Distinct brain-derived TDP-43 strains from FTLD-TDP subtypes induce diverse morphological TDP-43 aggregates and spreading patterns in vitro and in vivo

Transcellular transmission and molecular heterogeneity of aggregation-prone proteins in neurodegenerative diseases

The accumulation of aggregation-prone proteins in a specific neuronal population is a common feature of neurodegenerative diseases, which is correlated with the development of pathological lesions in diseased brains. The formation and progression of pathological protein aggregates in susceptible neurons induce cellular dysfunction, resulting in progressive degeneration. Moreover, recent evidence supports the notion that the cell-to-cell transmission of pathological protein aggregates may be involved in the onset and progression of many neurodegenerative diseases. Indeed, several studies have identified different pathological aggregate strains. Although how these different aggregate strains form remains unclear, a variety of biomolecular compositions or cross-seeding events promoted by the presence of other protein aggregates in the cellular environment may affect the formation of different strains of pathological aggregates, which in turn can influence complex pathologies in diseased brains. In this review, we summarize the recent results regarding cell-to-cell transmission and the molecular heterogeneity of pathological aggregate strains, raising key questions for future research directions.

Frontotemporal dementia-like disease progression elicited by seeded aggregation and spread of FUS

RNA binding proteins have emerged as central players in the mechanisms of many neurodegenerative diseases. In particular, a proteinopathy of fused in sarcoma (FUS) is present in some instances of familial Amyotrophic lateral sclerosis (ALS) and about 10% of sporadic Frontotemporal lobar degeneration (FTLD). Here we establish that focal injection of sonicated human FUS fibrils into brains of mice in which ALS-linked mutant or wild-type human FUS replaces endogenous mouse FUS is sufficient to induce focal cytoplasmic mislocalization and aggregation of mutant and wild-type FUS which with time spreads to distal regions of the brain. Human FUS fibril-induced FUS aggregation in the mouse brain of humanized FUS mice is accelerated by an ALS-causing FUS mutant relative to wild-type human FUS. Injection of sonicated human FUS fibrils does not induce FUS aggregation and subsequent spreading after injection into naïve mouse brains containing only mouse FUS, indicating a species barrier to human FUS aggregation and its prion-like spread. Fibril-induced human FUS aggregates recapitulate pathological features of FTLD including increased detergent insolubility of FUS and TAF15 and amyloid-like, cytoplasmic deposits of FUS that accumulate ubiquitin and p62, but not TDP-43. Finally, injection of sonicated FUS fibrils is shown to exacerbate age-dependent cognitive and behavioral deficits from mutant human FUS expression. Thus, focal seeded aggregation of FUS and further propagation through prion-like spread elicits FUS-proteinopathy and FTLD-like disease progression.

Frontotemporal dementia-like disease progression elicited by seeded aggregation and spread of FUS

RNA binding proteins have emerged as central players in the mechanisms of many neurodegenerative diseases. In particular, a proteinopathy of fu sed in s arcoma (FUS) is present in some instances of familial Amyotrophic lateral sclerosis (ALS) and about 10% of sporadic FTLD. Here we establish that focal injection of sonicated human FUS fibrils into brains of mice in which ALS-linked mutant or wild-type human FUS replaces endogenous mouse FUS is sufficient to induce focal cytoplasmic mislocalization and aggregation of mutant and wild-type FUS which with time spreads to distal regions of the brain. Human FUS fibril-induced FUS aggregation in the mouse brain of humanized FUS mice is accelerated by an ALS-causing FUS mutant relative to wild-type human FUS. Injection of sonicated human FUS fibrils does not induce FUS aggregation and subsequent spreading after injection into naïve mouse brains containing only mouse FUS, indicating a species barrier to human FUS aggregation and its prion-like spread. Fibril-induced human FUS aggregates recapitulate pathological features of FTLD including increased detergent insolubility of FUS and TAF15 and amyloid-like, cytoplasmic deposits of FUS that accumulate ubiquitin and p62, but not TDP-43. Finally, injection of sonicated FUS fibrils is shown to exacerbate age-dependent cognitive and behavioral deficits from mutant human FUS expression. Thus, focal seeded aggregation of FUS and further propagation through prion-like spread elicits FUS-proteinopathy and FTLD-like disease progression.

