Molecular dissection of TDP-43 proteinopathies

α-Synuclein emulsifies TDP-43 prion-like domain-RNA liquid droplets to promote heterotypic amyloid fibrils

Many neurodegenerative diseases including frontotemporal lobar degeneration (FTLD), Lewy body disease (LBD), multiple system atrophy (MSA), etc., show colocalized deposits of TDP-43 and α-synuclein (αS) aggregates. To understand whether these colocalizations are driven by specific molecular interactions between the two proteins, we previously showed that the prion-like C-terminal domain of TDP-43 (TDP-43PrLD) and αS synergistically interact to form neurotoxic heterotypic amyloids in homogeneous buffer conditions. However, it remains unclear if αS can modulate TDP-43 present within liquid droplets and biomolecular condensates called stress granules (SGs). Here, using cell culture and in vitro TDP-43PrLD - RNA liquid droplets as models along with microscopy, nanoscale AFM-IR spectroscopy, and biophysical analyses, we uncover the interactions of αS with phase-separated droplets. We learn that αS acts as a Pickering agent by forming clusters on the surface of TDP-43PrLD - RNA droplets. The aggregates of αS on these clusters emulsify the droplets by nucleating the formation of heterotypic TDP-43PrLD amyloid fibrils, structures of which are distinct from those derived from homogenous solutions. Together, these results reveal an intriguing property of αS to act as a Pickering agent while interacting with SGs and unmask the hitherto unknown role of αS in modulating TDP-43 proteinopathies.

Dysregulation of the progranulin-driven autophagy-lysosomal pathway mediates secretion of the nuclear protein TDP-43

The cytoplasmic accumulation of the nuclear protein transactive response DNA-binding protein 43 kDa (TDP-43) has been linked to the progression of amyotrophic lateral sclerosis and frontotemporal lobar degeneration. TDP-43 secreted into the extracellular space has been suggested to contribute to the cell-to-cell spread of the cytoplasmic accumulation of TDP-43 throughout the brain; however, the underlying mechanisms remain unknown. We herein demonstrated that the secretion of TDP-43 was stimulated by the inhibition of the autophagy-lysosomal pathway driven by progranulin (PGRN), a causal protein of frontotemporal lobar degeneration. Among modulators of autophagy, only vacuolar-ATPase inhibitors, such as bafilomycin A1 (Baf), increased the levels of the full-length and cleaved forms of TDP-43 and the autophagosome marker LC3-II (microtubule-associated proteins 1A/1B light chain 3B) in extracellular vesicle fractions prepared from the culture media of HeLa, SH-SY5Y, or NSC-34 cells, whereas vacuolin-1, MG132, chloroquine, rapamycin, and serum starvation did not. The C-terminal fragment of TDP-43 was required for Baf-induced TDP-43 secretion. The Baf treatment induced the translocation of the aggregate-prone GFP-tagged C-terminal fragment of TDP-43 and mCherry-tagged LC3 to the plasma membrane. The Baf-induced secretion of TDP-43 was attenuated in autophagy-deficient ATG16L1 knockout HeLa cells. The knockdown of PGRN induced the secretion of cleaved TDP-43 in an autophagy-dependent manner in HeLa cells. The KO of PGRN in mouse embryonic fibroblasts increased the secretion of the cleaved forms of TDP-43 and LC3-II. The treatment inducing TDP-43 secretion increased the nuclear translocation of GFP-tagged transcription factor EB, a master regulator of the autophagy-lysosomal pathway in SH-SY5Y cells. These results suggest that the secretion of TDP-43 is promoted by dysregulation of the PGRN-driven autophagy-lysosomal pathway.

