Axonal transport of TDP-43 mRNA granules is impaired by ALS-causing mutations

ELAV/Hu RNA-binding protein family: key regulators in neurological disorders, cancer, and other diseases

The ELAV/Hu family represents a crucial group of RNA-binding proteins predominantly expressed in neurons, playing significant roles in mRNA transcription and translation. These proteins bind to AU-rich elements in transcripts to regulate the expression of cytokines, growth factors, and the development and maintenance of neurons. Elav-like RNA-binding proteins exhibit remarkable molecular weight conservation across different species, highlighting their evolutionary conservation. Although these proteins are widely expressed in the nervous system and other cell types, variations in the DNA sequences of the four Elav proteins contribute to their distinct roles in neurological disorders, cancer, and other Diseases . Elavl1, a ubiquitously expressed family member, is integral to processes such as cell growth, ageing, tumorigenesis, and inflammatory diseases. Elavl2, primarily expressed in the nervous and reproductive systems, is critical for central nervous system and retinal development; its dysregulation has been implicated in neurodevelopmental disorders such as autism. Both Elavl3 and Elavl4 are restricted to the nervous system and are involved in neuronal differentiation and excitability. Elavl3 is essential for cerebellar function and has been associated with epilepsy, while Elavl4 is linked to neurodegenerative diseases, including Parkinson's and Alzheimer's diseases. This paper provides a comprehensive review of the ELAV/Hu family's role in nervous system development, neurological disorders, cancer, and other diseases.

Amyotrophic lateral sclerosis caused by TARDBP mutations: from genetics to TDP-43 proteinopathy

Mutations in the TARDBP gene, which encodes the TDP-43 protein, account for only 3-5% of familial cases of amyotrophic lateral sclerosis and less than 1% of cases that are apparently idiopathic. However, the discovery of neuronal inclusions of TDP-43 as the neuropathological hallmark in the majority of cases of amyotrophic lateral sclerosis has transformed our understanding of the pathomechanisms underlying neurodegeneration. An individual TARDBP mutation can cause phenotypic heterogeneity. Most mutations lie within the C-terminus of the TDP-43 protein. In pathological conditions, TDP-43 is mislocalised from the nucleus to the cytoplasm, where it can be phosphorylated, cleaved, and form insoluble aggregates. This mislocalisation leads to dysfunction of downstream pathways of RNA metabolism, proteostasis, mitochondrial function, oxidative stress, axonal transport, and local translation. Biomarkers for TDP-43 dysfunction and targeted therapies are being developed, justifying cautious optimism for personalised medicine approaches that could rescue the downstream effects of TDP-43 pathology.

Optogenetic induction of TDP-43 aggregation impairs neuronal integrity and behavior in Caenorhabditis elegans

BACKGROUND

Cytoplasmic aggregation of TAR DNA binding protein 43 (TDP-43) in neurons is one of the hallmarks of TDP-43 proteinopathy. Amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) are closely associated with TDP-43 proteinopathy; however, it remains uncertain whether TDP-43 aggregation initiates the pathology or is a consequence of it.

METHODS

To demonstrate the pathology of TDP-43 aggregation, we applied the optoDroplet technique in Caenorhabditis elegans (C. elegans), which allows spatiotemporal modulation of TDP-43 phase separation and assembly.

RESULTS

We demonstrate that optogenetically induced TDP-43 aggregates exhibited insolubility similar to that observed in TDP-43 proteinopathy. These aggregates increased the severity of neurodegeneration, particularly in GABAergic motor neurons, and exacerbated sensorimotor dysfunction in C. elegans.

CONCLUSIONS

We present an optogenetic C. elegans model of TDP-43 proteinopathy that provides insight into the neuropathological mechanisms of TDP-43 aggregates. Our model serves as a promising tool for identifying therapeutic targets for TDP-43 proteinopathy.

Pathogenic TDP-43 in amyotrophic lateral sclerosis

The aberrant expression of the transactive response DNA-binding protein of 43 kDa (TDP-43) has been closely associated with amyotrophic lateral sclerosis (ALS). Cytoplasmic inclusions containing TDP-43 can be found in the brain and spinal cord in up to 97% of ALS cases. Mutations in the TARDBP gene promote the nuclear export of TDP-43, increase cytoplasmic aggregation, and predispose TDP-43 to post-translational modifications. Cleavage of TDP-43 and the resulting C- and N-terminal fragments also contribute to the development of ALS. Cellularly, the resulting impairment of autophagy and mitochondria aggravates cellular damage and neurodegeneration. Given the contribution of pathogenic TDP-43 to the development of ALS, elucidating the mechanisms related to TDP-43 will facilitate finding therapeutic targets for the disease.

Amyotrophic Lateral Sclerosis: Focus on Cytoplasmic Trafficking and Proteostasis

Amyotrophic lateral sclerosis (ALS) is a progressive and fatal motor neuron disease characterized by the pathological loss of upper and lower motor neurons. Whereas most ALS cases are caused by a combination of environmental factors and genetic susceptibility, in a relatively small proportion of cases, the disorder results from mutations in genes that are inherited. Defects in several different cellular mechanisms and processes contribute to the selective loss of motor neurons (MNs) in ALS. Prominent among these is the accumulation of aggregates of misfolded proteins or peptides which are toxic to motor neurons. These accumulating aggregates stress the ability of the endoplasmic reticulum (ER) to function normally, cause defects in the transport of proteins between the ER and Golgi, and impair the transport of RNA, proteins, and organelles, such as mitochondria, within axons and dendrites, all of which contribute to the degeneration of MNs. Although dysfunction of a variety of cellular processes combines towards the pathogenesis of ALS, in this review, we focus on recent advances concerning the involvement of defective ER stress, vesicular transport between the ER and Golgi, and axonal transport.

The Regulation of TDP-43 Structure and Phase Transitions: A Review

The transactive response DNA binding protein 43 (TDP-43) is an RNA/DNA-binding protein that is involved in a number of cellular functions, including RNA processing and alternative splicing, RNA transport and translation, and stress granule assembly. It has attracted significant attention for being the primary component of cytoplasmic inclusions in patients with amyotrophic lateral sclerosis or frontotemporal dementia. Mounting evidence suggests that both cytoplasmic aggregation of TDP-43 and loss of nuclear TDP-43 function contribute to TDP-43 pathology. Furthermore, recent studies have demonstrated that TDP-43 is an important component of many constitutive or stress-induced biomolecular condensates. Dysregulation or liquid-to-gel transition of TDP-43 condensates can lead to alterations in TDP-43 function and the formation of TDP-43 amyloid fibrils. In this review, we summarize recent research progress on the structural characterization of TDP-43 and the TDP-43 phase transition. In particular, the roles that disease-associated genetic mutations, post-translational modifications, and extrinsic stressors play in the transitions among TDP-43 monomers, liquid condensates, solid condensates, and fibrils are discussed. Finally, we discuss the effectiveness of available regulators of TDP-43 phase separation and aggregation. Understanding the underlying mechanisms that drive the pathological transformation of TDP-43 could help develop therapeutic strategies for TDP-43 pathology.