Seeding competent TDP-43 persists in human patient and mouse muscle

TAR DNA-binding protein 43 (TDP-43) is an RNA binding protein that accumulates as aggregates in the central nervous system of some neurodegenerative diseases. However, TDP-43 aggregation is also a sensitive and specific pathologic feature found in a family of degenerative muscle diseases termed inclusion body myopathy (IBM). TDP-43 aggregates from ALS and FTD brain lysates may serve as self-templating aggregate seeds in vitro and in vivo, supporting a prion-like spread from cell to cell. Whether a similar process occurs in IBM patient muscle is not clear. We developed a mouse model of inducible, muscle-specific cytoplasmic localized TDP-43. These mice develop muscle weakness with robust accumulation of insoluble and phosphorylated sarcoplasmic TDP-43, leading to eosinophilic inclusions, altered proteostasis and changes in TDP-43-related RNA processing that resolve with the removal of doxycycline. Skeletal muscle lysates from these mice also have seeding competent TDP-43, as determined by a FRET-based biosensor, that persists for weeks upon resolution of TDP-43 aggregate pathology. Human muscle biopsies with TDP-43 pathology also contain TDP-43 aggregate seeds. Using lysates from muscle biopsies of patients with IBM, IMNM and ALS we found that TDP-43 seeding capacity was specific to IBM. Surprisingly, TDP-43 seeding capacity anti-correlated with TDP-43 aggregate and vacuole abundance. These data support that TDP-43 aggregate seeds are present in IBM skeletal muscle and represent a unique TDP-43 pathogenic species not previously appreciated in human muscle disease.

A model of human neural networks reveals NPTX2 pathology in ALS and FTLD

Human cellular models of neurodegeneration require reproducibility and longevity, which is necessary for simulating age-dependent diseases. Such systems are particularly needed for TDP-43 proteinopathies1, which involve human-specific mechanisms2-5 that cannot be directly studied in animal models. Here, to explore the emergence and consequences of TDP-43 pathologies, we generated induced pluripotent stem cell-derived, colony morphology neural stem cells (iCoMoNSCs) via manual selection of neural precursors6. Single-cell transcriptomics and comparison to independent neural stem cells7 showed that iCoMoNSCs are uniquely homogenous and self-renewing. Differentiated iCoMoNSCs formed a self-organized multicellular system consisting of synaptically connected and electrophysiologically active neurons, which matured into long-lived functional networks (which we designate iNets). Neuronal and glial maturation in iNets was similar to that of cortical organoids8. Overexpression of wild-type TDP-43 in a minority of neurons within iNets led to progressive fragmentation and aggregation of the protein, resulting in a partial loss of function and neurotoxicity. Single-cell transcriptomics revealed a novel set of misregulated RNA targets in TDP-43-overexpressing neurons and in patients with TDP-43 proteinopathies exhibiting a loss of nuclear TDP-43. The strongest misregulated target encoded the synaptic protein NPTX2, the levels of which are controlled by TDP-43 binding on its 3' untranslated region. When NPTX2 was overexpressed in iNets, it exhibited neurotoxicity, whereas correcting NPTX2 misregulation partially rescued neurons from TDP-43-induced neurodegeneration. Notably, NPTX2 was consistently misaccumulated in neurons from patients with amyotrophic lateral sclerosis and frontotemporal lobar degeneration with TDP-43 pathology. Our work directly links TDP-43 misregulation and NPTX2 accumulation, thereby revealing a TDP-43-dependent pathway of neurotoxicity.