α-Synuclein emulsifies TDP-43 prion-like domain - RNA liquid droplets to promote heterotypic amyloid fibrils

Many neurodegenerative diseases including frontotemporal lobar degeneration (FTLD), Lewy body disease (LBD), multiple system atrophy (MSA), etc., show colocalized deposits of TDP-43 and α-synuclein (αS) aggregates. To understand whether these colocalizations are driven by specific molecular interactions between the two proteins, we previously showed that the prion-like C-terminal domain of TDP-43 (TDP-43PrLD) and αS synergistically interact to form neurotoxic heterotypic amyloids in homogeneous buffer conditions. However, it remains unclear whether and how αS modulates TDP-43 present within liquid droplets and biomolecular condensates called stress granules (SGs). Here, using cell culture and in vitro TDP-43PrLD - RNA liquid droplets as models along with microscopy, nanoscale spatially-resolved spectroscopy, and other biophysical analyses, we uncover the interactions of αS with phase-separated droplets. We learn that αS acts as a Pickering agent by forming clusters on the surface of TDP-43PrLD - RNA droplets and emulsifying them. The 'hardening' of the droplets that follow by αS aggregates on the periphery, nucleates the formation of heterotypic TDP-43PrLD amyloid fibrils with structures distinct from those derived from homogenous solutions. Together, these results reveal an intriguing property of αS as a Pickering agent in interacting with SGs and unmask the hitherto unknown role of αS in modulating TDP-43 proteinopathies.

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.

Ultrastructural and biochemical classification of pathogenic tau, α-synuclein and TDP-43

Intracellular accumulation of abnormal proteins with conformational changes is the defining neuropathological feature of neurodegenerative diseases. The pathogenic proteins that accumulate in patients' brains adopt an amyloid-like fibrous structure and exhibit various ultrastructural features. The biochemical analysis of pathogenic proteins in sarkosyl-insoluble fractions extracted from patients' brains also shows disease-specific features. Intriguingly, these ultrastructural and biochemical features are common within the same disease group. These differences among the pathogenic proteins extracted from patients' brains have important implications for definitive diagnosis of the disease, and also suggest the existence of pathogenic protein strains that contribute to the heterogeneity of pathogenesis in neurodegenerative diseases. Recent experimental evidence has shown that prion-like propagation of these pathogenic proteins from host cells to recipient cells underlies the onset and progression of neurodegenerative diseases. The reproduction of the pathological features that characterize each disease in cellular and animal models of prion-like propagation also implies that the structural differences in the pathogenic proteins are inherited in a prion-like manner. In this review, we summarize the ultrastructural and biochemical features of pathogenic proteins extracted from the brains of patients with neurodegenerative diseases that accumulate abnormal forms of tau, α-synuclein, and TDP-43, and we discuss how these disease-specific properties are maintained in the brain, based on recent experimental insights.

Motor cortical patterns of upper motor neuron pathology in amyotrophic lateral sclerosis: A 3 T MRI study with iron-sensitive sequences

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M1 regions associated with the body site of onset are frequently affected at MRI.

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The simultaneous involvement of both homologous M1 regions is frequent.

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The T2* hypointensity in non-contiguous M1 regions seems rare.

Background

Patterns of initiation and propagation of disease in Amyotrophic Lateral Sclerosis (ALS) are still partly unknown. Single or multiple foci of neurodegeneration followed by disease diffusion to contiguous or connected regions have been proposed as mechanisms underlying symptom occurrence. Here, we investigated cortical patterns of upper motor neuron (UMN) pathology in ALS using iron-sensitive MR imaging.

Methods

Signal intensity and magnetic susceptibility of the primary motor cortex (M1), which are associated with clinical UMN burden and neuroinflammation, were assessed in 78 ALS patients using respectively T2*-weighted images and Quantitative Susceptibility Maps. The signal intensity of the whole M1 and each of its functional regions was rated as normal or reduced, and the magnetic susceptibility of each M1 region was measured.