Current amyloid inhibitors: Therapeutic applications and nanomaterial-based innovations

Amyloid proteins have long been in the spotlight for being involved in many degenerative diseases including Alzheimer´s, Parkinson´s or type 2 diabetes, which currently cannot be prevented and for which there is no effective treatment or cure. Here we provide a comprehensive review of inhibitors that act directly on the amyloidogenic pathway (at the monomer, oligomer or fibril level) of key pathological amyloids, focusing on the most representative amyloid-related diseases. We discuss the latest advances in preclinical and clinical trials, focusing on cutting-edge developments, particularly on nanomaterials-based inhibitors, which offer unprecedented opportunities to address the complexity of protein misfolding disorders and are revolutionizing the landscape of anti-amyloid therapeutics. Notably, nanomaterials are impacting critical areas such as bioavailability, penetrability and functionality of compounds currently used in biomedicine, paving the way for more specific therapeutic solutions tailored to various amyloid-related diseases. Finally, we highlight the window of opportunity opened by comparative analysis with so-called functional amyloids for the development of innovative therapeutic approaches for these devastating diseases.

Ribosome-associated pathological TDP-43 alters the expression of multiple mRNAs in the monkey brain

Cytoplasmic accumulation of TDP-43 is a pathological hallmark of amyotrophic lateral sclerosis (ALS) and other neurodegenerative diseases. While current studies have primarily focused on gene regulation mediated by full-length nuclear TDP-43, the potential effects of cytoplasmic TDP-43 fragments remain less explored. Our previous findings demonstrated that primate-specific cleavage of TDP-43 contributes to its cytoplasmic localization, prompting further investigation into its pathological effects. In the cynomolgus monkey brain, we observed that mutant or truncated TDP-43 was transported onto the ribosome organelle. Ribosome-associated transcriptomic analysis revealed dysregulation of apoptosis- and lysosome-related genes, indicating that cytoplasmic TDP-43 induces neurotoxicity by binding to ribosomes and disrupting mRNA expression. These findings provide mechanistic insights into the gain-of-function effects of pathological TDP-43.

Graphene Quantum Dots Attenuate TDP-43 Proteinopathy in Amyotrophic Lateral Sclerosis

Aberrant phase separation- and stress granule (SG)-mediated cytosolic aggregation of TDP-43 in motor neurons is the hallmark of amyotrophic lateral sclerosis (ALS). In this study, we found that graphene quantum dots (GQDs) potentially modulate TDP-43 aggregation during SG dynamics and phase separation. The intrinsically disordered region in the C-terminus of TDP-43 exhibited amyloid fibril formation; however, GQDs inhibited the formation of amyloid fibrils through direct intermolecular interactions with TDP-43. These effects were accompanied by attenuation of the ALS phenotype in animal models. Additionally, GQDs delayed the onset and survival of TDP-43 transgenic mouse models by enhancing motor neuron survival, reducing glial activation, and reducing the cytosolic aggregation of TDP-43 in motor neurons. In this research, we demonstrated the efficacy of GQDs on the SG-mediated aggregation of TDP-43 and the binding property of GQDs with TDP-43. Additionally, we demonstrated the clinical feasibility of GQDs using several animal models and other types of ALS caused by FUS and C9orf72. Therefore, GQDs could offer a new therapeutic approach for proteinopathy-associated ALS.

Decoding subcellular RNA localization one molecule at a time

Eukaryotic cells are highly structured and composed of multiple membrane-bound and membraneless organelles. Subcellular RNA localization is a critical regulator of RNA function, influencing various biological processes. At any given moment, RNAs must accurately navigate the three-dimensional subcellular environment to ensure proper localization and function, governed by numerous factors, including splicing, RNA stability, modifications, and localizing sequences. Aberrant RNA localization can contribute to the development of numerous diseases. Here, we explore diverse RNA localization mechanisms and summarize advancements in methods for determining subcellular RNA localization, highlighting imaging techniques transforming our ability to study RNA dynamics at the single-molecule level.

TDP-43 transports ferritin heavy chain mRNA to regulate oxidative stress in neuronal axons

Amyotrophic lateral sclerosis (ALS) is characterized by the mislocalization and abnormal deposition of TAR DNA-binding protein 43 (TDP-43). This protein plays important roles in RNA metabolism and transport in motor neurons and glial cells. In addition, abnormal iron accumulation and oxidative stress are observed in the brain and spinal cord of patients with ALS exhibiting TDP-43 pathology and in animal models of ALS. We have previously demonstrated that TDP-43 downregulation significantly affects the expression of ferritin heavy chain (Fth1) mRNA in the axonal regions of neurons. Nevertheless, the mechanisms by which TDP-43 contributes to oxidative stress and iron accumulation in the central nervous system remain elusive. In this study, we aimed to investigate whether Fth1 mRNA is a target transported to the axon by TDP-43 using biophysical and biochemical analyses. Our results revealed Fth1 mRNA as a target mRNA transported to axons by TDP-43. Moreover, we demonstrated that TDP-43 regulates iron homeostasis and oxidative stress in neurons via Fth1 mRNA transport to the axons, possibly followed by a local translation of the ferritin heavy chain in the axons. This study suggests that TDP-43 plays an important role in preventing iron-mediated oxidative stress in neurons, with its loss contributing to ALS pathogenesis.

Modeling amyotrophic lateral sclerosis with amniotic membrane-derived mesenchymal stem cells: A novel approach for disease modeling

Advancement of therapeutics for neurodegenerative diseases like amyotrophic lateral sclerosis (ALS) has been predominantly hampered by the dearth of relevant disease models. Despite numerous animal models, significant challenges remain in correlating these with human disease complexities. In this study, the ALS model was created using amniotic membrane-derived mesenchymal stem cells (AM-MSCs) which were differentiated into motor neurons (MN) with specific MN induction media and transiently transfected with mutated human SOD1 G93A plasmid to induce ALS-like condition. Characterization included gene expression analysis, immunocytochemistry, flow cytometry, and Western blot. Functional assays assessed the extent of degeneration and model efficiency. AM-MSCs demonstrated multipotency and were positive for MSC markers. Upon differentiation, the expression of MN markers like MNX1, Olig2, and ChAT were found to be elevated. SOD1 G93A overexpression, downregulated MN markers, upregulated NURR1 gene, reduced acetylcholine (ACh), reduced glutathione, and elevated oxidative stress markers. This robust in-vitro ALS model derived from AM-MSCs offers an alternative to animal models to provide an efficient and cost-effective platform to conduct rapid drug screening.

Opposing roles of p38α-mediated phosphorylation and PRMT1-mediated arginine methylation in driving TDP-43 proteinopathy

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disorder typically characterized by insoluble inclusions of hyperphosphorylated TDP-43. The mechanisms underlying toxic TDP-43 accumulation are not understood. Persistent activation of p38 mitogen-activated protein kinase (MAPK) is implicated in ALS. However, it is unclear how p38 MAPK affects TDP-43 proteinopathy. Here, we show that p38α MAPK inhibition reduces pathological TDP-43 phosphorylation, aggregation, cytoplasmic mislocalization, and neurotoxicity. Remarkably, p38α MAPK inhibition mitigates aberrant TDP-43 phenotypes in diverse ALS patient-derived motor neurons. p38α MAPK phosphorylates TDP-43 at pathological S409/S410 and S292, which reduces TDP-43 liquid-liquid phase separation (LLPS) but allows pathological TDP-43 aggregation. Moreover, we establish that PRMT1 methylates TDP-43 at R293. Importantly, S292 phosphorylation reduces R293 methylation, and R293 methylation reduces S409/S410 phosphorylation. Notably, R293 methylation permits TDP-43 LLPS and reduces pathological TDP-43 aggregation. Thus, strategies to reduce p38α-mediated TDP-43 phosphorylation and promote PRMT1-mediated R293 methylation could have therapeutic utility for ALS and related TDP-43 proteinopathies.