Cryo-EM observation of the amyloid key structure of polymorphic TDP-43 amyloid fibrils

The transactive response DNA-binding protein-43 (TDP-43) is a multi-facet protein involved in phase separation, RNA-binding, and alternative splicing. In the context of neurodegenerative diseases, abnormal aggregation of TDP-43 has been linked to amyotrophic lateral sclerosis and frontotemporal lobar degeneration through the aggregation of its C-terminal domain. Here, we report a cryo-electron microscopy (cryo-EM)-based structural characterization of TDP-43 fibrils obtained from the full-length protein. We find that the fibrils are polymorphic and contain three different amyloid structures. The structures differ in the number and relative orientation of the protofilaments, although they share a similar fold containing an amyloid key motif. The observed fibril structures differ from previously described conformations of TDP-43 fibrils and help to better understand the structural landscape of the amyloid fibril structures derived from this protein.

Targeting RACK1 to alleviate TDP-43 and FUS proteinopathy-mediated suppression of protein translation and neurodegeneration

TAR DNA-binding protein 43 (TDP-43) and Fused in Sarcoma/Translocated in Sarcoma (FUS) are ribonucleoproteins associated with pathogenesis of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). Under physiological conditions, TDP-43 and FUS are predominantly localized in the nucleus, where they participate in transcriptional regulation, RNA splicing and metabolism. In disease, however, they are typically mislocalized to the cytoplasm where they form aggregated inclusions. A number of shared cellular pathways have been identified that contribute to TDP-43 and FUS toxicity in neurodegeneration. In the present study, we report a novel pathogenic mechanism shared by these two proteins. We found that pathological FUS co-aggregates with a ribosomal protein, the Receptor for Activated C-Kinase 1 (RACK1), in the cytoplasm of spinal cord motor neurons of ALS, as previously reported for pathological TDP-43. In HEK293T cells transiently transfected with TDP-43 or FUS mutant lacking a functional nuclear localization signal (NLS; TDP-43ΔNLS and FUSΔNLS), cytoplasmic TDP-43 and FUS induced co-aggregation with endogenous RACK1. These co-aggregates sequestered the translational machinery through interaction with the polyribosome, accompanied by a significant reduction of global protein translation. RACK1 knockdown decreased cytoplasmic aggregation of TDP-43ΔNLS or FUSΔNLS and alleviated associated global translational suppression. Surprisingly, RACK1 knockdown also led to partial nuclear localization of TDP-43ΔNLS and FUSΔNLS in some transfected cells, despite the absence of NLS. In vivo, RACK1 knockdown alleviated retinal neuronal degeneration in transgenic Drosophila melanogaster expressing hTDP-43WT or hTDP-43Q331K and improved motor function of hTDP-43WT flies, with no observed adverse effects on neuronal health in control knockdown flies. In conclusion, our results revealed a novel shared mechanism of pathogenesis for misfolded aggregates of TDP-43 and FUS mediated by interference with protein translation in a RACK1-dependent manner. We provide proof-of-concept evidence for targeting RACK1 as a potential therapeutic approach for TDP-43 or FUS proteinopathy associated with ALS and FTLD.

Heterogeneity of cortical pTDP-43 inclusion morphologies in amyotrophic lateral sclerosis

BACKGROUND

Despite the presence of significant cortical pTDP-43 inclusions of heterogeneous morphologies in patients diagnosed with amyotrophic lateral sclerosis (ALS), pathological subclassification is routinely performed in the minority of patients with concomitant frontotemporal dementia (FTD).

OBJECTIVE

In order to improve current understanding of the presence and relevance of pathological pTDP-43 subtypes in ALS, the present study examined the pattern of cortical pTDP-43 aggregates in 61 ALS cases without FTD.

