review the various TDP-43 ribonucleoprotein complexes characterized so far, how they contribute to the diverse functions of TDP-43, and roles of post-translational modifications. Further understanding of the fluid dynamic properties of TDP-43 in ribonucleoprotein complexes, RNA granules, and self-assemblies will advance the understanding of RNA processing in cells and perhaps help to develop novel therapeutic approaches for TDPopathies.

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.

Uncovering Critical Roles for RNA in Neurodegeneration

RNA-binding proteins, in particular TDP-43, are key players in neurodegenerative disorders, mainly amyotrophic lateral sclerosis and frontotemporal dementia. We aim to elucidate how TDP-43 dysfunction alters cell metabolism and to identify mechanisms linked to aberrant behavior. We find that RNA binding plays a key role in maintaining TDP-43 homeostasis and in controlling cellular organization, two processes of essential importance to TDP-43 pathology. This research will provide insight into pathogenesis and help develop therapeutic interventions.

RNA-mediated ribonucleoprotein assembly controls TDP-43 nuclear retention

TDP-43 is an essential RNA-binding protein strongly implicated in the pathogenesis of neurodegenerative disorders characterized by cytoplasmic aggregates and loss of nuclear TDP-43. The protein shuttles between nucleus and cytoplasm, yet maintaining predominantly nuclear TDP-43 localization is important for TDP-43 function and for inhibiting cytoplasmic aggregation. We previously demonstrated that specific RNA binding mediates TDP-43 self-assembly and biomolecular condensation, requiring multivalent interactions via N- and C-terminal domains. Here, we show that these complexes play a key role in TDP-43 nuclear retention. TDP-43 forms macromolecular complexes with a wide range of size distribution in cells and we find that defects in RNA binding or inter-domain interactions, including phase separation, impair the assembly of the largest species. Our findings suggest that recruitment into these macromolecular complexes prevents cytoplasmic egress of TDP-43 in a size-dependent manner. Our observations uncover fundamental mechanisms controlling TDP-43 cellular homeostasis, whereby regulation of RNA-mediated self-assembly modulates TDP-43 nucleocytoplasmic distribution. Moreover, these findings highlight pathways that may be implicated in TDP-43 proteinopathies and identify potential therapeutic targets.

Arabidopsis Transcription Regulatory Factor Domain/Domain Interaction Analysis Tool-Liquid/Liquid Phase Separation, Oligomerization, GO Analysis: A Toolkit for Interaction Data-Based Domain Analysis

Although a large number of databases are available for regulatory elements, a bottleneck has been created by the lack of bioinformatics tools to predict the interaction modes of regulatory elements. To reduce this gap, we developed the Arabidopsis Transcription Regulatory Factor Domain/Domain Interaction Analysis Tool-liquid/liquid phase separation (LLPS), oligomerization, GO analysis (ART FOUNDATION-LOG), a useful toolkit for protein-nucleic acid interaction (PNI) and protein-protein interaction (PPI) analysis based on domain-domain interactions (DDIs). LLPS, protein oligomerization, the structural properties of protein domains, and protein modifications are major components in the orchestration of the spatiotemporal dynamics of PPIs and PNIs. Our goal is to integrate PPI/PNI information into the development of a prediction model for identifying important genetic variants in peaches. Our program unified interdatabase relational keys based on protein domains to facilitate inference from the model species. A key advantage of this program lies in the integrated information of related features, such as protein oligomerization, LOG analysis, structural characterizations of domains (e.g., domain linkers, intrinsically disordered regions, DDIs, domain-motif (peptide) interactions, beta sheets, and transmembrane helices), and post-translational modification. We provided simple tests to demonstrate how to use this program, which can be applied to other eukaryotic organisms.

RNA-binding proteins as a common ground for neurodegeneration and inflammation in amyotrophic lateral sclerosis and multiple sclerosis

The neurodegenerative and inflammatory illnesses of amyotrophic lateral sclerosis and multiple sclerosis were once thought to be completely distinct entities that did not share any remarkable features, but new research is beginning to reveal more information about their similarities and differences. Here, we review some of the pathophysiological features of both diseases and their experimental models: RNA-binding proteins, energy balance, protein transportation, and protein degradation at the molecular level. We make a thorough analysis on TDP-43 and hnRNP A1 dysfunction, as a possible common ground in both pathologies, establishing a potential link between neurodegeneration and pathological immunity. Furthermore, we highlight the putative variations that diverge from a common ground in an atemporal course that proposes three phases for all relevant molecular events.

