Super-Resolution Microscopy Reveals Presynaptic Localization of the ALS/FTD Related Protein FUS in Hippocampal Neurons

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.

Emerging biophysical techniques for probing synaptic transmission in neurodegenerative disorders

Plethora of research has shed light on the critical role of synaptic dysfunction in various neurodegenerative disorders (NDDs), including Alzheimer's disease (AD), Parkinson's disease (PD), Amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD). Synapses, the fundamental units for neural communication in the brain, are highly vulnerable to pathological conditions and are central to the progression of neurological diseases. The presynaptic terminal, a key component of synapses responsible for neurotransmitter release and synaptic communication, undergoes structural and functional alterations in these disorders. Understanding synaptic transmission abnormalities is crucial for unravelling the pathophysiological mechanisms underlying neurodegeneration. In the quest to probe synaptic transmission in NDDs, emerging biophysical techniques play a pivotal role. These advanced methods offer insights into the structural and functional changes occurring at nerve terminals in conditions like AD, PD, HD & ALS. By investigating synaptic plasticity and alterations in neurotransmitter release dynamics, researchers can uncover valuable information about disease progression and potential therapeutic targets. The review articles highlighted provide a comprehensive overview of how synaptic vulnerability and pathology are shared mechanisms across a spectrum of neurological disorders. In major neurodegenerative diseases, synaptic dysfunction is a common thread linking these conditions. The intricate molecular machinery involved in neurotransmitter release, synaptic vesicle dynamics, and presynaptic protein regulation are key areas of focus for understanding synaptic alterations in neurodegenerative diseases.

Synaptopathy: presynaptic convergence in frontotemporal dementia and amyotrophic lateral sclerosis

Frontotemporal dementia and amyotrophic lateral sclerosis are common forms of neurodegenerative disease that share overlapping genetics and pathologies. Crucially, no significantly disease-modifying treatments are available for either disease. Identifying the earliest changes that initiate neuronal dysfunction is important for designing effective intervention therapeutics. The genes mutated in genetic forms of frontotemporal dementia and amyotrophic lateral sclerosis have diverse cellular functions, and multiple disease mechanisms have been proposed for both. Identification of a convergent disease mechanism in frontotemporal dementia and amyotrophic lateral sclerosis would focus research for a targetable pathway, which could potentially effectively treat all forms of frontotemporal dementia and amyotrophic lateral sclerosis (both familial and sporadic). Synaptopathies are diseases resulting from physiological dysfunction of synapses, and define the earliest stages in multiple neuronal diseases, with synapse loss a key feature in dementia. At the presynapse, the process of synaptic vesicle recruitment, fusion and recycling is necessary for activity-dependent neurotransmitter release. The unique distal location of the presynaptic terminal means the tight spatio-temporal control of presynaptic homeostasis is dependent on efficient local protein translation and degradation. Recently, numerous publications have shown that mutations associated with frontotemporal dementia and amyotrophic lateral sclerosis present with synaptopathy characterized by presynaptic dysfunction. This review will describe the complex local signalling and membrane trafficking events that occur at the presynapse to facilitate neurotransmission and will summarize recent publications linking frontotemporal dementia/amyotrophic lateral sclerosis genetic mutations to presynaptic function. This evidence indicates that presynaptic synaptopathy is an early and convergent event in frontotemporal dementia and amyotrophic lateral sclerosis and illustrates the need for further research in this area, to identify potential therapeutic targets with the ability to impact this convergent pathomechanism.

RNA-binding proteins in pain

RNAs are meticulously controlled by proteins. Through direct and indirect associations, every facet in the brief life of an mRNA is subject to regulation. RNA-binding proteins (RBPs) permeate biology. Here, we focus on their roles in pain. Chronic pain is among the largest challenges facing medicine and requires new strategies. Mounting pharmacologic and genetic evidence obtained in pre-clinical models suggests fundamental roles for a broad array of RBPs. We describe their diverse roles that span RNA modification, splicing, stability, translation, and decay. Finally, we highlight opportunities to expand our understanding of regulatory interactions that contribute to pain signaling. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications Translation > Regulation RNA in Disease and Development > RNA in Disease.

Animal Models of FUS-Proteinopathy: A Systematic Review

Mutations that disrupt the function of the DNA/RNA-binding protein FUS could cause amyotrophic lateral sclerosis (ALS) and other neurodegenerative diseases. One of the key features in ALS pathogenesis is the formation of insoluble protein aggregates containing aberrant isoforms of the FUS protein in the cytoplasm of upper and lower motor neurons. Reproduction of human pathology in animal models is the main tool for studying FUS-associated pathology and searching for potential therapeutic agents for ALS treatment. In this review, we provide a systematic analysis of the role of FUS protein in ALS pathogenesis and an overview of the results of modelling FUS-proteinopathy in animals.

Mimicking hypomethylation of FUS requires liquid-liquid phase separation to induce synaptic dysfunctions

The hypomethylation of fused in sarcoma (FUS) in frontotemporal lobar degeneration promotes the formation of irreversible condensates of FUS. However, the mechanisms by which these hypomethylated FUS condensates cause neuronal dysfunction are unknown. Here we report that expression of FUS constructs mimicking hypomethylated FUS causes aberrant dendritic FUS condensates in CA1 neurons. These hypomethylated FUS condensates exhibit spontaneous, and activity induced movement within the dendrite. They impair excitatory synaptic transmission, postsynaptic density-95 expression, and dendritic spine plasticity. These neurophysiological defects are dependent upon both the dendritic localisation of the condensates, and their ability to undergo liquid-liquid phase separation. These results indicate that the irreversible liquid-liquid phase separation is a key component of hypomethylated FUS pathophysiology in sporadic FTLD, and this can cause synapse dysfunction in sporadic FTLD.

Integrative proteomics highlight presynaptic alterations and c-Jun misactivation as convergent pathomechanisms in ALS

Amyotrophic Lateral Sclerosis (ALS) is a fatal neurodegenerative disease mainly affecting upper and lower motoneurons. Several functionally heterogeneous genes have been associated with the familial form of this disorder (fALS), depicting an extremely complex pathogenic landscape. This heterogeneity has limited the identification of an effective therapy, and this bleak prognosis will only improve with a greater understanding of convergent disease mechanisms. Recent evidence from human post-mortem material and diverse model systems has highlighted the synapse as a crucial structure actively involved in disease progression, suggesting that synaptic aberrations might represent a shared pathological feature across the ALS spectrum. To test this hypothesis, we performed the first comprehensive analysis of the synaptic proteome from post-mortem spinal cord and human iPSC-derived motoneurons carrying mutations in the major ALS genes. This integrated approach highlighted perturbations in the molecular machinery controlling vesicle release as a shared pathomechanism in ALS. Mechanistically, phosphoproteomic analysis linked the presynaptic vesicular phenotype to an accumulation of cytotoxic protein aggregates and to the pro-apoptotic activation of the transcription factor c-Jun, providing detailed insights into the shared pathobiochemistry in ALS. Notably, sub-chronic treatment of our iPSC-derived motoneurons with the fatty acid docosahexaenoic acid exerted a neuroprotective effect by efficiently rescuing the alterations revealed by our multidisciplinary approach. Together, this study provides strong evidence for the central and convergent role played by the synaptic microenvironment within the ALS spinal cord and highlights a potential therapeutic target that counteracts degeneration in a heterogeneous cohort of human motoneuron cultures.