RESULTS

Based on the presence, morphology and composition of pTDP-43 pathology, three distinct ALS-TDP subtypes were delineated: (1) A predominant pattern of pTDP-43 granulofilamentous neuronal inclusions (GFNIs) and grains that were immuno-negative for p62 was identified in 18% of cases designated ALS-TDP type E; (2) neuronal cytoplasmic inclusions (NCIs) that were immuno-positive for both pTDP-43 and p62 were observed in 67% of cases assigned ALS-TDP type B; and (3) scarce cortical pTDP-43 and p62 aggregates were identified in 15% of cases coined ALS-TDP type SC (scarce cortical). Quantitative analyses revealed a significantly greater burden of pTDP-43 GFNI and grains in ALS-TDP type E. Principal component analysis demonstrated significant relationships between GFNIs, grains and ALS-TDP subtypes to support the distinction of subtypes E and B. No significant difference in age at death or disease duration was found between ALS-TDP subgroups to suggest that these subtypes represent earlier or later stages of the same disease process. Instead, a significantly higher ALS-TDP stage, indicating greater topographical spread of pTDP-43, was identified in ALS-TDP type E. Alzheimer's disease neuropathological change (ABC score ≥ intermediate) and Lewy body disease (Braak stage ≥ IV) was more prevalent in the ALS-TDP type SC cohort, which also demonstrated a significantly lower overall cognitive score.

CONCLUSION

In summary, the present study demonstrates that ALS-TDP does not represent a single homogenous neuropathology. We propose the subclassification of ALS-TDP into three distinct subtypes using standard immuno-stains for pTDP-43 and p62 in the motor cortex, which is routinely sampled and evaluated for diagnostic neuropathological characterisation of ALS. We propose that future studies specify both clinicopathological group and pTDP-43 subtype to advance current understanding of the pathogenesis of clinical phenotypes in pTDP-43 proteinopathies, which will have significant relevance to the development of targeted therapies for this heterogeneous disorder.

Frontotemporal Degeneration with Transactive Response DNA-Binding Protein Type C at the Anterior Temporal Lobe

The anatomical distribution of most neurodegenerative diseases shows considerable interindividual variations. In contrast, frontotemporal lobar degeneration with transactive response DNA-binding protein type C (TDP-C) shows a consistent predilection for the anterior temporal lobe (ATL). The relatively selective atrophy of ATL in TDP-C patients has highlighted the importance of this region for complex cognitive and behavioral functions. This review includes observations on 28 TDP-C patients, 18 with semantic primary progressive aphasia and 10 with other syndromes. Longitudinal imaging allowed the delineation of progression trajectories. At post-mortem examination, the pathognomonic feature of TDP-C consisted of long, thick neurites found predominantly in superficial cortical layers. These neurites may represent dystrophic apical dendrites of layer III and V pyramidal neurons that are known to play pivotal roles in complex cortical computations. Other types of frontotemporal lobar degeneration TDP, such as TDP-A and TDP-B, are not associated with long dystrophic neurites in the cerebral cortex, and do not show similar predilection patterns for ATL. Research is beginning to identify molecular, structural, and immunological differences between pathological TDP-43 in TDP-C versus TDP-A and B. Parallel investigations based on proteomics, somatic mutations, and genome-wide association studies are detecting molecular features that could conceivably mediate the selective vulnerability of ATL to TDP-C. Future work will focus on characterizing the distinctive features of the abnormal TDP-C neurites, the mechanisms of neurotoxicity, initial cellular targets within the ATL, trajectory of spread, and the nature of ATL-specific markers that modulate vulnerability to TDP-C. ANN NEUROL 2023;94:1-12.

Seeding the aggregation of TDP-43 requires post-fibrillization proteolytic cleavage

Despite the strong evidence linking the transactive response DNA-binding protein 43 (TDP-43) aggregation to the pathogenesis of frontotemporal lobar degeneration with TDP-43, amyotrophic lateral sclerosis and several neurodegenerative diseases, our knowledge of the sequence and structural determinants of its aggregation and neurotoxicity remains incomplete. Herein, we present a new method for producing recombinant full-length TDP-43 filaments that exhibit sequence and morphological features similar to those of brain-derived TDP-43 filaments. We show that TDP-43 filaments contain a β-sheet-rich helical amyloid core that is fully buried by the flanking structured domains of the protein. We demonstrate that the proteolytic cleavage of TDP-43 filaments and exposure of this amyloid core are necessary for propagating TDP-43 pathology and enhancing the seeding of brain-derived TDP-43 aggregates. Only TDP-43 filaments with exposed amyloid core efficiently seeded the aggregation of endogenous TDP-43 in cells. These findings suggest that inhibiting the enzymes mediating cleavage of TDP-43 aggregates represents a viable disease-modifying strategy to slow the progression of amyotrophic lateral sclerosis and other TDP-43 proteinopathies.