Nemo-like kinase disrupts nuclear import and drives TDP43 mislocalization in ALS

Cytoplasmic TDP43 mislocalization and aggregation are pathological hallmarks of amyotrophic lateral sclerosis (ALS). However, the initial cellular insults that lead to TDP43 mislocalization remain unclear. In this study, we demonstrate that Nemo-like kinase (NLK)-a proline-directed serine/threonine kinase-promotes the mislocalization of TDP43 and other RNA-binding proteins by disrupting nuclear import. NLK levels are selectively elevated in neurons exhibiting TDP43 mislocalization in ALS patient tissues, while genetic reduction of NLK reduces toxicity in human neuron models of ALS. Our findings suggest that NLK is a promising therapeutic target for neurodegenerative diseases.

Ataxin-2 polyglutamine expansions aberrantly sequester TDP-43 ribonucleoprotein condensates disrupting mRNA transport and local translation in neurons

Altered RNA metabolism and misregulation of transactive response DNA-binding protein of 43 kDa (TDP-43), an essential RNA-binding protein (RBP), define amyotrophic lateral sclerosis (ALS). Intermediate-length polyglutamine (polyQ) expansions of Ataxin-2, a like-Sm (LSm) RBP, are associated with increased risk for ALS, but the underlying biological mechanisms remain unknown. Here, we studied the spatiotemporal dynamics and mRNA regulatory functions of TDP-43 and Ataxin-2 ribonucleoprotein (RNP) condensates in rodent (rat) primary cortical neurons and mouse motor neuron axons in vivo. We report that Ataxin-2 polyQ expansions aberrantly sequester TDP-43 within RNP condensates and disrupt both its motility along the axon and liquid-like properties. We provide evidence that Ataxin-2 governs motility and translation of neuronal RNP condensates and that Ataxin-2 polyQ expansions fundamentally perturb spatial localization of mRNA and suppress local translation. Overall, our results support a model in which Ataxin-2 polyQ expansions disrupt stability, localization, and/or translation of critical axonal and cytoskeletal mRNAs, particularly important for motor neuron integrity.

Molecular switch of the dendrite-to-spine transport of TDP-43/FMRP-bound neuronal mRNAs and its impairment in ASD

BACKGROUND

Regulation of messenger RNA (mRNA) transport and translation in neurons is essential for dendritic plasticity and learning/memory development. The trafficking of mRNAs along the hippocampal neuron dendrites remains translationally silent until they are selectively transported into the spines upon glutamate-induced receptor activation. However, the molecular mechanism(s) behind the spine entry of dendritic mRNAs under metabotropic glutamate receptor (mGluR)-mediated neuroactivation and long-term depression (LTD) as well as the fate of these mRNAs inside the spines are still elusive.

METHOD

Different molecular and imaging techniques, e.g., immunoprecipitation (IP), RNA-IP, Immunofluorescence (IF)/fluorescence in situ hybridization (FISH), live cell imaging, live cell tracking of RNA using beacon, and mouse model study are used to elucidate a novel mechanism regulating dendritic spine transport of mRNAs in mammalian neurons.

RESULTS

We demonstrate here that brief mGluR1 activation-mediated dephosphorylation of pFMRP (S499) results in the dissociation of FMRP from TDP-43 and handover of TDP-43/Rac1 mRNA complex from the dendritic transport track on microtubules to myosin V track on the spine actin filaments. Rac1 mRNA thus enters the spines for translational reactivation and increases the mature spine density. In contrast, during mGluR1-mediated neuronal LTD, FMRP (S499) remains phosphorylated and the TDP-43/Rac1 mRNA complex, being associated with kinesin 1-FMRP/cortactin/drebrin, enters the spines owing to Ca2+-dependent microtubule invasion into spines, but without translational reactivation. In a VPA-ASD mouse model, this regulation become anomalous.

CONCLUSIONS

This study, for the first time, highlights the importance of posttranslational modification of RBPs, such as the neurodevelopmental disease-related protein FMRP, as the molecular switch regulating the dendrite-to-spine transport of specific mRNAs under mGluR1-mediated neurotransmissions. The misregulation of this switch could contribute to the pathogenesis of FMRP-related neurodisorders including the autism spectrum disorder (ASD). It also could indicate a molecular connection between ASD and neurodegenerative disease-related protein TDP-43 and opens up a new perspective of research to elucidate TDP-43 proteinopathy among patients with ASD.

Gain-of-function ANXA11 mutation cause late-onset ALS with aberrant protein aggregation, neuroinflammation and autophagy impairment

Mutations in the ANXA11 gene, encoding an RNA-binding protein, have been implicated in the pathogenesis of amyotrophic lateral sclerosis (ALS), but the underlying in vivo mechanisms remain unclear. This study examines the clinical features of ALS patients harboring the ANXA11 hotspot mutation p.P36R, characterized by late-onset motor neuron disease and occasional multi-system involvement. To elucidate the pathogenesis, we developed a knock-in mouse model carrying the p.P36R mutation. In both heterozygous and homozygous mutant mice, ANXA11 protein levels were comparable to those in wild-type. Both groups exhibited late-onset motor dysfunction at approximately 10 months of age, with similar survival rates to wild-type (> 24 months) and no signs of dementia. Pathological analysis revealed early abnormal aggregates in spinal cord motor neurons, cortical neurons, and muscle cells of homozygous mice. From 2 months of age, we observed mislocalized ANXA11 aggregates, SQSTM1/p62-positive inclusions, and cytoplasmic TDP-43 mislocalization, which intensified with disease progression. Importantly, mutant ANXA11 co-aggregated with TDP-43 and SQSTM1/p62-positive inclusions. Electron microscopy of the gastrocnemius muscle uncovered myofibrillar abnormalities, including sarcomeric disorganization, Z-disc dissolution, and subsarcolemmal electron-dense structures within autophagic vacuoles. Autophagic flux, initially intact at 2 months, was impaired by 9 months, as evidenced by decreased Beclin-1 and LC3BII/I levels and increased SQSTM1/p62 expression, coinciding with mTORC1 hyperactivation. Significant motor neuron loss and neuroinflammation were detected by 9 months, with marked muscle dystrophy apparent by 12 months compared to wild-type controls. These findings implicate the gain-of-function ANXA11 mutation drives late-onset motor neuron disease by early presymptomatic proteinopathy, progressive neuronal degeneration, neuroinflammation, and autophagic dysfunction.