Results

The highest frequencies of T2* hypointensity were found in M1 regions associated with the body sites of symptom onset. Homologous M1 regions were both hypointense in 80–93 % of patients with cortical abnormalities, and magnetic susceptibility values measured in homologous M1 regions were strongly correlated with each other (ρ = 0.88; p < 0.0001). In some cases, the T2* hypointensity was detectable in two non-contiguous M1 regions but spared the cortex in between.

Conclusions

M1 regions associated with the body site of onset are frequently affected at imaging. The simultaneous involvement of both homologous M1 regions is frequent, followed by that of adjacent regions; the affection of non-contiguous regions, instead, seems rare. This type of cortical involvement suggests the interhemispheric connections as one of the preferential paths for the UMN pathology diffusion in ALS.

Tau-tubulin kinase 1 phosphorylates TDP-43 at disease-relevant sites and exacerbates TDP-43 pathology

TDP-43 pathology is a hallmark of Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal lobar degeneration (FTLD). Namely, both diseases feature aggregated and phosphorylated TDP-43 containing inclusions in the cytoplasm and a loss of nuclear TDP-43 in affected neurons. It has been reported that tau tubulin kinase (TTBK)1/2 phosphorylate TDP-43 and TTBK1/2 overexpression induced neuronal loss and behavioral deficits in a C. elegans model of ALS. Here we aimed to elucidate the molecular mechanisms of TTBK1 in TDP-43 pathology. TTBK1 levels were observed to be elevated in ALS patients' post-mortem motor cortex. Also, TTBK1 was found to phosphorylate TDP-43 at disease-relevant sites in vitro directly, and this phosphorylation accelerated TDP-43 formation of high molecular species. Overexpression of TTBK1 in mammalian cells induced TDP-43 phosphorylation and the construction of high molecular species, concurrent with TDP-43 mis-localization and cytoplasmic inclusions. In addition, when TTBK1 was knocked down or pharmacologically inhibited, TDP-43 phosphorylation and aggregation were significantly alleviated. Functionally, TTBK1 knockdown could rescue TDP-43 overexpression-induced neurite and neuronal loss in iPSC-derived GABAergic neurons. These findings suggest that phosphorylation plays a critical role in the pathogenesis of TDP-43 pathology and that TTBK1 inhibition may have therapeutic potential for the treatment of ALS and FTLD.

Phenotypic diversity in ALS and the role of poly-conformational protein misfolding

In many types of familial amyotrophic lateral sclerosis (fALS), mutations cause proteins to gain toxic properties that mediate neurodegenerative processes. It is becoming increasingly clear that the proteins involved in ALS, and those responsible for a host of other neurodegenerative diseases, share many characteristics with a growing number of prion diseases. ALS is a heterogenous disease in which the majority of cases are sporadic in their etiology. Studies investigating the inherited forms of the disease are now beginning to provide evidence that some of this heterogeneity may be due to the existence of distinct conformations that ALS-linked proteins can adopt to produce the equivalent of prion strains. In this review, we discuss the in vitro and in vivo evidence that has been generated to better understand the characteristics of these proteins and how their tertiary structure may impact the disease phenotype.

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

AIM

The heterogeneity in the distribution and morphological features of TAR DNA-binding protein-43 (TDP-43) pathology in the brains of frontotemporal lobar degeneration (FTLD-TDP) patients and their different clinical manifestations suggest that distinct pathological TDP-43 strains could play a role in this heterogeneity between different FTLD-TDP subtypes (A-E). Our aim was to evaluate the existence of distinct TDP-43 strains in the brains of different FTLD-TDP subtypes and characterise their specific seeding properties in vitro and in vivo.