Tandem detergent-extraction and immunoprecipitation of proteinopathy: Scalable enrichment of ALS-associated TDP-43 aggregates

Transactive response DNA-binding protein of 43 kDa (TDP-43) is a highly conserved, ubiquitously expressed nucleic acid-binding protein that regulates DNA/RNA metabolism. Genetics and neuropathology studies have linked TDP-43 to several neuromuscular and neurological disorders including amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). Under pathological conditions, TDP-43 mislocalizes to the cytoplasm where it forms insoluble, hyper-phosphorylated aggregates during disease progression. Here, we optimized a scalable in vitro immuno-purification strategy referred to as tandem detergent-extraction and immunoprecipitation of proteinopathy (TDiP) to isolate TDP-43 aggregates that recapitulate those identified in postmortem ALS tissue. Moreover, we demonstrate that these purified aggregates can be utilized in biochemical, proteomics, and live-cell assays. This platform offers a rapid, accessible, and streamlined approach to study ALS disease mechanisms, while overcoming many limitations that have hampered TDP-43 disease modeling and therapeutic drug discovery efforts.

ECLiPSE: A Versatile Classification Technique for Structural and Morphological Analysis of Super-Resolution Microscopy Data

We introduce a new automated machine learning analysis pipeline to precisely classify cellular structures captured through single molecule localization microscopy, which we call ECLiPSE (Enhanced Classification of Localized Pointclouds by Shape Extraction). ECLiPSE leverages 67 comprehensive shape descriptors encompassing geometric, boundary, skeleton and other properties, the majority of which are directly extracted from the localizations to accurately characterize individual structures. We validate ECLiPSE through unsupervised and supervised classification on a dataset featuring five distinct cellular structures, achieving exceptionally high classification accuracies nearing 100%. Moreover, we demonstrate the versatility of our approach by applying it to two novel biological applications: quantifying the clearance of tau protein aggregates, a critical marker for neurodegenerative diseases, and differentiating between two distinct morphological features (morphotypes) of TAR DNA-binding protein 43 proteinopathy, potentially associated to different TDP-43 strains, each exhibiting unique seeding and spreading properties. We anticipate that this versatile approach will significantly enhance the way we study cellular structures across various biological contexts, elucidating their roles in disease development and progression.

The unique neuropathological vulnerability of the human brain to aging

Alzheimer's disease (AD)-related neurofibrillary tangles (NFT), argyrophilic grain disease (AGD), aging-related tau astrogliopathy (ARTAG), limbic predominant TDP-43 proteinopathy (LATE), and amygdala-predominant Lewy body disease (LBD) are proteinopathies that, together with hippocampal sclerosis, progressively appear in the elderly affecting from 50% to 99% of individuals aged 80 years, depending on the disease. These disorders usually converge on the same subject and associate with additive cognitive impairment. Abnormal Tau, TDP-43, and α-synuclein pathologies progress following a pattern consistent with an active cell-to-cell transmission and abnormal protein processing in the host cell. However, cell vulnerability and transmission pathways are specific for each disorder, albeit abnormal proteins may co-localize in particular neurons. All these alterations are unique or highly prevalent in humans. They all affect, at first, the archicortex and paleocortex to extend at later stages to the neocortex and other regions of the telencephalon. These observations show that the phylogenetically oldest areas of the human cerebral cortex and amygdala are not designed to cope with the lifespan of actual humans. New strategies aimed at reducing the functional overload of the human telencephalon, including optimization of dream repair mechanisms and implementation of artificial circuit devices to surrogate specific brain functions, appear promising.