Severe dynein dysfunction in cholinergic neurons exacerbates ALS-like phenotypes in a new mouse model

Cytoplasmic dynein 1, a motor protein essential for retrograde axonal transport, is increasingly implicated in the pathogenesis of neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). In this study, we developed a novel mouse model that combines the Legs at odd angles (Loa, F580Y) point mutation in the dynein heavy chain with a cholinergic neuron-specific knockout of the dynein heavy chain. This model, for the first time, allows us to investigate the impact of Loa allele exclusivity in these neurons into adulthood. Our findings reveal that this selective increase in dynein dysfunction exacerbated the phenotypes observed in heterozygous Loa mice including pre-wean survival, reduced body weight and grip strength. Additionally, it induced ALS-like pathology in neuromuscular junctions (NMJs) not seen in heterozygous Loa mice. Notably, we also found a previously unobserved significant increase in neurons displaying TDP-43 puncta in both Loa mutants, suggesting early TDP-43 mislocalisation - a hallmark of ALS. The novel model also exhibited a concurrent rise in p62 puncta that did not co-localise with TDP-43, indicating broader impairments in autophagic clearance mechanisms. Overall, this new model underscores the fact that dynein impairment alone can induce ALS-like pathology and provides a valuable platform to further explore the role of dynein in ALS.

A robust evaluation of TDP-43, poly GP, cellular pathology and behavior in an AAV-C9ORF72 (G4C2)66 mouse model

The G4C2 hexanucleotide repeat expansion in C9ORF72 is the major genetic cause of both amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) (C9-ALS/FTD). Despite considerable efforts, the development of mouse models of C9-ALS/FTD useful for therapeutic development has proven challenging due to the intricate interplay of genetic and molecular factors underlying this neurodegenerative disorder, in addition to species differences. This study presents a robust investigation of the cellular pathophysiology and behavioral outcomes in a previously described AAV mouse model of C9-ALS expressing 66 G4C2 hexanucleotide repeats. The model displays key molecular ALS pathological markers including RNA foci, dipeptide repeat (DPR) protein aggregation, p62 positive stress granule formation as well as mild gliosis. However, the AAV-(G4C2)66 mouse model in this study has marginal neurodegeneration with negligible neuronal loss, or clinical deficits. Human C9orf72 is typically associated with altered TAR DNA-binding protein (TDP-43) function, yet studies of this rodent model revealed no significant evidence of TDP-43 dysfunction. While our findings indicate and support that this is a highly valuable robust and pharmacologically tractable model for investigating the molecular mechanisms and cellular consequences of (G4C2) repeat driven DPR pathology, it is not suitable for investigating the development of disease- associated TDP-43 dysfunction or clinical impairment. Our findings underscore the complexity of ALS pathogenesis involving genetic mutations and protein dysregulation and highlight the need for more comprehensive model systems that reliably replicate the multifaceted cellular and behavioral aspects of C9-ALS.

Condensates act as translation hubs to coordinate multinucleate cell growth

Coordination between growth and division is a fundamental feature of cells. In many syncytia, cell growth must couple with multiple nuclear divisions in one cytoplasm. In the fungus, Ashbya gossypii, cell-cycle progression and hyphal elongation require condensates formed by the protein Whi3 in complex with distinct mRNA species. We hypothesized the condensates may act through local translation regulation and find that Whi3 target mRNAs show distinct spatial biases in translation in vivo. Whi3-RNA condensates can both promote and repress RNA translation in an RNA- and condensate size-dependent manner in vitro. Interestingly, we observe a sub-condensate enrichment of translation that is tunable by RNA valency and protein phospho-state. Together, these data suggest that Whi3 condensates generate a continuum of translation states, resulting in asynchronous nuclear divisions coordinated with growth. This local regulation requires a minimal complement of molecular components at the nano scale to support global coordination at the cell scale.

Research progress on astrocyte-derived extracellular vesicles in the pathogenesis and treatment of neurodegenerative diseases

Neurodegenerative disorders, including Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD), pose significant global health risks and represent a substantial public health concern in the contemporary era. A primary factor in the pathophysiology of these disorders is aberrant accumulation and aggregation of pathogenic proteins within the brain and spinal cord. Recent investigations have identified extracellular vesicles (EVs) in the central nervous system (CNS) as potential carriers for intercellular transport of misfolded proteins associated with neurodegenerative diseases. EVs are involved in pathological processes that contribute to various brain disorders including neurodegenerative disorders. Proteins linked to neurodegenerative disorders are secreted and distributed from cell to cell via EVs, serving as a mechanism for direct intercellular communication through the transfer of biomolecules. Astrocytes, as active participants in CNS intercellular communication, release astrocyte-derived extracellular vesicles (ADEVs) that are capable of interacting with diverse target cells. This review primarily focuses on the involvement of ADEVs in the development of neurological disorders and explores their potential dual roles - both advantageous and disadvantageous in the context of neurological disorders. Furthermore, this review examines the current studies investigating ADEVs as potential biomarkers for the diagnosis and treatment of neurodegenerative diseases. The prospects and challenges associated with the application of ADEVs in clinical settings were also comprehensively reviewed.

A robust evaluation of TDP-43, poly GP, cellular pathology and behavior in a AAV- C9ORF72 (G 4 C 2) 66 mouse model

The G4C2 hexanucleotide repeat expansion in C9ORF72is the major genetic cause of both amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) (C9-ALS/FTD). Despite considerable efforts, the development of mouse models of C9-ALS/FTD useful for therapeutic development has proven challenging due to the intricate interplay of genetic and molecular factors underlying this neurodegenerative disorder, in addition to species differences. This study presents a robust investigation of the cellular pathophysiology and behavioral outcomes in a previously described AAV mouse model of C9-ALS expressing 66 G4C2 hexanucleotide repeats. The model displays key molecular ALS pathological markers including RNA foci, dipeptide repeat (DPR) protein aggregation, p62 positive stress granule formation as well as mild gliosis. However, the AAV-(G4C2)66 mouse model in this study has marginal neurodegeneration with negligible neuronal loss, or clinical deficits. Human C9orf72 is typically associated with altered TAR DNA-binding protein (TDP-43) function, yet studies of this rodent model revealed no significant evidence of TDP-43 dysfunction. While our findings indicate and support that this is a highly valuable robust and pharmacologically tractable model for investigating the molecular mechanisms and cellular consequences of (G4C2) repeat driven DPR pathology, it is not suitable for investigating the development of disease- associated TDP-43 dysfunction or clinical impairment. Our findings underscore the complexity of ALS pathogenesis involving genetic mutations and protein dysregulation and highlight the need for more comprehensive model systems that reliably replicate the multifaceted cellular and behavioral aspects of C9-ALS.

Decoding protein-RNA interactions using CLIP-based methodologies

Protein-RNA interactions are central to all RNA processing events, with pivotal roles in the regulation of gene expression and cellular functions. Dysregulation of these interactions has been increasingly linked to the pathogenesis of human diseases. High-throughput approaches to identify RNA-binding proteins and their binding sites on RNA - in particular, ultraviolet crosslinking followed by immunoprecipitation (CLIP) - have helped to map the RNA interactome, yielding transcriptome-wide protein-RNA atlases that have contributed to key mechanistic insights into gene expression and gene-regulatory networks. Here, we review these recent advances, explore the effects of cellular context on RNA binding, and discuss how these insights are shaping our understanding of cellular biology. We also review the potential therapeutic applications arising from new knowledge of protein-RNA interactions.