METHODS AND RESULTS

We used an inducible stable cell line expressing a mutant cytoplasmic TDP-43 (iGFP-NLSm) to evaluate the seeding properties of distinct pathological TDP-43 strains. Brain-derived TDP-43 protein extracts from FTLD-TDP types A (n = 6) and B (n = 3) cases induced the formation of round/spherical phosphorylated TDP-43 aggregates that morphologically differed from the linear and wavy wisps and bigger heterogeneous filamentous (skein-like) aggregates induced by type E (n = 3) cases. These morphological differences correlated with distinct biochemical banding patterns of sarkosyl-insoluble TDP-43 protein recovered from the transduced cells. Moreover, brain-derived TDP-43 extracts from type E cases showed higher susceptibility to PK digestion of full-length TDP-43 and the most abundant C-terminal fragments that characterise type E extracts. Finally, we showed that intracerebral injections of different TDP-43 strains induced a distinctive morphological and subcellular distribution of TDP-43 pathology and different spreading patterns in the brains of CamKIIa-hTDP-43_NLSm Tg mice.

CONCLUSIONS

We show the existence of distinct TDP-43 strains in the brain of different FTLD-TDP subtypes with distinctive seeding and spreading properties in the brains of experimental animal models.

Multi-phaseted problems of TDP-43 in selective neuronal vulnerability in ALS

Transactive response DNA-binding protein 43 kDa (TDP-43) encoded by the TARDBP gene is an evolutionarily conserved heterogeneous nuclear ribonucleoprotein (hnRNP) that regulates multiple steps of RNA metabolism, and its cytoplasmic aggregation characterizes degenerating motor neurons in amyotrophic lateral sclerosis (ALS). In most ALS cases, cytoplasmic TDP-43 aggregation occurs in the absence of mutations in the coding sequence of TARDBP. Thus, a major challenge in ALS research is to understand the nature of pathological changes occurring in wild-type TDP-43 and to explore upstream events in intracellular and extracellular milieu that promote the pathological transition of TDP-43. Despite the inherent obstacles to analyzing TDP-43 dynamics in in vivo motor neurons due to their anatomical complexity and inaccessibility, recent studies using cellular and animal models have provided important mechanistic insights into potential links between TDP-43 and motor neuron vulnerability in ALS. This review is intended to provide an overview of the current literature on the function and regulation of TDP-43-containing RNP granules or membraneless organelles, as revealed by various models, and to discuss the potential mechanisms by which TDP-43 can cause selective vulnerability of motor neurons in ALS.

Frontotemporal Lobar Degeneration TDP-43-Immunoreactive Pathological Subtypes: Clinical and Mechanistic Significance

Frontotemporal lobar degeneration with TPD-43-immunoreactive pathology (FTLD-TDP) is subclassified based on the type and cortical laminar distribution of neuronal inclusions. The relevance of these pathological subtypes is supported by the presence of relatively specific clinical and genetic correlations. Recent evidence suggests that the different patterns of pathology are a reflection of biochemical differences in the pathological TDP-43 species, each of which is influenced by differing genetic factors. As a result, patient FTLD-TDP subtype may be an important factor to consider when developing biomarkers and targeted therapies for frontotemporal dementia. In this chapter, we first describe the pathological features, clinical and genetic correlations of the currently recognized FTLD-TDP subtypes. We then discuss a number of novel patterns of TDP-43 pathology. Finally, we provide an overview of what is currently known about the biochemical basis of the different FTLD-TDP subtypes and how this may explain the observed phenotypic and pathological heterogeneity.

TDP-43 and Limbic-Predominant Age-Related TDP-43 Encephalopathy

Through a number of an extensive autopsy, biomarker, and genomics studies, researchers have recently defined a novel type of dementia known as limbic-predominant age-related TDP-43 encephalopathy (LATE). LATE is perhaps best characterized by the presence of hyperphosphorylated TDP-43, which plays multi-functional roles through interactions with DNA and RNA, leading to significant alterations in the transcription and translation of particular genes. As individuals of advanced age represent a rapidly growing demographic group globally, there is a steadily increasing rate of LATE incidence that has to date received insufficient recognition despite its serious implications for public health. TDP-43 is the common pathology of various age-related dementia, therefore, it may be a potential and promising therapeutic target for such diseases. In the present review, we discuss the pathways regulating TDP-43 expression, metabolism, and disease activity in order to better understand the link between TDP-43 proteinopathy and LATE at the genetic, pathological, and clinical levels.