Synaptic expression of TAR-DNA-binding protein 43 in the mouse spinal cord determined using super-resolution microscopy

Amyotrophic Lateral Sclerosis (ALS) is characterised by a loss of motor neurons in the brain and spinal cord that is preceded by early-stage changes in synapses that may be associated with TAR-DNA-Binding Protein 43 (TDP-43) pathology. Cellular inclusions of hyperphosphorylated TDP-43 (pTDP-43) are a key hallmark of neurodegenerative diseases such ALS. However, there has been little characterisation of the synaptic expression of TDP-43 inside subpopulations of spinal cord synapses. This study utilises a range of high-resolution and super-resolution microscopy techniques with immunolabelling, as well as an aptamer-based TDP-43 labelling strategy visualised with single-molecule localisation microscopy, to characterise and quantify the presence of pTDP-43 in populations of excitatory synapses near where motor neurons reside in the lateral ventral horn of the mouse lumbar spinal cord. We observe that TDP-43 is expressed in approximately half of spinal cord synapses as nanoscale clusters. Synaptic TDP-43 clusters are found most abundantly at synapses associated with VGLUT1-positive presynaptic terminals, compared to VGLUT2-associated synapses. Our nanoscopy techniques showed no difference in the subsynaptic expression of pTDP-43 in the ALS mouse model, SOD1G93a, compared to healthy controls, despite prominent structural deficits in VGLUT1-associated synapses in SOD1G93a mice. This research characterises the basic synaptic expression of TDP-43 with nanoscale precision and provides a framework with which to investigate the potential relationship between TDP-43 pathology and synaptic pathology in neurodegenerative diseases.

CLARITY increases sensitivity and specificity of fluorescence immunostaining in long-term archived human brain tissue

BACKGROUND

Post mortem human brain tissue is an essential resource to study cell types, connectivity as well as subcellular structures down to the molecular setup of the central nervous system especially with respect to the plethora of brain diseases. A key method is immunostaining with fluorescent dyes, which allows high-resolution imaging in three dimensions of multiple structures simultaneously. Although there are large collections of formalin-fixed brains, research is often limited because several conditions arise that complicate the use of human brain tissue for high-resolution fluorescence microscopy.

RESULTS

In this study, we developed a clearing approach for immunofluorescence-based analysis of perfusion- and immersion-fixed post mortem human brain tissue, termed human Clear Lipid-exchanged Acrylamide-hybridized Rigid Imaging / Immunostaining / In situ hybridization-compatible Tissue-hYdrogel (hCLARITY). hCLARITY is optimized for specificity by reducing off-target labeling and yields very sensitive stainings in human brain sections allowing for super-resolution microscopy with unprecedented imaging of pre- and postsynaptic compartments. Moreover, hallmarks of Alzheimer's disease were preserved with hCLARITY, and importantly classical 3,3'-diaminobenzidine (DAB) or Nissl stainings are compatible with this protocol. hCLARITY is very versatile as demonstrated by the use of more than 30 well performing antibodies and allows for de- and subsequent re-staining of the same tissue section, which is important for multi-labeling approaches, e.g., in super-resolution microscopy.

CONCLUSIONS

Taken together, hCLARITY enables research of the human brain with high sensitivity and down to sub-diffraction resolution. It therefore has enormous potential for the investigation of local morphological changes, e.g., in neurodegenerative diseases.

Genetics of amyotrophic lateral sclerosis: seeking therapeutic targets in the era of gene therapy

Amyotrophic lateral sclerosis (ALS) is an intractable disease that causes respiratory failure leading to mortality. The main locus of ALS is motor neurons. The success of antisense oligonucleotide (ASO) therapy in spinal muscular atrophy (SMA), a motor neuron disease, has triggered a paradigm shift in developing ALS therapies. The causative genes of ALS and disease-modifying genes, including those of sporadic ALS, have been identified one after another. Thus, the freedom of target choice for gene therapy has expanded by ASO strategy, leading to new avenues for therapeutic development. Tofersen for superoxide dismutase 1 (SOD1) was a pioneer in developing ASO for ALS. Improving protocols and devising early interventions for the disease are vital. In this review, we updated the knowledge of causative genes in ALS. We summarized the genetic mutations identified in familial ALS and their clinical features, focusing on SOD1, fused in sarcoma (FUS), and transacting response DNA-binding protein. The frequency of the C9ORF72 mutation is low in Japan, unlike in Europe and the United States, while SOD1 and FUS are more common, indicating that the target mutations for gene therapy vary by ethnicity. A genome-wide association study has revealed disease-modifying genes, which could be the novel target of gene therapy. The current status and prospects of gene therapy development were discussed, including ethical issues. Furthermore, we discussed the potential of axonal pathology as new therapeutic targets of ALS from the perspective of early intervention, including intra-axonal transcription factors, neuromuscular junction disconnection, dysregulated local translation, abnormal protein degradation, mitochondrial pathology, impaired axonal transport, aberrant cytoskeleton, and axon branching. We simultaneously discuss important pathological states of cell bodies: persistent stress granules, disrupted nucleocytoplasmic transport, and cryptic splicing. The development of gene therapy based on the elucidation of disease-modifying genes and early intervention in molecular pathology is expected to become an important therapeutic strategy in ALS.

Synaptic Dysfunction and Plasticity in Amyotrophic Lateral Sclerosis

Recent evidence has supported the hypothesis that amyotrophic lateral sclerosis (ALS) is a multi-step disease, as the onset of symptoms occurs after sequential exposure to a defined number of risk factors. Despite the lack of precise identification of these disease determinants, it is known that genetic mutations may contribute to one or more of the steps leading to ALS onset, the remaining being linked to environmental factors and lifestyle. It also appears evident that compensatory plastic changes taking place at all levels of the nervous system during ALS etiopathogenesis may likely counteract the functional effects of neurodegeneration and affect the timing of disease onset and progression. Functional and structural events of synaptic plasticity probably represent the main mechanisms underlying this adaptive capability, causing a significant, although partial and transient, resiliency of the nervous system affected by a neurodegenerative disease. On the other hand, the failure of synaptic functions and plasticity may be part of the pathological process. The aim of this review was to summarize what it is known today about the controversial involvement of synapses in ALS etiopathogenesis, and an analysis of the literature, although not exhaustive, confirmed that synaptic dysfunction is an early pathogenetic process in ALS. Moreover, it appears that adequate modulation of structural and functional synaptic plasticity may likely support function sparing and delay disease progression.

Interleukin-13 and its receptor are synaptic proteins involved in plasticity and neuroprotection

Immune system molecules are expressed by neurons, yet their functions are often unknown. We have identified IL-13 and its receptor IL-13Ra1 as neuronal, synaptic proteins in mouse, rat, and human brains, whose engagement upregulates the phosphorylation of NMDAR and AMPAR subunits and, in turn, increases synaptic activity and CREB-mediated transcription. We demonstrate that increased IL-13 is a hallmark of traumatic brain injury (TBI) in male mice as well as in two distinct cohorts of human patients. We also provide evidence that IL-13 upregulation protects neurons from excitotoxic death. We show IL-13 upregulation occurring in several cohorts of human brain samples and in cerebrospinal fluid (CSF). Thus, IL-13 is a physiological modulator of synaptic physiology of neuronal origin, with implications for the establishment of synaptic plasticity and the survival of neurons under injury conditions. Furthermore, we suggest that the neuroprotection afforded through the upregulation of IL-13 represents an entry point for interventions in the pathophysiology of TBI.

Stress induced TDP-43 mobility loss independent of stress granules

TAR DNA binding protein 43 (TDP-43) is closely related to the pathogenesis of amyotrophic lateral sclerosis (ALS) and translocates to stress granules (SGs). The role of SGs as aggregation-promoting “crucibles” for TDP-43, however, is still under debate. We analyzed TDP-43 mobility and localization under different stress and recovery conditions using live cell single-molecule tracking and super-resolution microscopy. Besides reduced mobility within SGs, a stress induced decrease of TDP-43 mobility in the cytoplasm and the nucleus was observed. Stress removal led to a recovery of TDP-43 mobility, which strongly depended on the stress duration. ‘Stimulated-emission depletion microscopy’ (STED) and ‘tracking and localization microscopy’ (TALM) revealed not only TDP-43 substructures within stress granules but also numerous patches of slow TDP-43 species throughout the cytoplasm. This work provides insights into the aggregation of TDP-43 in living cells and provide evidence suggesting that TDP-43 oligomerization and aggregation takes place in the cytoplasm separate from SGs.