In vivo analysis of aggregation propensity of low levels of mislocalized TDP-43 on cytopathological and behavioral phenotype of ALS/FTLD

Mislocalization and aggregate formation of TAR DNA-biding protein of 43kD (TDP-43) in the cytoplasm are signatures of amyotrophic lateral sclerosis(ALS) and frontotemporal lobar degeneration (FTLD). However, the role of two cytopathologies in ALS/FTLD pathogenesis is unclear. This study aims to elucidate the difference in their causality of TDP-43 in ALS/FTLD in vivo, using transgenic mice expressing human TDP-43 with defective nuclear localizing signals in neurons (Cyto-TDP) and those with aggregation propensity (Cyto-aggTDP). The expression levels of both proteins are less than half of endogenous TDP-43. Despite the low amount of Cyto-aggTDP, the TDP-43 phosphorylation is more evident than Cyto-TDP. Histopathological study showed accelerated astrogliosis in the anterior cerebral cortex of both mice. Cyto-aggTDP mice demonstrated significant but faint loss of neurons in the perirhinal(PERI) and ectorhinal(ECT) areas and higher Iba1-staining in the spinal cord than aged control. Despite the lack of locomotor dysfunctions in both mice, the open-field test showed enhanced exploratory behavior, indicating that the perpetual mislocalization of TDP-43 may suffice to trigger FTLD behavior. Besides, the aggregation propensity of TDP-43 promotes phosphorylation, but its role in the clinicopathological phenotype may not be primary.

Metamorphism in TDP-43 prion-like domain determines chaperone recognition

The RNA binding protein TDP-43 forms cytoplasmic inclusions via its C-terminal prion-like domain in several neurodegenerative diseases. Aberrant TDP-43 aggregation arises upon phase de-mixing and transitions from liquid to solid states, following still unknown structural conversions which are primed by oxidative stress and chaperone inhibition. Despite the well-established protective roles for molecular chaperones against protein aggregation pathologies, knowledge on the determinants of chaperone recognition in disease-related prions is scarce. Here we show that chaperones and co-chaperones primarily recognize the structured elements in TDP-43´s prion-like domain. Significantly, while HSP70 and HSP90 chaperones promote TDP-43 phase separation, co-chaperones from the three classes of the large human HSP40 family (namely DNAJA2, DNAJB1, DNAJB4 and DNAJC7) show strikingly different effects on TDP-43 de-mixing. Dismantling of the second helical element in TDP-43 prion-like domain by methionine sulfoxidation impacts phase separation and amyloid formation, abrogates chaperone recognition and alters phosphorylation by casein kinase-1δ. Our results show that metamorphism in the post-translationally modified TDP-43 prion-like domain encodes determinants that command mechanisms with major relevance in disease.

Limbic-Predominant Age-Related TDP-43 Encephalopathy: LATE-Breaking Updates in Clinicopathologic Features and Biomarkers

PURPOSE OF REVIEW

Limbic-predominant age-related TDP-43 encephalopathy (LATE) is a recently defined neurodegenerative disease characterized by amnestic phenotype and pathological inclusions of TAR DNA-binding protein 43 (TDP-43). LATE is distinct from rarer forms of TDP-43 diseases such as frontotemporal lobar degeneration with TDP-43 but is also a common copathology with Alzheimer's disease (AD) and cerebrovascular disease and accelerates cognitive decline. LATE contributes to clinicopathologic heterogeneity in neurodegenerative diseases, so it is imperative to distinguish LATE from other etiologies.

RECENT FINDINGS

Novel biomarkers for LATE are being developed with magnetic resonance imaging (MRI) and positron emission tomography (PET). When cooccurring with AD, LATE exhibits identifiable patterns of limbic-predominant atrophy on MRI and hypometabolism on 18F-fluorodeoxyglucose PET that are greater than expected relative to levels of local AD pathology. Efforts are being made to develop TDP-43-specific radiotracers, molecularly specific biofluid measures, and genomic predictors of TDP-43. LATE is a highly prevalent neurodegenerative disease distinct from previously characterized cognitive disorders.