Disrupting pathologic phase transitions in neurodegeneration

Solid-like protein deposits found in aged and diseased human brains have revealed a relationship between insoluble protein accumulations and the resulting deficits in neurologic function. Clinically diverse neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, frontotemporal lobar degeneration, and amyotrophic lateral sclerosis, exhibit unique and disease-specific biochemical protein signatures and abnormal protein depositions that often correlate with disease pathogenesis. Recent evidence indicates that many pathologic proteins assemble into liquid-like protein phases through the highly coordinated process of liquid-liquid phase separation. Over the last decade, biomolecular phase transitions have emerged as a fundamental mechanism of cellular organization. Liquid-like condensates organize functionally related biomolecules within the cell, and many neuropathology-associated proteins reside within these dynamic structures. Thus, examining biomolecular phase transitions enhances our understanding of the molecular mechanisms mediating toxicity across diverse neurodegenerative diseases. This Review explores the known mechanisms contributing to aberrant protein phase transitions in neurodegenerative diseases, focusing on tau and TDP-43 proteinopathies and outlining potential therapeutic strategies to regulate these pathologic events.

Lost in local translation: TDP-43 and FUS in axonal/neuromuscular junction maintenance and dysregulation in amyotrophic lateral sclerosis

Key early features of amyotrophic lateral sclerosis (ALS) are denervation of neuromuscular junctions and axonal degeneration. Motor neuron homeostasis relies on local translation through controlled regulation of axonal mRNA localization, transport, and stability. Yet the composition of the local transcriptome, translatome (mRNAs locally translated), and proteome during health and disease remains largely unexplored. This review covers recent discoveries on axonal translation as a critical mechanism for neuronal maintenance/survival. We focus on two RNA binding proteins, transactive response DNA binding protein-43 (TDP-43) and fused in sarcoma (FUS), whose mutations cause ALS and frontotemporal dementia (FTD). Emerging evidence points to their essential role in the maintenance of axons and synapses, including mRNA localization, transport, and local translation, and whose dysfunction may contribute to ALS. Finally, we describe recent advances in omics-based approaches mapping compartment-specific local RNA and protein compositions, which will be invaluable to elucidate fundamental local processes and identify key targets for therapy development.

Aggregation-prone TDP-43 sequesters and drives pathological transitions of free nuclear TDP-43

Aggregation of the RNA-binding protein, TDP-43, is the unifying hallmark of amyotrophic lateral sclerosis and frontotemporal dementia. TDP-43-related neurodegeneration involves multiple changes to normal physiological TDP-43, which undergoes nuclear depletion, cytoplasmic mislocalisation, post-translational modification, and aberrant liquid-liquid phase separation, preceding inclusion formation. Along with toxic cytoplasmic aggregation, concurrent depletion and dysfunction of normal nuclear TDP-43 in cells with TDP-43 pathology is likely a key potentiator of neurodegeneration, but is not well understood. To define processes driving TDP-43 dysfunction, we used CRISPR/Cas9-mediated fluorescent tagging to investigate how disease-associated stressors and pathological TDP-43 alter abundance, localisation, self-assembly, aggregation, solubility, and mobility dynamics of normal nuclear TDP-43 over time in live cells. Oxidative stress stimulated liquid-liquid phase separation of endogenous TDP-43 into droplet-like puncta, or spherical shell-like anisosomes. Further, nuclear RNA-binding-ablated or acetylation-mimicking TDP-43 readily sequestered and depleted free normal nuclear TDP-43 into dynamic anisosomes, in which recruited endogenous TDP-43 proteins remained soluble and highly mobile. Large, phosphorylated inclusions formed by nuclear or cytoplasmic aggregation-prone TDP-43 mutants also caused sequestration, but rendered endogenous TDP-43 immobile and insoluble, indicating pathological transition. These findings suggest that RNA-binding deficiency and post-translational modifications including acetylation exacerbate TDP-43 aggregation and dysfunction by driving sequestration, mislocalisation, and depletion of normal nuclear TDP-43 in neurodegenerative diseases.

The Advance on Frontotemporal Dementia (FTD)’s Neuropathology and Molecular Genetics

The morbidity of frontotemporal dementia (FTD), one of the most prevalent dementias praccox, is second to Alzheimer disease (AD). It is different with AD that FTD has a rapider course and a higher mortality. FTD has not yet been fully understood in terms of etiology or pathogenesis, but genetic factors are believed to be involved. In this paper, we were committed to providing a comprehensive overview to FTD in aspects of the neuropathology features and the relevant molecular genetics advances, so that there would be insights to those researchers in search of novel approaches in FTD diagnosis and treatment.