Amyotrophic Lateral Sclerosis related TDP-43 protein translocates to stress granules with a concomitant reduction in mobility. Here, the authors use single molecule tracking and find a stress-induced reduction in TDP-43 mobility also in the cytoplasm potentially relevant for TDP-43 aggregation.

Single-molecule tracking (SMT) and localization of SRF and MRTF transcription factors during neuronal stimulation and differentiation

In cells, proteins encoded by the same gene do not all behave uniformly but engage in functional subpopulations induced by spatial or temporal segregation. While conventional microscopy has limitations in revealing such spatial and temporal diversity, single-molecule tracking (SMT) microscopy circumvented this problem and allows for high-resolution imaging and quantification of dynamic single-molecule properties. Particularly in the nucleus, SMT has identified specific DNA residence times of transcription factors (TFs), DNA-bound TF fractions and positions of transcriptional hot-spots upon cell stimulation. By contrast to cell stimulation, SMT has not been employed to follow dynamic TF changes along stages of cell differentiation. Herein, we analysed the serum response factor (SRF), a TF involved in the differentiation of many cell types to study nuclear single-molecule dynamics in neuronal differentiation. Our data in living mouse hippocampal neurons show dynamic changes in SRF DNA residence time and SRF DNA-bound fraction between the stages of adhesion, neurite growth and neurite differentiation in axon and dendrites. Using TALM (tracking and localization microscopy), we identified nuclear positions of SRF clusters and observed changes in their numbers and size during differentiation. Furthermore, we show that the SRF cofactor MRTF-A (myocardin-related TF or MKL1) responds to cell activation by enhancing the long-bound DNA fraction. Finally, a first SMT colocalization study of two proteins was performed in living cells showing enhanced SRF/MRTF-A colocalization upon stimulation. In summary, SMT revealed modulation of dynamic TF properties during cell stimulation and differentiation.

Liquid-Liquid Phase Separation of TDP-43 and FUS in Physiology and Pathology of Neurodegenerative Diseases

Liquid-liquid phase separation of RNA-binding proteins mediates the formation of numerous membraneless organelles with essential cellular function. However, aberrant phase transition of these proteins leads to the formation of insoluble protein aggregates, which are pathological hallmarks of neurodegenerative diseases including ALS and FTD. TDP-43 and FUS are two such RNA-binding proteins that mislocalize and aggregate in patients of ALS and FTD. They have similar domain structures that provide multivalent interactions driving their phase separation in vitro and in the cellular environment. In this article, we review the factors that mediate and regulate phase separation of TDP-43 and FUS. We also review evidences that connect the phase separation property of TDP-43 and FUS to their functional roles in cells. Aberrant phase transition of TDP-43 and FUS leads to protein aggregation and disrupts their regular cell function. Therefore, restoration of functional protein phase of TDP-43 and FUS could be beneficial for neuronal cells. We discuss possible mechanisms for TDP-43 and FUS aberrant phase transition and aggregation while reviewing the methods that are currently being explored as potential therapeutic strategies to mitigate aberrant phase transition and aggregation of TDP-43 and FUS.

Identification of RNA-RBP Interactions in Subcellular Compartments by CLIP-Seq

Cross-linking immunoprecipitation and high-throughput sequencing (CLIP-seq) allows the identification of RNA targets bound by a specific RNA-binding protein (RBP) in in vivo and ex vivo experimental models with high specificity. Due to the little RNA yield obtained after cross-linking, immunoprecipitation, polyacrylamide gel electrophoresis, membrane transfer, and RNA extraction, CLIP-seq is usually performed from relatively large amounts of starting material, like cell lysates or tissue homogenates. However, RBP binding of its specific RNA targets depends on its subcellular localization, and a different set of RNAs may be bound by the same RBP within distinct subcellular sites. To uncover these RNA subsets, preparation of CLIP-seq libraries from specific subcellular compartments and comparison to CLIP-seq datasets from total lysates is necessary, yet there are currently no available protocols for this. Here we describe the adaptation of CLIP-seq to identify the specific RNA targets of an RBP (FUS) at a small subcompartment, that is, neuronal synapses, including subcompartment isolation, RBP-RNA complex enrichment, and upscaling steps.

Super-resolution imaging to reveal the nanostructure of tripartite synapses

Even though neurons are the main drivers of information processing in the brain and spinal cord, other cell types are important to mediate adequate flow of information. These include electrically passive glial cells such as microglia and astrocytes, which recently emerged as active partners facilitating proper signal transduction. In disease, these cells undergo pathophysiological changes that propel disease progression and change synaptic connections and signal transmission. In the healthy brain, astrocytic processes contact pre- and postsynaptic structures. These processes can be nanoscopic, and therefore only electron microscopy has been able to reveal their structure and morphology. However, electron microscopy is not suitable in revealing dynamic changes, and it is labour- and time-intensive. The dawn of super-resolution microscopy, techniques that 'break' the diffraction limit of conventional light microscopy, over the last decades has enabled researchers to reveal the nanoscopic synaptic environment. In this review, we highlight and discuss recent advances in our understanding of the nano-world of the so-called tripartite synapses, the relationship between pre- and postsynapse as well as astrocytic processes. Overall, novel super-resolution microscopy methods are needed to fully illuminate the intimate relationship between glia and neuronal cells that underlies signal transduction in the brain and that might be affected in diseases such as Alzheimer's disease and epilepsy.

Epitope-preserving magnified analysis of proteome (eMAP)

Synthetic tissue-hydrogel methods have enabled superresolution investigation of biological systems using diffraction-limited microscopy. However, chemical modification by fixatives can cause loss of antigenicity, limiting molecular interrogation of the tissue gel. Here, we present epitope-preserving magnified analysis of proteome (eMAP) that uses purely physical tissue-gel hybridization to minimize the loss of antigenicity while allowing permanent anchoring of biomolecules. We achieved success rates of 96% and 94% with synaptic antibodies for mouse and marmoset brains, respectively. Maximal preservation of antigenicity allows imaging of nanoscopic architectures in 1000-fold expanded tissues without additional signal amplification. eMAP-processed tissue gel can endure repeated staining and destaining without epitope loss or structural damage, enabling highly multiplexed proteomic analysis. We demonstrated the utility of eMAP as a nanoscopic proteomic interrogation tool by investigating molecular heterogeneity in inhibitory synapses in the mouse brain neocortex and characterizing the spatial distributions of synaptic proteins within synapses in mouse and marmoset brains.

Identification of a novel interaction of FUS and syntaphilin may explain synaptic and mitochondrial abnormalities caused by ALS mutations

Aberrantly expressed fused in sarcoma (FUS) is a hallmark of FUS-related amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Wildtype FUS localises to synapses and interacts with mitochondrial proteins while mutations have been shown to cause to pathological changes affecting mitochondria, synapses and the neuromuscular junction (NMJ). This indicates a crucial physiological role for FUS in regulating synaptic and mitochondrial function that is currently poorly understood. In this paper we provide evidence that mislocalised cytoplasmic FUS causes mitochondrial and synaptic changes and that FUS plays a vital role in maintaining neuronal health in vitro and in vivo. Overexpressing mutant FUS altered synaptic numbers and neuronal complexity in both primary neurons and zebrafish models. The degree to which FUS was mislocalised led to differences in the synaptic changes which was mirrored by changes in mitochondrial numbers and transport. Furthermore, we showed that FUS co-localises with the mitochondrial tethering protein Syntaphilin (SNPH), and that mutations in FUS affect this relationship. Finally, we demonstrated mutant FUS led to changes in global protein translation. This localisation between FUS and SNPH could explain the synaptic and mitochondrial defects observed leading to global protein translation defects. Importantly, our results support the ‘gain-of-function’ hypothesis for disease pathogenesis in FUS-related ALS.