Distinct neurotoxic TDP-43 fibril polymorphs are generated by heterotypic interactions with α-Synuclein

Amyloid aggregates of specific proteins constitute important pathological hallmarks in many neurodegenerative diseases, defining neuronal degeneration and disease onset. Recently, increasing numbers of patients show comorbidities and overlaps between multiple neurodegenerative diseases, presenting distinct phenotypes. Such overlaps are often accompanied by colocalizations of more than one amyloid protein, prompting the question of whether direct interactions between different amyloid proteins could generate heterotypic amyloids. To answer this question, we investigated the effect of α-synuclein (αS) on the DNA-binding protein TDP-43 aggregation inspired by their coexistence in pathologies such as Lewy body dementia and limbic predominant age-related TDP-43 encephalopathy. We previously showed αS and prion-like C-terminal domain (PrLD) of TDP-43 synergistically interact to generate toxic heterotypic aggregates. Here, we extend these studies to investigate whether αS induces structurally and functionally distinct polymorphs of PrLD aggregates. Using αS-PrLD heterotypic aggregates generated in two different stoichiometric proportions, we show αS can affect PrLD fibril forms. PrLD fibrils show distinctive residue level signatures determined by solid state NMR, dye-binding capability, proteinase K (PK) stability, and thermal stability toward SDS denaturation. Furthremore, by gold nanoparticle labeling and transmission electron microscopy, we show the presence of both αS and PrLD proteins within the same fibrils, confirming the existence of heterotypic amyloid fibrils. We also observe αS and PrLD colocalize in the cytosol of neuroblastoma cells and show that the heterotypic PrLD fibrils selectively induce synaptic dysfunction in primary neurons. These findings establish the existence of heterotypic amyloid and provide a molecular basis for the observed overlap between synucleinopathies and TDP-43 proteinopathies.

pTDP-43 aggregates accumulate in non-central nervous system tissues prior to symptom onset in amyotrophic lateral sclerosis: a case series linking archival surgical biopsies with clinical phenotypic data

Neurodegenerative diseases such as Parkinson's disease (PD), Alzheimer's disease (AD), and amyotrophic lateral sclerosis (ALS) are traditionally considered strictly neurological disorders. However, clinical presentation is not restricted to neurological systems, and non-central nervous system (CNS) manifestations, particularly gastrointestinal (GI) symptoms, are common. Our objective was to understand the systemic distribution of pathology in archived non-CNS tissues, taken as part of routine clinical practice during life from people with ALS. We examined tissue from 13 people who went on to develop ALS; including sporadic ALS (n = 12) and C9orf72 hexanucleotide repeat expansion (n = 1). The tissue cohort consisted of 68 formalin-fixed paraffin embedded samples from 21 surgical cases (some patients having more than one case over their lifetimes), from 8 organ systems, which we examined for evidence of phosphorylated TDP-43 (pTDP-43) pathology. We identified pTDP-43 aggregates in multiple cell types of the GI tract, including macrophages and dendritic cells within the lamina propria; as well as ganglion/neuronal and glial cells of the myenteric plexus. Aggregates were also noted within lymph node parenchyma, blood vessel endothelial cells, and chondrocytes. We note that in all cases with non-CNS pTDP-43 pathology, aggregates were present prior to ALS diagnosis and in some instances preceded neurological symptom onset by more than 10 years. These data imply that patients with microscopically unexplained non-CNS symptoms could have occult protein aggregation that could be detected many years prior to neurological involvement.

Neuropathology and neuroanatomy of TDP-43 amyotrophic lateral sclerosis

PURPOSE OF REVIEW

Intracellular inclusions consisting of the abnormal TDP-43 protein and its nucleocytoplasmic mislocalization in selected cell types are hallmark pathological features of sALS. Descriptive (histological, morphological), anatomical, and molecular studies all have improved our understanding of the neuropathology of sporadic amyotrophic lateral sclerosis (sALS). This review highlights some of the latest developments in the field.

RECENT FINDINGS

Increasing evidence exists from experimental models for the prion-like nature of abnormal TDP-43, including a strain-effect, and with the help of neuroimaging-based studies, for spreading of disease along corticofugal connectivities in sALS. Progress has also been made with respect to finding and establishing reliable biomarkers (neurofilament levels, diffusor tensor imaging).