Pathogenic TDP-43 accelerates the generation of toxic exon1 HTT in Huntington's disease knock-in mice

Huntington's disease (HD) is caused by a CAG repeat expansion in exon1 of the HTT gene that encodes a polyglutamine tract in huntingtin protein. The formation of HTT exon1 fragments with an expanded polyglutamine repeat has been implicated as a key step in the pathogenesis of HD. It was reported that the CAG repeat length-dependent aberrant splicing of exon1 HTT results in a short polyadenylated mRNA that is translated into an exon1 HTT protein. Under normal conditions, TDP-43 is predominantly found in the nucleus, where it regulates gene expression. However, in various pathological conditions, TDP-43 is mislocalized in the cytoplasm. By investigating HD knock-in mice, we explore whether the pathogenic TDP-43 in the cytoplasm contributes to HD pathogenesis, through expressing the cytoplasmic TDP-43 without nuclear localization signal. We found that the cytoplasmic TDP-43 is increased in the HD mouse brain and that its mislocalization could deteriorate the motor and gait behavior. Importantly, the cytoplasmic TDP-43, via its binding to the intron1 sequence (GU/UG)n of the mouse Htt pre-mRNA, promotes the transport of exon1-intron1 Htt onto ribosome, resulting in the aberrant generation of exon1 Htt. Our findings suggest that cytoplasmic TDP-43 contributes to HD pathogenesis via its binding to and transport of nuclear un-spliced mRNA to the ribosome for the generation of a toxic protein product.

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.

Membraneless organelles in health and disease: exploring the molecular basis, physiological roles and pathological implications

Once considered unconventional cellular structures, membraneless organelles (MLOs), cellular substructures involved in biological processes or pathways under physiological conditions, have emerged as central players in cellular dynamics and function. MLOs can be formed through liquid-liquid phase separation (LLPS), resulting in the creation of condensates. From neurodegenerative disorders, cardiovascular diseases, aging, and metabolism to cancer, the influence of MLOs on human health and disease extends widely. This review discusses the underlying mechanisms of LLPS, the biophysical properties that drive MLO formation, and their implications for cellular function. We highlight recent advances in understanding how the physicochemical environment, molecular interactions, and post-translational modifications regulate LLPS and MLO dynamics. This review offers an overview of the discovery and current understanding of MLOs and biomolecular condensate in physiological conditions and diseases. This article aims to deliver the latest insights on MLOs and LLPS by analyzing current research, highlighting their critical role in cellular organization. The discussion also covers the role of membrane-associated condensates in cell signaling, including those involving T-cell receptors, stress granules linked to lysosomes, and biomolecular condensates within the Golgi apparatus. Additionally, the potential of targeting LLPS in clinical settings is explored, highlighting promising avenues for future research and therapeutic interventions.

Co-Aggregation of TDP-43 with Other Pathogenic Proteins and Their Co-Pathologies in Neurodegenerative Diseases

Pathological aggregation of a specific protein into insoluble aggregates is a common hallmark of various neurodegenerative diseases (NDDs). In the earlier literature, each NDD is characterized by the aggregation of one or two pathogenic proteins, which can serve as disease-specific biomarkers. The aggregation of these specific proteins is thought to be a major cause of or deleterious result in most NDDs. However, accumulating evidence shows that a pathogenic protein can interact and co-aggregate with other pathogenic proteins in different NDDs, thereby contributing to disease onset and progression synergistically. During the past years, more than one type of NDD has been found to co-exist in some individuals, which may increase the complexity and pathogenicity of these diseases. This article reviews and discusses the biochemical characteristics and molecular mechanisms underlying the co-aggregation and co-pathologies associated with TDP-43 pathology. The TDP-43 aggregates, as a hallmark of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD), can often be detected in other NDDs, such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD) and spinocerebellar ataxia type 2 (SCA2). In many cases, TDP-43 is shown to interact and co-aggregate with multiple pathogenic proteins in vitro and in vivo. Furthermore, the co-occurrence and co-aggregation of TDP-43 with other pathogenic proteins have important consequences that may aggravate the diseases. Thus, the current viewpoint that the co-aggregation of TDP-43 with other pathogenic proteins in NDDs and their relevance to disease progression may gain insights into the patho-mechanisms and therapeutic potential of various NDDs.

Prion-like Spreading of Disease in TDP-43 Proteinopathies

TDP-43 is a ubiquitous nuclear protein that plays a central role in neurodegenerative disorders collectively known as TDP-43 proteinopathies. Under physiological conditions, TDP-43 is primarily localized to the nucleus, but in its pathological form it aggregates in the cytoplasm, contributing to neuronal death. Given its association with numerous diseases, particularly ALS and FTLD, the mechanisms underlying TDP-43 aggregation and its impact on neuronal function have been extensively investigated. However, little is still known about the spreading of this pathology from cell to cell. Recent research has unveiled the possibility that TDP-43 may possess prion-like properties. Specifically, misfolded TDP-43 aggregates can act as templates inducing conformational changes in native TDP-43 molecules and propagating the misfolded state across neural networks. This review summarizes the mounting and most recent evidence from in vitro and in vivo studies supporting the prion-like hypothesis and its underlying mechanisms. The prion-like behavior of TDP-43 has significant implications for diagnostics and therapeutics. Importantly, emerging strategies such as small molecule inhibitors, immunotherapies, and gene therapies targeting TDP-43 propagation offer promising avenues for developing effective treatments. By elucidating the mechanisms of TDP-43 spreading, we therefore aim to pave the way for novel therapies for TDP-43-related neurodegenerative diseases.

Axonopathy Underlying Amyotrophic Lateral Sclerosis: Unraveling Complex Pathways and Therapeutic Insights

Amyotrophic Lateral Sclerosis (ALS) is a complex neurodegenerative disorder characterized by progressive axonopathy, jointly leading to the dying back of the motor neuron, disrupting both nerve signaling and motor control. In this review, we highlight the roles of axonopathy in ALS progression, driven by the interplay of multiple factors including defective trafficking machinery, protein aggregation, and mitochondrial dysfunction. Dysfunctional intracellular transport, caused by disruptions in microtubules, molecular motors, and adaptors, has been identified as a key contributor to disease progression. Aberrant protein aggregation involving TDP-43, FUS, SOD1, and dipeptide repeat proteins further amplifies neuronal toxicity. Mitochondrial defects lead to ATP depletion, oxidative stress, and Ca2+ imbalance, which are regarded as key factors underlying the loss of neuromuscular junctions and axonopathy. Mitigating these defects through interventions including neurotrophic treatments offers therapeutic potential. Collaborative research efforts aim to unravel ALS complexities, opening avenues for holistic interventions that target diverse pathological mechanisms.

RNA granules in flux: dynamics to balance physiology and pathology

The life cycle of an mRNA is a complex process that is tightly regulated by interactions between the mRNA and RNA-binding proteins, forming molecular machines known as RNA granules. Various types of these membrane-less organelles form inside cells, including neurons, and contribute critically to various physiological processes. RNA granules are constantly in flux, change dynamically and adapt to their local environment, depending on their intracellular localization. The discovery that RNA condensates can form by liquid-liquid phase separation expanded our understanding of how compartments may be generated in the cell. Since then, a plethora of new functions have been proposed for distinct condensates in cells that await their validation in vivo. The finding that dysregulation of RNA granules (for example, stress granules) is likely to affect neurodevelopmental and neurodegenerative diseases further boosted interest in this topic. RNA granules have various physiological functions in neurons and in the brain that we would like to focus on. We outline examples of state-of-the-art experiments including timelapse microscopy in neurons to unravel the precise functions of various types of RNA granule. Finally, we distinguish physiologically occurring RNA condensation from aberrant aggregation, induced by artificial RNA overexpression, and present visual examples to discriminate both forms in neurons.