Synaptic FUS accumulation triggers early misregulation of synaptic RNAs in a mouse model of ALS

Mutations disrupting the nuclear localization of the RNA-binding protein FUS characterize a subset of amyotrophic lateral sclerosis patients (ALS-FUS). FUS regulates nuclear RNAs, but its role at the synapse is poorly understood. Using super-resolution imaging we determined that the localization of FUS within synapses occurs predominantly near the vesicle reserve pool of presynaptic sites. Using CLIP-seq on synaptoneurosomes, we identified synaptic FUS RNA targets, encoding proteins associated with synapse organization and plasticity. Significant increase of synaptic FUS during early disease in a mouse model of ALS was accompanied by alterations in density and size of GABAergic synapses. mRNAs abnormally accumulated at the synapses of 6-month-old ALS-FUS mice were enriched for FUS targets and correlated with those depicting increased short-term mRNA stability via binding primarily on multiple exonic sites. Our study indicates that synaptic FUS accumulation in early disease leads to synaptic impairment, potentially representing an initial trigger of neurodegeneration.

Cytoplasmic FUS triggers early behavioral alterations linked to cortical neuronal hyperactivity and inhibitory synaptic defects

Gene mutations causing cytoplasmic mislocalization of the RNA-binding protein FUS lead to severe forms of amyotrophic lateral sclerosis (ALS). Cytoplasmic accumulation of FUS is also observed in other diseases, with unknown consequences. Here, we show that cytoplasmic mislocalization of FUS drives behavioral abnormalities in knock-in mice, including locomotor hyperactivity and alterations in social interactions, in the absence of widespread neuronal loss. Mechanistically, we identified a progressive increase in neuronal activity in the frontal cortex of Fus knock-in mice in vivo, associated with altered synaptic gene expression. Synaptic ultrastructural and morphological defects were more pronounced in inhibitory than excitatory synapses and associated with increased synaptosomal levels of FUS and its RNA targets. Thus, cytoplasmic FUS triggers synaptic deficits, which is leading to increased neuronal activity in frontal cortex and causing related behavioral phenotypes. These results indicate that FUS mislocalization may trigger deleterious phenotypes beyond motor neuron impairment in ALS, likely relevant also for other neurodegenerative diseases characterized by FUS mislocalization.

The Interplay of RNA Binding Proteins, Oxidative Stress and Mitochondrial Dysfunction in ALS

RNA binding proteins fulfil a wide number of roles in gene expression. Multiple mechanisms of RNA binding protein dysregulation have been implicated in the pathomechanisms of several neurodegenerative diseases including amyotrophic lateral sclerosis (ALS). Oxidative stress and mitochondrial dysfunction also play important roles in these diseases. In this review, we highlight the mechanistic interplay between RNA binding protein dysregulation, oxidative stress and mitochondrial dysfunction in ALS. We also discuss different potential therapeutic strategies targeting these pathways.

Mechanism of karyopherin-β2 binding and nuclear import of ALS variants FUS(P525L) and FUS(R495X)

Mutations in the RNA-binding protein FUS cause familial amyotropic lateral sclerosis (ALS). Several mutations that affect the proline-tyrosine nuclear localization signal (PY-NLS) of FUS cause severe juvenile ALS. FUS also undergoes liquid-liquid phase separation (LLPS) to accumulate in stress granules when cells are stressed. In unstressed cells, wild type FUS resides predominantly in the nucleus as it is imported by the importin Karyopherin-β2 (Kapβ2), which binds with high affinity to the C-terminal PY-NLS of FUS. Here, we analyze the interactions between two ALS-related variants FUS(P525L) and FUS(R495X) with importins, especially Kapβ2, since they are still partially localized to the nucleus despite their defective/missing PY-NLSs. The crystal structure of the Kapβ2·FUS(P525L)PY-NLS complex shows the mutant peptide making fewer contacts at the mutation site, explaining decreased affinity for Kapβ2. Biochemical analysis revealed that the truncated FUS(R495X) protein, although missing the PY-NLS, can still bind Kapβ2 and suppresses LLPS. FUS(R495X) uses its C-terminal tandem arginine-glycine-glycine regions, RGG2 and RGG3, to bind the PY-NLS binding site of Kapβ2 for nuclear localization in cells when arginine methylation is inhibited. These findings suggest the importance of the C-terminal RGG regions in nuclear import and LLPS regulation of ALS variants of FUS that carry defective PY-NLSs.

STochastic Optical Reconstruction Microscopy (STORM) reveals the nanoscale organization of pathological aggregates in human brain

AIMS

Histological analysis of brain tissue samples provides valuable information about the pathological processes leading to common neurodegenerative disorders. In this context, the development of novel high-resolution imaging approaches is a current challenge in neuroscience.

METHODS

To this end, we used a recent super-resolution imaging technique called STochastic Optical Reconstruction Microscopy (STORM) to analyse human brain sections. We combined STORM cell imaging protocols with neuropathological techniques to image cryopreserved brain samples from control subjects and patients with neurodegenerative diseases.

RESULTS

This approach allowed us to perform 2D-, 3D- and two-colour-STORM in neocortex, white matter and brainstem samples. STORM proved to be particularly effective at visualizing the organization of dense protein inclusions and we imaged with a <50 nm resolution pathological aggregates within the central nervous system of patients with Alzheimer's disease, Parkinson's disease, Lewy body dementia and fronto-temporal lobar degeneration. Aggregated Aβ branches appeared reticulated and cross-linked in the extracellular matrix, with widths from 60 to 240 nm. Intraneuronal Tau and TDP-43 inclusions were denser, with a honeycomb pattern in the soma and a filamentous organization in the axons. Finally, STORM imaging of α-synuclein pathology revealed the internal organization of Lewy bodies that could not be observed by conventional fluorescence microscopy.

CONCLUSIONS

STORM imaging of human brain samples opens further gates to a more comprehensive understanding of common neurological disorders. The convenience of this technique should open a straightforward extension of its application for super-resolution imaging of the human brain, with promising avenues to current challenges in neuroscience.

TDP-43 proteinopathy impairs mRNP granule mediated postsynaptic translation and mRNA metabolism

Background: Local protein synthesis and mRNA metabolism mediated by mRNP granules in the dendrites and the postsynaptic compartment is essential for synaptic remodeling and plasticity in neuronal cells. Dysregulation of these processes caused by TDP-43 proteinopathy leads to neurodegenerative diseases, such as frontotemporal lobar degeneration and amyotrophic lateral sclerosis. Methods: Using biochemical analysis and imaging techniques, including super-resolution microscopy, we provide evidence, for the first time, for the postsynaptic localization of TDP-43 in mammalian synapses and we show that TDP-43 is a component of neuronal mRNP granules. Results: With activity stimulation and various molecular approaches, we further demonstrate activity-dependent mRNP granule dynamics involving disassembly of mRNP granules, release of mRNAs, activation of local protein translation, and the impairment of granule disassembly in cellular, animal and human models of TDP-43 proteinopathy. Conclusion: Our study elucidates the interplay between TDP-43 and neuronal mRNP granules in normal physiology and TDP-43 proteinopathy in the regulation of local protein translation and mRNA metabolism in the postsynaptic compartment.

ALS Genetics: Gains, Losses, and Implications for Future Therapies

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder caused by the loss of motor neurons from the brain and spinal cord. The ALS community has made remarkable strides over three decades by identifying novel familial mutations, generating animal models, elucidating molecular mechanisms, and ultimately developing promising new therapeutic approaches. Some of these approaches reduce the expression of mutant genes and are in human clinical trials, highlighting the need to carefully consider the normal functions of these genes and potential contribution of gene loss-of-function to ALS. Here, we highlight known loss-of-function mechanisms underlying ALS, potential consequences of lowering levels of gene products, and the need to consider both gain and loss of function to develop safe and effective therapeutic strategies.