SUMMARY

The latest findings may help to elucidate the preclinical phase of sALS and to define possible mechanisms for delaying or halting disease development and progression.

FTLD-TDP assemblies seed neoaggregates with subtype-specific features via a prion-like cascade

Morphologically distinct TDP-43 aggregates occur in clinically different FTLD-TDP subtypes, yet the mechanism of their emergence and contribution to clinical heterogeneity are poorly understood. Several lines of evidence suggest that pathological TDP-43 follows a prion-like cascade, but the molecular determinants of this process remain unknown. We use advanced microscopy techniques to compare the seeding properties of pathological FTLD-TDP-A and FTLD-TDP-C aggregates. Upon inoculation of patient-derived aggregates in cells, FTLD-TDP-A seeds amplify in a template-dependent fashion, triggering neoaggregation more efficiently than those extracted from FTLD-TDP-C patients, correlating with the respective disease progression rates. Neoaggregates are sequentially phosphorylated with N-to-C directionality and with subtype-specific timelines. The resulting FTLD-TDP-A neoaggregates are large and contain densely packed fibrils, reminiscent of the pure compacted fibrils present within cytoplasmic inclusions in postmortem brains. In contrast, FTLD-TDP-C dystrophic neurites show less dense fibrils mixed with cellular components, and their respective neoaggregates are small, amorphous protein accumulations. These cellular seeding models replicate aspects of the patient pathological diversity and will be a useful tool in the quest for subtype-specific therapeutics.

Structural and Functional Insights into α-Synuclein Fibril Polymorphism

Abnormal accumulation of aggregated α-synuclein (α-Syn) is seen in a variety of neurodegenerative diseases, including Parkinson's disease (PD), multiple system atrophy (MSA), dementia with Lewy body (DLB), Parkinson's disease dementia (PDD), and even subsets of Alzheimer's disease (AD) showing Lewy-body-like pathology. These synucleinopathies exhibit differences in their clinical and pathological representations, reminiscent of prion disorders. Emerging evidence suggests that α-Syn self-assembles and polymerizes into conformationally diverse polymorphs in vitro and in vivo, similar to prions. These α-Syn polymorphs arising from the same precursor protein may exhibit strain-specific biochemical properties and the ability to induce distinct pathological phenotypes upon their inoculation in animal models. In this review, we discuss clinical and pathological variability in synucleinopathies and several aspects of α-Syn fibril polymorphism, including the existence of high-resolution molecular structures and brain-derived strains. The current review sheds light on the recent advances in delineating the structure-pathogenic relationship of α-Syn and how diverse α-Syn molecular polymorphs contribute to the existing clinical heterogeneity in synucleinopathies.

The Cellular Prion Protein Increases the Uptake and Toxicity of TDP-43 Fibrils

Cytoplasmic aggregation of the primarily nuclear TAR DNA-binding protein 43 (TDP-43) affects neurons in most amyotrophic lateral sclerosis (ALS) and approximately half of frontotemporal lobar degeneration (FTLD) cases. The cellular prion protein, PrP^C, has been recognized as a common receptor and downstream effector of circulating neurotoxic species of several proteins involved in neurodegeneration. Here, capitalizing on our recently adapted TDP-43 real time quaking induced reaction, we set reproducible protocols to obtain standardized preparations of recombinant TDP-43 fibrils. We then exploited two different cellular systems (human SH-SY5Y and mouse N2a neuroblastoma cells) engineered to express low or high PrP^C levels to investigate the link between PrP^C expression on the cell surface and the internalization of TDP-43 fibrils. Fibril uptake was increased in cells overexpressing either human or mouse prion protein. Increased internalization was associated with detrimental consequences in all PrP-overexpressing cell lines but was milder in cells expressing the human form of the prion protein. As described for other amyloids, treatment with TDP-43 fibrils induced a reduction in the accumulation of the misfolded form of PrP^C, PrP^Sc, in cells chronically infected with prions. Our results expand the list of misfolded proteins whose uptake and detrimental effects are mediated by PrP^C, which encompass almost all pathological amyloids involved in neurodegeneration.