Spliceosomal GTPase Eftud2 deficiency-triggered ferroptosis leads to Purkinje cell degeneration

Spliceosomal GTPase elongation factor Tu GTP binding domain containing 2 (EFTUD2) is a causative gene for mandibulofacial dysostosis with microcephaly (MFDM) syndrome comprising cerebellar hypoplasia and motor dysfunction. How EFTUD2 deficiency contributes to these symptoms remains elusive. Here, we demonstrate that specific ablation of Eftud2 in cerebellar Purkinje cells (PCs) in mice results in severe ferroptosis, PC degeneration, dyskinesia, and cerebellar atrophy, which recapitulates phenotypes observed in patients with MFDM. Mechanistically, Eftud2 promotes Scd1 and Gch1 expression, upregulates monounsaturated fatty acid phospholipids, and enhances antioxidant activity, thereby suppressing PC ferroptosis. Importantly, we identified transcription factor Atf4 as a downstream target to regulate anti-ferroptosis effects in PCs in a p53-independent manner. Inhibiting ferroptosis efficiently rescued cerebellar deficits in Eftud2 cKO mice. Our data reveal an important role of Eftud2 in maintaining PC survival, showing that pharmacologically or genetically inhibiting ferroptosis may be a promising therapeutic strategy for EFTUD2 deficiency-induced disorders.

Potential role of solid lipid curcumin particle (SLCP) as estrogen replacement therapy in mitigating TDP-43-related neuropathy in the mouse model of ALS disease

BACKGROUND

Amyotrophic lateral sclerosis (ALS) was first identified in 1869, but it wasn't until the 2014 Ice Bucket Challenge that widespread attention was drawn to the disease. Since then, substantial research has been dedicated to developing treatments for ALS. Despite this, only three drugs - riluzole, edaravone and AMX0035, have been approved for clinical use, and they can only temporarily alleviate mild symptoms without significant disease modification or cure. Therefore, there remains a critical unmet need to identify disease modifying or curative therapies for ALS. The higher incidence and more severe progression of ALS and FTLD (frontotemporal lobar degeneration) observed in men and postmenopausal woman compared to young women suggests that sex hormones may significantly influence disease onset and progression. In both animal models and human clinical studies, 17β estradiol (E2) has been shown to delay and improve the outcomes of many neurodegenerative diseases. Here, we examined the role of TDP-43 in the regulation of estrogen-related enzymes, CYP19A1 and CYP3A4. In addition, we examined the impact of curcumin on the regulation of estrogen E2 levels and TDP-43-associated neuropathy as a potential therapeutic strategy for the treatment of FTLD and ALS.

METHODS

Prp-TDP-43A315T mice was used as a model of ALS/FTLD to examine the expression patterns of E2 and its biosynthesis and degradation enzymes, CYP19A1 and CYP3A4. Moreover, the molecular mechanisms and the potency of solid lipid curcumin particles (SLCP) as an E2 replacement therapy for TDP-43 associated neuropathy was analyzed. We further examined the survival rates and the pathological TDP43 patterns in female and male Prp-TDP-43A315T mice administrated with or without SLCP. In addition, the changed expression levels of enzymes corresponding to E2 biosynthesis and degradation in the spinal cord of female and male Prp-TDP-43A315T mice with or without SLCP were determined.

RESULTS

We found that in addition to E2, the expression patterns of CYP19A1 and CYP3A4 proteins differed between Prp-TDP-43A315T mice compared to wild-type control, suggesting that toxic phosphorylated TDP43 oligomers may disrupt the balance between CYP19A1 and CYP3A4 expression, leading to reduced estrogen biosynthesis and accelerated degradation. In addition, we found that oral administration of SLCP prolonged the survival rates in female Prp-TDP-43A315T mice and significantly reduced the pathological insoluble phosphorylated TDP-43 species. Furthermore, SLCP attenuated disease progression associated with TDP-43-related neuropathies through modulating estrogen biosynthesis and the activity of CYP450 enzymes.

CONCLUSIONS

Our results showed that Prp-TDP-43A315T mice exhibit altered estradiol levels. Moreover, we demonstrated the efficacy of SLCP as an estrogen replacement therapy in mitigating TDP-43-associated disease progression and pathogenesis. These findings suggest that SLCP could be a promising strategy to induce E2 expression for the treatment of ALS and FTLD.

Altered mRNA transport and local translation in iNeurons with RNA binding protein knockdown

Neurons rely on mRNA transport and local translation to facilitate rapid protein synthesis in processes far from the cell body. These processes allow precise spatial and temporal control of translation and are mediated by RNA binding proteins (RBPs), including those known to be associated with neurodegenerative diseases. Here, we use proteomics, transcriptomics, and microscopy to investigate the impact of RBP knockdown on mRNA transport and local translation in iPSC-derived neurons. We find thousands of transcripts enriched in neurites and that many of these transcripts are locally translated, possibly due to the shorter length of transcripts in neurites. Loss of frontotemporal dementia/amyotrophic lateral sclerosis (FTD/ALS)-associated RBPs TDP-43 and hnRNPA1 lead to distinct alterations in the neuritic proteome and transcriptome. TDP-43 knockdown (KD) leads to increased neuritic mRNA and translation. In contrast, hnRNPA1 leads to increased neuritic mRNA, but not translation, and more moderate effects on local mRNA profiles, possibly due to compensation by hnRNPA3. These results highlight the crucial role of FTD/ALS-associated RBPs in mRNA transport and local translation in neurons and the importance of these processes in neuron health and disease.

Association of cortical morphology, white matter hyperintensity, and glymphatic function in frontotemporal dementia variants

INTRODUCTION

Frontotemporal dementia (FTD) can be phenotypically divided into behavioral variant FTD (bvFTD), nonfluent variant primary progressive aphasia (nfvPPA), and semantic variant PPA (svPPA). However, the neural underpinnings of this phenotypic heterogeneity remain elusive.

METHODS

Cortical morphology, white matter hyperintensities (WMH), diffusion tensor image analysis along the perivascular space (DTI-ALPS), and their interrelationships were assessed in subtypes of FTD. Neuroimaging-transcriptional analyses on the regional cortical morphological deviances among subtypes were also performed.

RESULTS

Changes in cortical thickness, surface area, gyrification, WMH, and DTI-ALPS were subtype-specific in FTD. The three morphologic indices are related to whole-brain WMH volume and cognitive performance, while cortical thickness is related to DTI-ALPS. Neuroimaging-transcriptional analyses identified key biological pathways linked to the formation and/or spread of TDP-43/tau pathologies.

DISCUSSION

We found subtype-specific changes in cortical morphology, WMH, and glymphatic function in FTD. Our findings have the potential to contribute to the development of personalized predictions and treatment strategies for this disorder.

HIGHLIGHTS

Cortical morphologic changes, white matter hyperintensities (WMH), and glymphatic dysfunction are subtype-specific. Cortical morphologic changes, WMH, and glymphatic dysfunction are inter-correlated. Cortical morphologic changes and WMH burden contribute to cognitive impairments.