RNA-binding proteins in neurological development and disease

RNA-binding proteins are a critical group of multifunctional proteins that precisely regulate all aspects of gene expression, from alternative splicing to mRNA trafficking, stability, and translation. Converging evidence highlights aberrant RNA metabolism as a common pathogenic mechanism in several neurodevelopmental and neurodegenerative diseases. However, dysregulation of disease-linked RNA-binding proteins results in widespread, often tissue-specific and/or pleiotropic effects on the transcriptome, making it challenging to determine the underlying cellular and molecular mechanisms that contribute to disease pathogenesis. Understanding how splicing misregulation as well as alterations of mRNA stability and localization impact the activity and function of neuronal proteins is fundamental to addressing neurodevelopmental defects and synaptic dysfunction in disease. Here we highlight recent exciting studies that use high-throughput transcriptomic analysis and advanced genetic, cell biological, and imaging approaches to dissect the role of disease-linked RNA-binding proteins on different RNA processing steps. We focus specifically on efforts to elucidate the functional consequences of aberrant RNA processing on neuronal morphology, synaptic activity and plasticity in development and disease. We also consider new areas of investigation that will elucidate the molecular mechanisms RNA-binding proteins use to achieve spatiotemporal control of gene expression for neuronal homeostasis and plasticity.

Distribution of Acid Sensing Ion Channels in Axonal Growth Cones and Presynaptic Membrane of Cultured Hippocampal Neurons

Although acid-sensing ion channels (ASICs) are widely expressed in the central nervous system, their distribution and roles in axonal growth cones remain unclear. In this study, we examined ASIC localization and function in the axonal growth cones of cultured immature hippocampal neurons. Our immunocytochemical data showed that native and overexpressed ASIC1a and ASIC2a are both localized in growth cones of cultured young hippocampal neurons. Calcium imaging and electrophysiological assay results were utilized to validate their function. The calcium imaging test results indicated that the ASICs (primarily ASIC1a) present in growth cones mediate calcium influx despite the addition of voltage-gated Ca²⁺ channels antagonists and the depletion of intracellular calcium stores. The electrophysiological tests results suggested that a rapid decrease in extracellular pH at the growth cones of voltage-clamped neurons elicits inward currents that were blocked by bath application of the ASIC antagonist amiloride, showing that the ASICs expressed at growth cones are functional. The subsequent immuno-colocalization test results demonstrated that ASIC1a and ASIC2a are both colocalized with Neurofilament-H and Bassoon in mature hippocampal neurons. This finding demonstrated that after reaching maturity, ASIC1a and ASIC2a are both distributed in axons and the presynaptic membrane. Our data reveal the distribution of functional ASICs in growth cones of immature hippocampal neurons and the presence of ASICs in the axons and presynaptic membrane of mature hippocampal neurons, indicating a possible role for ASICs in axonal guidance, synapse formation and neurotransmitter release.

Local Translation in Growth Cones and Presynapses, Two Axonal Compartments for Local Neuronal Functions

During neural development, growth cones, very motile compartments of tips of axons, lead axonal extension to the correct targets. Subsequently, presynapses, another axonal compartment with vigorous trafficking of synaptic vesicles, emerge to form functional synapses with postsynapses. In response to extracellular stimuli, the immediate supply of proteins by local translation within these two axonal compartments far from cell bodies confers high motility of growth cones and active vesicle trafficking in presynapses. Although local translation in growth cones and presynapses occurs at a very low level compared with cell bodies and even dendrites, recent progress in omics and visualization techniques with subcellular fractionation of these compartments has revealed the actual situation of local translation within these two axonal compartments. Here, the increasing evidence for local protein synthesis in growth cones and presynapses for axonal and synaptic functions has been reviewed. Furthermore, the mechanisms regulating local translation in these two compartments and pathophysiological conditions caused by dysregulated local translation are highlighted.

The depletion of ubiquilin in Drosophila melanogaster disturbs neurochemical regulation to drive activity and behavioral deficits

Drosophila melanogaster is a useful and highly tractable model organism for understanding the molecular mechanisms of human diseases. We previously characterized a new dUbqn knockdown model that induces learning-memory and locomotive deficits mediated by impaired proteostasis. Although proteinopathies are the main causes of neurodegenerative diseases, limited information is currently available on the relationship between proteostasis and neurodegenerative-related behavioral perturbations, such as locomotion, wakefulness, and sexual activities. Thus, the present study aimed to elucidate the mechanisms by which dUbqn depletion which is known to cause proteinopathies, affects neurodegenerative-related behavioral perturbations. Pan-neuronal dUbqn-depleted flies showed significantly reduced evening activity along with altered pre- and postsynaptic structural NMJ's proteins by attenuating signals of Bruchpilot puncta and GluRIIA clustering. In addition, the neurochemical profiles of GABA, glutamate, dopamine, and serotonin were disturbed and these changes also affected courtship behaviors in dUbqn-depleted flies. Collectively, these results extend our understanding on how dUbqn depletion affects neurochemical regulation to drive behavioral disturbances that are generally found in the early stage of neurodegenerative diseases. Moreover, the present study may contribute a novel finding to the design of new agents that prevent disease progression or even treat diseases related to neurodegeneration.

Omics Approach to Axonal Dysfunction of Motor Neurons in Amyotrophic Lateral Sclerosis (ALS)

Amyotrophic lateral sclerosis (ALS) is an intractable adult-onset neurodegenerative disease that leads to the loss of upper and lower motor neurons (MNs). The long axons of MNs become damaged during the early stages of ALS. Genetic and pathological analyses of ALS patients have revealed dysfunction in the MN axon homeostasis. However, the molecular pathomechanism for the degeneration of axons in ALS has not been fully elucidated. This review provides an overview of the proposed axonal pathomechanisms in ALS, including those involving the neuronal cytoskeleton, cargo transport within axons, axonal energy supply, clearance of junk protein, neuromuscular junctions (NMJs), and aberrant axonal branching. To improve understanding of the global changes in axons, the review summarizes omics analyses of the axonal compartments of neurons in vitro and in vivo, including a motor nerve organoid approach that utilizes microfluidic devices developed by this research group. The review also discusses the relevance of intra-axonal transcription factors frequently identified in these omics analyses. Local axonal translation and the relationship among these pathomechanisms should be pursued further. The development of novel strategies to analyze axon fractions provides a new approach to establishing a detailed understanding of resilience of long MN and MN pathology in ALS.

Low Level of Expression of C-Terminally Truncated Human FUS Causes Extensive Changes in the Spinal Cord Transcriptome of Asymptomatic Transgenic Mice

A number of mutations in a gene encoding RNA-binding protein FUS have been linked to the development of a familial form of amyotrophic lateral sclerosis known as FUS-ALS. C-terminal truncations of FUS by either nonsense or frameshift mutations lead to the development of FUS-ALS with a particularly early onset and fast progression. However, even in patients bearing these highly pathogenic mutations the function of motor neurons is not noticeably compromised for at least a couple of decades, suggesting that until cytoplasmic levels of FUS lacking its C-terminal nuclear localisation signal reaches a critical threshold, motor neurons are able to tolerate its permanent production. In order to identify how the nervous system responds to low levels of pathogenic variants of FUS we produced and characterised a mouse line, L-FUS[1-359], with a low neuronal expression level of a highly aggregation-prone and pathogenic form of C-terminally truncated FUS. In contrast to mice that express substantially higher level of the same FUS variant and develop severe early onset motor neuron pathology, L-FUS[1-359] mice do not develop any clinical or histopathological signs of motor neuron deficiency even at old age. Nevertheless, we detected substantial changes in the spinal cord transcriptome of these mice compared to their wild type littermates. We suggest that at least some of these changes reflect activation of cellular mechanisms compensating for the potentially damaging effect of pathogenic FUS production. Further studies of these mechanism might reveal effective targets for therapy of FUS-ALS and possibly, other forms of ALS.