Influence of ALS-linked M337V mutation on the conformational ensembles of TDP-43321-340 peptide monomer and dimer

The transactive response (TAR) DNA/RNA-binding protein 43 (TDP-43) can self-assemble into both functional stress granules via liquid-liquid phase separation (LLPS) and pathogenic amyloid fibrillary aggregates that are closely linked to amyotrophic lateral sclerosis. Previous experimental studies reported that the low complexity domain (LCD) of TDP-43 plays an essential role in the LLPS and aggregation of the full-length protein, and it alone can also undergo LLPS to form liquid droplets mainly via intermolecular interactions in the 321-340 region. And the ALS-associated M337V mutation impairs LCD's LLPS and facilitates liquid-solid phase transition. However, the underlying atomistic mechanism is not well understood. Herein, as a first step to understand the M337V-caused LLPS disruption of TDP-43 LCD mediated by the 321-340 region and the fibrillization enhancement, we investigated the conformational properties of monomer/dimer of TDP-43321-340 peptide and its M337V mutant by performing extensive all-atom explicit-solvent replica exchange molecular dynamic simulations. Our simulations demonstrate that M337V mutation alters the residue regions with high helix/β-structure propensities and thus affects the conformational ensembles of both monomer and dimer. M337V mutation inhibits helix formation in the N-terminal Ala-rich region and the C-terminal mutation site region, while facilitating their long β-sheet formation, albeit with a minor impact on the average probability of both helix structure and β-structure. Further analysis of dimer system shows that M337V mutation disrupts inter-molecular helix-helix interactions and W334-W334 π-π stacking interactions which were reported to be important for the LLPS of TDP-43 LCD, whereas enhances the overall peptide residue-residue interactions and weakens peptide-water interactions, which is conducive to peptide fibrillization. This study provides mechanistic insights into the M337V-mutation-induced impairment of phase separation and facilitation of fibril formation of TDP-43 LCD.

Microtubules, Membranes, and Movement: New Roles for Stathmin-2 in Axon Integrity

Neurons establish functional connections responsible for how we perceive and react to the world around us. Communication from a neuron to its target cell occurs through a long projection called an axon. Axon distances can exceed 1 m in length in humans and require a dynamic microtubule cytoskeleton for growth during development and maintenance in adulthood. Stathmins are microtubule-associated proteins that function as relays between kinase signaling and microtubule polymerization. In this review, we describe the prolific role of Stathmins in microtubule homeostasis with an emphasis on emerging roles for Stathmin-2 (Stmn2) in axon integrity and neurodegeneration. Stmn2 levels are altered in Amyotrophic Lateral Sclerosis and loss of Stmn2 provokes motor and sensory neuropathies. There is growing potential for employing Stmn2 as a disease biomarker or even a therapeutic target. Meeting this potential requires a mechanistic understanding of emerging complexity in Stmn2 function. In particular, Stmn2 palmitoylation has a surprising contribution to axon maintenance through undefined mechanisms linking membrane association, tubulin interaction, and axon transport. Exploring these connections will reveal new insight on neuronal cell biology and novel opportunities for disease intervention.

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.

A robust evaluation of TDP-43, poly GP, cellular pathology and behavior in a AAV-C9ORF72 (G4C2)66 mouse model

The G4C2 hexanucleotide repeat expansion in C9ORF72 is the major genetic cause of both amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) (C9-ALS/FTD). Despite considerable efforts, the development of mouse models of C9-ALS/FTD useful for therapeutic development has proven challenging due to the intricate interplay of genetic and molecular factors underlying this neurodegenerative disorder, in addition to species differences. This study presents a robust investigation of the cellular pathophysiology and behavioral outcomes in a previously described AAV mouse model of C9-ALS expressing 66 G4C2 hexanucleotide repeats. Despite displaying key molecular ALS pathological markers including RNA foci, dipeptide repeat (DPR) protein aggregation, p62 positive stress granule formation as well as mild gliosis, the AAV-(G4C2)66 mouse model in this study exhibits negligible neuronal loss, no motor deficits, and functionally unimpaired TAR DNA-binding protein-43 (TDP-43). While our findings indicate and support that this is a robust and pharmacologically tractable model for investigating the molecular mechanisms and cellular consequences of (G4C2) repeat driven DPR pathology, it is not suitable for investigating the development of disease associated neurodegeneration, TDP-43 dysfunction, gliosis, and motor performance. Our findings underscore the complexity of ALS pathogenesis involving genetic mutations and protein dysregulation and highlight the need for more comprehensive model systems that reliably replicate the multifaceted cellular and behavioral aspects of C9-ALS.

Aberrant protein aggregation in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal disease. As its pathological mechanisms are not well understood, there are no efficient therapeutics for it at present. While it is highly heterogenous both etiologically and clinically, it has a common salient hallmark, i.e., aberrant protein aggregation (APA). The upstream pathogenesis and the downstream effects of APA in ALS are sophisticated and the investigation of this pathology would be of consequence for understanding ALS. In this paper, the pathomechanism of APA in ALS and the candidate treatment strategies for it are discussed.

Stress-induced TDP-43 nuclear condensation causes splicing loss of function and STMN2 depletion

TDP-43 protein is dysregulated in several neurodegenerative diseases, which often have a multifactorial nature and may have extrinsic stressors as a "second hit." TDP-43 undergoes reversible nuclear condensation in stressed cells including neurons. Here, we demonstrate that stress-inducible nuclear TDP-43 condensates are RNA-depleted, non-liquid assemblies distinct from the known nuclear bodies. Their formation requires TDP-43 oligomerization and ATP and is inhibited by RNA. Using a confocal nanoscanning assay, we find that amyotrophic lateral sclerosis (ALS)-linked mutations alter stress-induced TDP-43 condensation by changing its affinity to liquid-like ribonucleoprotein assemblies. Stress-induced nuclear condensation transiently inactivates TDP-43, leading to loss of interaction with its protein binding partners and loss of function in splicing. Splicing changes are especially prominent and persisting for STMN2 RNA, and STMN2 protein becomes rapidly depleted early during stress. Our results point to early pathological changes to TDP-43 in the nucleus and support therapeutic modulation of stress response in ALS.

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.

Long way up: rethink diseases in light of phase separation and phase transition

Biomolecular condensation, driven by multivalency, serves as a fundamental mechanism within cells, facilitating the formation of distinct compartments, including membraneless organelles that play essential roles in various cellular processes. Perturbations in the delicate equilibrium of condensation, whether resulting in gain or loss of phase separation, have robustly been associated with cellular dysfunction and physiological disorders. As ongoing research endeavors wholeheartedly embrace this newly acknowledged principle, a transformative shift is occurring in our comprehension of disease. Consequently, significant strides have been made in unraveling the profound relevance and potential causal connections between abnormal phase separation and various diseases. This comprehensive review presents compelling recent evidence that highlight the intricate associations between aberrant phase separation and neurodegenerative diseases, cancers, and infectious diseases. Additionally, we provide a succinct summary of current efforts and propose innovative solutions for the development of potential therapeutics to combat the pathological consequences attributed to aberrant phase separation.