The Y172 Monoclonal Antibody Against p-c-Jun (Ser63) Is a Marker of the Postsynaptic Compartment of C-Type Cholinergic Afferent Synapses on Motoneurons

C-bouton-type cholinergic afferents exert an important function in controlling motoneuron (MN) excitability. During the immunocytochemical analysis of the role of c-Jun in MNs with a monoclonal (clone Y172) antibody against phospho (p)-c-Jun (serine [Ser]63), unexpected labeling was identified in the cell body cytoplasm. As predicted for c-Jun in adult spinal cord, very few, if any MNs exhibited nuclear immunoreactivity with the Y172 antibody; conversely, virtually all MNs displayed strong Y172 immunostaining in cytoplasmic structures scattered throughout the soma and proximal dendrites. The majority of these cytoplasmic Y172-positive profiles was closely associated with VAChT-positive C-boutons, but not with other types of nerve afferents contacting MNs. Ultrastructural analysis revealed that cytoplasmic Y172 immunostaining was selectively located at the subsurface cistern (SSC) of C-boutons and also in the inner areas of the endoplasmic reticulum (ER). We also described changes in cytoplasmic Y172 immunoreactivity in injured and degenerating MNs. Moreover, we noticed that MNs from NRG1 type III-overexpressing transgenic mice, which show abnormally expanded SSCs, exhibited an increase in the density and size of peripherally located Y172-positive profiles. A similar immunocytochemical pattern to that of the Y172 antibody in MNs was found with a polyclonal antibody against p-c-Jun (Ser63) but not with another polyclonal antibody that recognizes c-Jun phosphorylated at a different site. No differential band patterns were found by western blotting with any of the antibodies against c-Jun or p-c-Jun used in our study. In cultured MNs, Y172-positive oval profiles were distributed in the cell body and proximal dendrites. The in vitro lentiviral-based knockdown of c-Jun resulted in a dramatic decrease in nuclear Y172 immunostaining in MNs without any reduction in the density of cytoplasmic Y172-positive profiles, suggesting that the synaptic antigen recognized by the antibody corresponds to a C-bouton-specific protein other than p-c-Jun. Our results lay the foundation for further studies aimed at identifying this protein and determining its role in this particular type of synapse.

FUS Recognizes G Quadruplex Structures Within Neuronal mRNAs

Fused in sarcoma (FUS), identified as the heterogeneous nuclear ribonuclear protein P2, is expressed in neuronal and non-neuronal tissue, and among other functions, has been implicated in messenger RNA (mRNA) transport and possibly local translation regulation. Although FUS is mainly localized to the nucleus, in the neurons FUS has also been shown to localize to the post-synaptic density, as well as to the pre-synapse. Additionally, the FUS deletion in cultured hippocampal cells results in abnormal spine and dendrite morphology. Thus, FUS may play a role in synaptic function regulation, mRNA localization, and local translation. Many dendritic mRNAs have been shown to form G quadruplex structures in their 3'-untranslated region (3'-UTR). Since FUS contains three arginine-glycine-glycine (RGG) boxes, an RNA binding domain shown to bind with high affinity and specificity to RNA G quadruplex structures, in this study we hypothesized that FUS recognizes these structural elements in its neuronal mRNA targets. Two neuronal mRNAs found in the pre- and post-synapse are the post-synaptic density protein 95 (PSD-95) and Shank1 mRNAs, which encode for proteins involved in synaptic plasticity, maintenance, and function. These mRNAs have been shown to form 3'-UTR G quadruplex structures and were also enriched in FUS hydrogels. In this study, we used native gel electrophoresis and steady-state fluorescence spectroscopy to demonstrate specific nanomolar binding of the FUS C-terminal RGG box and of full-length FUS to the RNA G quadruplex structures formed in the 3'-UTR of PSD-95 and Shank1a mRNAs. These results point toward a novel mechanism by which FUS targets neuronal mRNA and given that these PSD-95 and Shank1 3'-UTR G quadruplex structures are also targeted by the fragile X mental retardation protein (FMRP), they raise the possibility that FUS and FMRP might work together to regulate the translation of these neuronal mRNA targets.

The Role of RNA Binding Proteins for Local mRNA Translation: Implications in Neurological Disorders

As neurons are one of the most highly polarized cells in our body, they require sophisticated cellular mechanisms to maintain protein homeostasis in their subcellular compartments such as axons and dendrites. When neuronal protein homeostasis is disturbed due to genetic mutations or deletions, this often results in degeneration of neurons leading to devastating outcome such as spinal muscular atrophy (SMA), amyotrophic lateral sclerosis (ALS), and fragile X syndrome (FXS). Ribonucleoprotein (RNP) complexes are macromolecular complexes composed of RNA binding proteins (RBPs) and their target RNAs. RBPs contain RNA binding domains and bind to RNA molecules via specific sequence motifs. RNP complexes have various functions in gene expression including messenger RNA (mRNA) trafficking, RNA processing and silencing. In neurons, RBPs deliver specific sets of mRNAs to subcellular compartments such as axons and dendrites to be locally translated. Mutations or deletions in genes coding for RNPs have been reported as causes for neurological disorders such as SMA, ALS, and FXS. As RBPs determine axonal or dendritic mRNA repertoires as well as proteomes by trafficking selective mRNAs and regulating local protein synthesis, they play a crucial role for neuronal function. In this review, we summarize the role of well-known RBPs, SMN, TDP-43, FUS, and FMRP, and review their function for local protein synthesis in neurons. Furthermore, we discuss their pathological contribution to the neurological disorders.

Vesicular nucleotide transporter-immunoreactive type I cells associated with P2X3-immunoreactive nerve endings in the rat carotid body

ATP is the major excitatory transmitter from chemoreceptor type I cells to sensory nerve endings in the carotid body, and has been suggested to be released by exocytosis from these cells. We investigated the mRNA expression and immunohistochemical localization of vesicular nucleotide transporter (VNUT) in the rat carotid body. RT-PCR detected mRNA expression of VNUT in extracts of the tissue. Immunoreactivity for VNUT was localized in a part of type I cells immunoreactive for synaptophysin (SYN), but not in glial-like type II cells immunoreactive for S100 and S100B. Among SYN-immunoreactive type I cells, VNUT immunoreactivity was selectively localized in the sub-population of tyrosine hydroxylase (TH)-immunorective type I cells associated with nerve endings immunoreactive for the P2X3 purinoceptor; however, it was not detected in the sub-population of type I cells immunoreactive for dopamine beta-hydroxylase. Multi-immunolabeling for VNUT, P2X3, and Bassoon revealed that Bassoon-immunoreactive products were localized in type I cells with VNUT immunoreactivity, and accumulated on the contact side of P2X3-immunoreactive nerve endings. These results revealed the selective localization of VNUT in the subpopulation of TH-immunoreactive type I cells attached to sensory nerve endings and suggested that these cells release ATP by exocytosis for chemosensory transmission in the carotid body.

Interrogating Synaptic Architecture: Approaches for Labeling Organelles and Cytoskeleton Components

Synaptic transmission has been studied for decades, as a fundamental step in brain function. The structure of the synapse, and its changes during activity, turned out to be key aspects not only in the transfer of information between neurons, but also in cognitive processes such as learning and memory. The overall synaptic morphology has traditionally been studied by electron microscopy, which enables the visualization of synaptic structure in great detail. The changes in the organization of easily identified structures, such as the presynaptic active zone, or the postsynaptic density, are optimally studied via electron microscopy. However, few reliable methods are available for labeling individual organelles or protein complexes in electron microscopy. For such targets one typically relies either on combination of electron and fluorescence microscopy, or on super-resolution fluorescence microscopy. This review focuses on approaches and techniques used to specifically reveal synaptic organelles and protein complexes, such as cytoskeletal assemblies. We place the strongest emphasis on methods detecting the targets of interest by affinity binding, and we discuss the advantages and limitations of each method.

Aberrant axon branching via Fos-B dysregulation in FUS-ALS motor neurons

Background

The characteristic structure of motor neurons (MNs), particularly of the long axons, becomes damaged in the early stages of amyotrophic lateral sclerosis (ALS). However, the molecular pathophysiology of axonal degeneration remains to be fully elucidated.

Method

Two sets of isogenic human-induced pluripotent stem cell (hiPSCs)-derived MNs possessing the single amino acid difference (p.H517D) in the fused in sarcoma (FUS) were constructed. By combining MN reporter lentivirus, MN specific phenotype was analyzed. Moreover, RNA profiling of isolated axons were conducted by applying the microfluidic devices that enable axon bundles to be produced for omics analysis. The relationship between the target gene, which was identified as a pathological candidate in ALS with RNA-sequencing, and the MN phenotype was confirmed by intervention with si-RNA or overexpression to hiPSCs-derived MNs and even in vivo. The commonality was further confirmed with other ALS-causative mutant hiPSCs-derived MNs and human pathology.

Findings

We identified aberrant increasing of axon branchings in FUS-mutant hiPSCs-derived MN axons compared with isogenic controls as a novel phenotype. We identified increased level of Fos-B mRNA, the binding target of FUS, in FUS-mutant MNs. While Fos-B reduction using si-RNA or an inhibitor ameliorated the observed aberrant axon branching, Fos-B overexpression resulted in aberrant axon branching even in vivo. The commonality of those phenotypes was further confirmed with other ALS causative mutation than FUS.

Interpretation

Analyzing the axonal fraction of hiPSC-derived MNs using microfluidic devices revealed that Fos-B is a key regulator of FUS-mutant axon branching.

Fund

Japan Agency for Medical Research and development; Japanese Ministry of Education, Culture, Sports, Science and Technology Clinical Research, Innovation and Education Center, Tohoku University Hospital; Japan Intractable Diseases (Nanbyo) Research Foundation; the Kanae Foundation for the Promotion of Medical Science; and “Inochi-no-Iro” ALS research grant.

Synaptic FUS Localization During Motoneuron Development and Its Accumulation in Human ALS Synapses

Mutations in the fused in Sarcoma (FUS) gene induce cytoplasmic FUS aggregations, contributing to the neurodegenerative disease amyotrophic lateral sclerosis (ALS) in certain cases. While FUS is mainly a nuclear protein involved in transcriptional processes with limited cytoplasmic functions, it shows an additional somatodendritic localization in neurons. In this study we analyzed the localization of FUS in motoneuron synapses, these being the most affected neurons in ALS, using super-resolution microscopy to distinguish between the pre- and postsynaptic compartments. We report a maturation-based variation of FUS localization in rodent synapses where a predominantly postsynaptic FUS was observed in the early stages of synaptic development, while in mature synapses the protein was entirely localized in the axonal terminal. Likewise, we also show that at the synapse of human motoneurons derived from induced pluripotent stem cells of a healthy control, FUS is mainly postsynaptic in the early developmental stages. In motoneurons derived from ALS patients harboring a very aggressive juvenile FUS mutation, increased synaptic accumulation of mutated FUS was observed. Moreover increased aggregation of other synaptic proteins Bassoon and Homer1 was also detected in these abnormal synapses. Having demonstrated changes in the FUS localization during synaptogenesis, a role of synaptic FUS in both dendritic and axonal cellular compartments is probable, and we propose a gain-of-toxic function due to the synaptic aggregation of mutant FUS in ALS.

Cytoplasmic functions of TDP-43 and FUS and their role in ALS

TAR DNA-binding protein of 43 kDa (TDP-43) and fused in sarcoma (FUS) are RNA binding proteins (RBPs) primarily located in the nucleus, and involved in numerous aspects of RNA metabolism. Both proteins can be found to be depleted from the nucleus and accumulated in cytoplasmic inclusions in two major neurodegenerative conditions, amyotrophic lateral sclerosis and frontotemporal dementia. Recent evidences suggest that, in addition to their nuclear functions, both TDP-43 and FUS are involved in multiple processes in the cytoplasm, including mRNA stability and transport, translation, the stress response, mitochondrial function and autophagy regulation. Here, we review the most recent advances in understanding their functions in the cytoplasm and how these are affected in disease.

Mitochondrial abnormalities and disruption of the neuromuscular junction precede the clinical phenotype and motor neuron loss in hFUSWT transgenic mice

FUS (fused in sarcoma) mislocalization and cytoplasmic aggregation are hallmark pathologies in FUS-related amyotrophic lateral sclerosis and frontotemporal dementia. Many of the mechanistic hypotheses have focused on a loss of nuclear function in the FUS-opathies, implicating dysregulated RNA transcription and splicing in driving neurodegeneration. Recent studies describe an additional somato-dendritic localization for FUS in the cerebral cortex implying a regulatory role in mRNA transport and local translation at the synapse. Here, we report that FUS is also abundant at the pre-synaptic terminal of the neuromuscular junction (NMJ), suggesting an important function for this protein at peripheral synapses. We have previously reported dose and age-dependent motor neuron degeneration in transgenic mice overexpressing human wild-type FUS, resulting in a motor phenotype detected by ∼28 days and death by ∼100 days. Now, we report the earliest structural events using electron microscopy and quantitative immunohistochemistry. Mitochondrial abnormalities in the pre-synaptic motor nerve terminals are detected at postnatal day 6, which are more pronounced at P15 and accompanied by a loss of synaptic vesicles and synaptophysin protein coupled with NMJs of a smaller size at a time when there is no detectable motor neuron loss. These changes occur in the presence of abundant FUS and support a peripheral toxic gain of function. This appearance is typical of a ‘dying-back’ axonopathy, with the earliest manifestation being mitochondrial disruption. These findings support our hypothesis that FUS has an important function at the NMJ, and challenge the ‘loss of nuclear function’ hypothesis for disease pathogenesis in the FUS-opathies.

ALS/FTD-Linked Mutation in FUS Suppresses Intra-axonal Protein Synthesis and Drives Disease Without Nuclear Loss-of-Function of FUS

Through the generation of humanized FUS mice expressing full-length human FUS, we identify that when expressed at near endogenous murine FUS levels, both wild-type and ALS-causing and frontotemporal dementia (FTD)-causing mutations complement the essential function(s) of murine FUS. Replacement of murine FUS with mutant, but not wild-type, human FUS causes stress-mediated induction of chaperones, decreased expression of ion channels and transporters essential for synaptic function, and reduced synaptic activity without loss of nuclear FUS or its cytoplasmic aggregation. Most strikingly, accumulation of mutant human FUS is shown to activate an integrated stress response and to inhibit local, intra-axonal protein synthesis in hippocampal neurons and sciatic nerves. Collectively, our evidence demonstrates that human ALS/FTD-linked mutations in FUS induce a gain of toxicity that includes stress-mediated suppression in intra-axonal translation, synaptic dysfunction, and progressive age-dependent motor and cognitive disease without cytoplasmic aggregation, altered nuclear localization, or aberrant splicing of FUS-bound pre-mRNAs. VIDEO ABSTRACT.

mRNP assembly, axonal transport, and local translation in neurodegenerative diseases

The development, maturation, and maintenance of the mammalian nervous system rely on complex spatiotemporal patterns of gene expression. In neurons, this is achieved by the expression of differentially localized isoforms and specific sets of mRNA-binding proteins (mRBPs) that regulate RNA processing, mRNA trafficking, and local protein synthesis at remote sites within dendrites and axons. There is growing evidence that axons contain a specialized transcriptome and are endowed with the machinery that allows them to rapidly alter their local proteome via local translation and protein degradation. This enables axons to quickly respond to changes in their environment during development, and to facilitate axon regeneration and maintenance in adult organisms. Aside from providing autonomy to neuronal processes, local translation allows axons to send retrograde injury signals to the cell soma. In this review, we discuss evidence that disturbances in mRNP transport, granule assembly, axonal localization, and local translation contribute to pathology in various neurodegenerative diseases, including spinal muscular atrophy (SMA), amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and Alzheimer's disease (AD).