Annexin A11 aggregation in FTLD-TDP type C and related neurodegenerative disease proteinopathies

TAR DNA-binding protein 43 (TDP-43) is an RNA binding protein found within ribonucleoprotein granules tethered to lysosomes via annexin A11. TDP-43 protein forms inclusions in many neurodegenerative diseases including amyotrophic lateral sclerosis (ALS), frontotemporal lobar degeneration with TDP-43 inclusions (FTLD-TDP) and limbic predominant age-related TDP-43 encephalopathy neuropathologic change (LATE-NC). Annexin A11 is also known to form aggregates in ALS cases with pathogenic variants in ANXA11. Annexin A11 aggregation has not been described in sporadic ALS, FTLD-TDP or LATE-NC cases. To explore the relationship between TDP-43 and annexin A11, genetic analysis of 822 autopsy cases was performed to identify rare ANXA11 variants. In addition, an immunohistochemical study of 368 autopsy cases was performed to identify annexin A11 aggregates. Insoluble annexin A11 aggregates which colocalize with TDP-43 inclusions were present in all FTLD-TDP Type C cases. Annexin A11 inclusions were also seen in a small proportion (3-6%) of sporadic and genetic forms of FTLD-TDP types A and B, ALS, and LATE-NC. In addition, we confirm the comingling of annexin A11 and TDP-43 aggregates in an ALS case with the pathogenic ANXA11 p.G38R variant. Finally, we found abundant annexin A11 inclusions as the primary pathologic finding in a case of progressive supranuclear palsy-like frontotemporal dementia with prominent striatal vacuolization due to a novel variant, ANXA11 p.P75S. By immunoblot, FTLD-TDP with annexinopathy and ANXA11 variant cases show accumulation of insoluble ANXA11 including a truncated fragment. These results indicate that annexin A11 forms a diverse and heterogeneous range of aggregates in both sporadic and genetic forms of TDP-43 proteinopathies. In addition, the finding of a primary vacuolar annexinopathy due to ANXA11 p.P75S suggests that annexin A11 aggregation is sufficient to cause neurodegeneration.

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.

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.

Targeting 14-3-3θ-mediated TDP-43 pathology in amyotrophic lateral sclerosis and frontotemporal dementia mice

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are characterized by cytoplasmic deposition of the nuclear TAR-binding protein 43 (TDP-43). Although cytoplasmic re-localization of TDP-43 is a key event in the pathogenesis of ALS/FTD, the underlying mechanisms remain unknown. Here, we identified a non-canonical interaction between 14-3-3θ and TDP-43, which regulates nuclear-cytoplasmic shuttling. Neuronal 14-3-3θ levels were increased in sporadic ALS and FTD with TDP-43 pathology. Pathogenic TDP-43 showed increased interaction with 14-3-3θ, resulting in cytoplasmic accumulation, insolubility, phosphorylation, and fragmentation of TDP-43, resembling pathological changes in disease. Harnessing this increased affinity of 14-3-3θ for pathogenic TDP-43, we devised a gene therapy vector targeting TDP-43 pathology, which mitigated functional deficits and neurodegeneration in different ALS/FTD mouse models expressing mutant or non-mutant TDP-43, including when already symptomatic at the time of treatment. Our study identified 14-3-3θ as a mediator of cytoplasmic TDP-43 localization with implications for ALS/FTD pathogenesis and therapy.

The pathogenic mechanism of TAR DNA-binding protein 43 (TDP-43) in amyotrophic lateral sclerosis

The onset of amyotrophic lateral sclerosis is usually characterized by focal death of both upper and/or lower motor neurons occurring in the motor cortex, basal ganglia, brainstem, and spinal cord, and commonly involves the muscles of the upper and/or lower extremities, and the muscles of the bulbar and/or respiratory regions. However, as the disease progresses, it affects the adjacent body regions, leading to generalized muscle weakness, occasionally along with memory, cognitive, behavioral, and language impairments; respiratory dysfunction occurs at the final stage of the disease. The disease has a complicated pathophysiology and currently, only riluzole, edaravone, and phenylbutyrate/taurursodiol are licensed to treat amyotrophic lateral sclerosis in many industrialized countries. The TAR DNA-binding protein 43 inclusions are observed in 97% of those diagnosed with amyotrophic lateral sclerosis. This review provides a preliminary overview of the potential effects of TAR DNA-binding protein 43 in the pathogenesis of amyotrophic lateral sclerosis, including the abnormalities in nucleoplasmic transport, RNA function, post-translational modification, liquid-liquid phase separation, stress granules, mitochondrial dysfunction, oxidative stress, axonal transport, protein quality control system, and non-cellular autonomous functions (e.g., glial cell functions and prion-like propagation).

Rational Design of TDP-43 Derived α-Helical Peptide Inhibitors: An In Silico Strategy to Prevent TDP-43 Aggregation in Neurodegenerative Disorders

TDP-43, an essential RNA/DNA-binding protein, is central to the pathology of neurodegenerative diseases, such as amyotrophic lateral sclerosis and frontotemporal dementia. Pathological mislocalization and aggregation of TDP-43 disrupt RNA splicing, mRNA stability, and mRNA transport, thereby impairing neuronal function and survival. The formation of amyloid-like TDP-43 filaments is largely facilitated by the destabilization of an α-helical segment within the disordered C-terminal region. In this study, we hypothesized that preventing the destabilization of the α-helical domain could potentially halt the growth of these pathological filaments. To explore this, we utilized a range of in silico techniques to design and evaluate peptide-based therapeutics that bind to pathological TDP-43 amyloid-like filament crystal structures and resist β sheet conversion. Our computational approaches, including biophysical and secondary structure property prediction, molecular docking, 3D structure prediction, and molecular dynamics simulations, were used to assess the structure, stability, and binding affinity of these peptides in relation to pathological TDP-43 filaments. The results of our in silico analyses identified a selection of promising peptides which displayed a stable α-helical structure, exhibited an increased number of intramolecular hydrogen bonds within the helical domain, and demonstrated high binding affinities for pathological TDP-43 amyloid-like filaments. Molecular dynamics simulations provided further support for the structural and thermodynamic stability of these peptides, as they exhibited lower root-mean-square deviation and more favorable free energy landscapes over 300 ns. These findings establish α-helical propensity peptides as potential lead molecules for the development of novel therapeutics against TDP-43 aggregation. This structure-based computational approach for the rational design of peptide inhibitors opens a new direction in the search for effective interventions for ALS, FTD, and other related neurodegenerative diseases. The peptides identified as the most promising candidates in this study are currently subject to further testing and validation through both in vitro and in vivo experiments.

The implications of physiological biomolecular condensates in amyotrophic lateral sclerosis

In recent years, there has been an emphasis on the role of phase-separated biomolecular condensates, especially stress granules, in neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). This is largely due to several ALS-associated mutations occurring in genes involved in stress granule assembly and observations that pathological inclusions detected in ALS patient neurons contain stress granule proteins, including the ALS-linked proteins TDP-43 and FUS. However, protein components of stress granules are also found in numerous other phase-separated biomolecular condensates under physiological conditions which are inadequately discussed in the context of ALS. In this review, we look beyond stress granules and describe the roles of TDP-43 and FUS in physiological condensates occurring in the nucleus and neurites, such as the nucleolus, Cajal bodies, paraspeckles and neuronal RNA transport granules. We also discuss the consequences of ALS-linked mutations in TDP-43 and FUS on their ability to phase separate into these stress-independent biomolecular condensates and perform their respective functions. Importantly, biomolecular condensates sequester multiple overlapping protein and RNA components, and their dysregulation could contribute to the observed pleiotropic effects of both sporadic and familial ALS on RNA metabolism.