FUS causes synaptic hyperexcitability in Drosophila dendritic arborization neurons

Mutations in the nuclear localization signal of the RNA binding protein FUS cause both Frontotemporal Dementia (FTD) and Amyotrophic Lateral Sclerosis (ALS). These mutations result in a loss of FUS from the nucleus and the formation of FUS-containing cytoplasmic aggregates in patients. To better understand the role of cytoplasmic FUS mislocalization in the pathogenesis of ALS, we identified a population of cholinergic neurons in Drosophila that recapitulate these pathologic hallmarks. Expression of mutant FUS or the Drosophila homolog, Cabeza (Caz), in class IV dendritic arborization neurons results in cytoplasmic mislocalization and axonal transport to presynaptic terminals. Interestingly, overexpression of FUS or Caz causes the progressive loss of neuronal projections, reduction of synaptic mitochondria, and the appearance of large calcium transients within the synapse. Additionally, we find that overexpression of mutant but not wild type FUS results in a reduction in presynaptic Synaptotagmin, an integral component of the neurotransmitter release machinery, and mutant Caz specifically disrupts axonal transport and induces hyperexcitability. These results suggest that FUS/Caz overexpression disrupts neuronal function through multiple mechanisms, and that ALS-causing mutations impair the transport of synaptic vesicle proteins and induce hyperexcitability.

Unraveling the Pathways to Neuronal Homeostasis and Disease: Mechanistic Insights into the Role of RNA-Binding Proteins and Associated Factors

The timing, dosage and location of gene expression are fundamental determinants of brain architectural complexity. In neurons, this is, primarily, achieved by specific sets of trans-acting RNA-binding proteins (RBPs) and their associated factors that bind to specific cis elements throughout the RNA sequence to regulate splicing, polyadenylation, stability, transport and localized translation at both axons and dendrites. Not surprisingly, misregulation of RBP expression or disruption of its function due to mutations or sequestration into nuclear or cytoplasmic inclusions have been linked to the pathogenesis of several neuropsychiatric and neurodegenerative disorders such as fragile-X syndrome, autism spectrum disorders, spinal muscular atrophy, amyotrophic lateral sclerosis and frontotemporal dementia. This review discusses the roles of Pumilio, Staufen, IGF2BP, FMRP, Sam68, CPEB, NOVA, ELAVL, SMN, TDP43, FUS, TAF15, and TIA1/TIAR in RNA metabolism by analyzing their specific molecular and cellular function, the neurological symptoms associated with their perturbation, and their axodendritic transport/localization along with their target mRNAs as part of larger macromolecular complexes termed ribonucleoprotein (RNP) granules.

Linking hnRNP Function to ALS and FTD Pathology

Following years of rapid progress identifying the genetic underpinnings of amyotrophic lateral sclerosis (ALS) and related diseases such as frontotemporal dementia (FTD), remarkable consistencies have emerged pointing to perturbed biology of heterogeneous nuclear ribonucleoproteins (hnRNPs) as a central driver of pathobiology. To varying extents these RNA-binding proteins are deposited in pathological inclusions in affected tissues in ALS and FTD. Moreover, mutations in hnRNPs account for a significant number of familial cases of ALS and FTD. Here we review the normal function and potential pathogenic contribution of TDP-43, FUS, hnRNP A1, hnRNP A2B1, MATR3, and TIA1 to disease. We highlight recent evidence linking the low complexity sequence domains (LCDs) of these hnRNPs to the formation of membraneless organelles and discuss how alterations in the dynamics of these organelles could contribute to disease. In particular, we discuss the various roles of disease-associated hnRNPs in stress granule assembly and disassembly, and examine the emerging hypothesis that disease-causing mutations in these proteins lead to accumulation of persistent stress granules.

Importance of Functional Loss of FUS in FTLD/ALS

Fused in sarcoma (FUS) is an RNA binding protein that regulates RNA metabolism including alternative splicing, transcription, and RNA transportation. FUS is genetically and pathologically involved in frontotemporal lobar degeneration (FTLD)/amyotrophic lateral sclerosis (ALS). Multiple lines of evidence across diverse models suggest that functional loss of FUS can lead to neuronal dysfunction and/or neuronal cell death. Loss of FUS in the nucleus can impair alternative splicing and/or transcription, whereas dysfunction of FUS in the cytoplasm, especially in the dendritic spines of neurons, can cause mRNA destabilization. Alternative splicing of the MAPT gene at exon 10, which generates 4-repeat Tau (4R-Tau) and 3-repeat Tau (3R-Tau), is one of the most impactful targets regulated by FUS. Additionally, loss of FUS function can affect dendritic spine maturations by destabilizing mRNAs such as Glutamate receptor 1 (GluA1), a major AMPA receptor, and Synaptic Ras GTPase-activating protein 1 (SynGAP1). Moreover, FUS is involved in axonal transport and morphological maintenance of neurons. These findings indicate that a biological link between loss of FUS function, Tau isoform alteration, aberrant post-synaptic function, and phenotypic expression might lead to the sequential cascade culminating in FTLD. Thus, to facilitate development of early disease markers and/or therapeutic targets of FTLD/ALS it is critical that the functions of FUS and its downstream pathways are unraveled.

Synaptic Paths to Neurodegeneration: The Emerging Role of TDP-43 and FUS in Synaptic Functions

TAR DNA-binding protein-43 KDa (TDP-43) and fused in sarcoma (FUS) as the defining pathological hallmarks for amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), coupled with ALS-FTD-causing mutations in both genes, indicate that their dysfunctions damage the motor system and cognition. On the molecular level, TDP-43 and FUS participate in the biogenesis and metabolism of coding and noncoding RNAs as well as in the transport and translation of mRNAs as part of cytoplasmic mRNA-ribonucleoprotein (mRNP) granules. Intriguingly, many of the RNA targets of TDP-43 and FUS are involved in synaptic transmission and plasticity, indicating that synaptic dysfunction could be an early event contributing to motor and cognitive deficits in ALS and FTD. Furthermore, the ability of the low-complexity prion-like domains of TDP-43 and FUS to form liquid droplets suggests a potential mechanism for mRNP assembly and conversion. This review will discuss the role of TDP-43 and FUS in RNA metabolism, with an emphasis on the involvement of this process in synaptic function and neuroprotection. This will be followed by a discussion of the potential phase separation mechanism for forming RNP granules and pathological inclusions.

Isogenic FUS-eGFP iPSC Reporter Lines Enable Quantification of FUS Stress Granule Pathology that Is Rescued by Drugs Inducing Autophagy

Perturbations in stress granule (SG) dynamics may be at the core of amyotrophic lateral sclerosis (ALS). Since SGs are membraneless compartments, modeling their dynamics in human motor neurons has been challenging, thus hindering the identification of effective therapeutics. Here, we report the generation of isogenic induced pluripotent stem cells carrying wild-type and P525L FUS-eGFP. We demonstrate that FUS-eGFP is recruited into SGs and that P525L profoundly alters their dynamics. With a screening campaign, we demonstrate that PI3K/AKT/mTOR pathway inhibition increases autophagy and ameliorates SG phenotypes linked to P525L FUS by reducing FUS-eGFP recruitment into SGs. Using a Drosophila model of FUS-ALS, we corroborate that induction of autophagy significantly increases survival. Finally, by screening clinically approved drugs for their ability to ameliorate FUS SG phenotypes, we identify a number of brain-penetrant anti-depressants and anti-psychotics that also induce autophagy. These drugs could be repurposed as potential ALS treatments.

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Generation of isogenic WT and P525L FUS-eGFP reporter iPSCs

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P525L FUS-eGFP SGs are more numerous, more intense, and larger than WT

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Increasing PI3K/AKT/mTOR-regulated autophagy reduces FUS-eGFP recruitment to SGs

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Brain-penetrant drugs that induce autophagy ameliorate the FUS SG phenotype