Modulating ALS-Related Amyloidogenic TDP-43307-319 Oligomeric Aggregates with Computationally Derived Therapeutic Molecules

TDP-43 aggregates are a salient feature of amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and a variety of other neurodegenerative diseases, including Alzheimer's disease (AD). With an anticipated growth in the most susceptible demographic, projections predict neurodegenerative diseases will potentially affect 15 million people in the United States by 2050. Currently, there are no cures for ALS, FTD, or AD. Previous studies of the amyloidogenic core of TDP-43 have demonstrated that oligomers greater than a trimer are associated with toxicity. Utilizing a joint pharmacophore space (JPS) method, potential drugs have been designed specifically for amyloid-related diseases. These molecules were generated on the basis of key chemical features necessary for blood-brain barrier permeability, low adverse side effects, and target selectivity. Combining ion-mobility mass spectrometry and atomic force microscopy with the JPS computational method allows us to more efficiently evaluate a potential drug's efficacy in disrupting the development of putative toxic species. Our results demonstrate the dissociation of higher-order oligomers in the presence of these novel JPS-generated inhibitors into smaller oligomer species. Additionally, drugs approved by the Food and Drug Administration for the treatment of ALS were also evaluated and demonstrated to maintain higher-order oligomeric assemblies. Possible mechanisms for the observed action of the JPS molecules are discussed.

Prion-like properties of assembled TDP-43

A neuropathological hallmark of most neurodegenerative diseases is the appearance of characteristic inclusions composed of misfolded proteins in brains of patients. Increasing evidence shows that aggregation-prone proteins such as tau, α-synuclein and TDP-43 are accumulated in a seed-dependent and self-templating manner in vitro and in vivo, suggesting that pathological protein aggregates found in these diseases function like abnormal prion protein. Indeed, insoluble tau and α-synuclein aggregates are transferred from cell to cell both in vitro and in vivo, indicating that prion-like propagation of aberrant protein aggregates may play a key role in the pathogenesis of most neurodegenerative diseases. Here, we will review the prion-like properties of TDP-43, and discuss the molecular mechanisms underlying the propagation of these accumulated proteins. The idea that aberrant protein aggregates propagate in a prion-like manner between cells opens up the possibility of novel therapeutic strategies to block the spread of these aggregates throughout the brain.

Limbic-predominant age-related TDP-43 encephalopathy (LATE): consensus working group report

We describe a recently recognized disease entity, limbic-predominant age-related TDP-43 encephalopathy (LATE). LATE neuropathological change (LATE-NC) is defined by a stereotypical TDP-43 proteinopathy in older adults, with or without coexisting hippocampal sclerosis pathology. LATE-NC is a common TDP-43 proteinopathy, associated with an amnestic dementia syndrome that mimicked Alzheimer's-type dementia in retrospective autopsy studies. LATE is distinguished from frontotemporal lobar degeneration with TDP-43 pathology based on its epidemiology (LATE generally affects older subjects), and relatively restricted neuroanatomical distribution of TDP-43 proteinopathy. In community-based autopsy cohorts, ∼25% of brains had sufficient burden of LATE-NC to be associated with discernible cognitive impairment. Many subjects with LATE-NC have comorbid brain pathologies, often including amyloid-β plaques and tauopathy. Given that the 'oldest-old' are at greatest risk for LATE-NC, and subjects of advanced age constitute a rapidly growing demographic group in many countries, LATE has an expanding but under-recognized impact on public health. For these reasons, a working group was convened to develop diagnostic criteria for LATE, aiming both to stimulate research and to promote awareness of this pathway to dementia. We report consensus-based recommendations including guidelines for diagnosis and staging of LATE-NC. For routine autopsy workup of LATE-NC, an anatomically-based preliminary staging scheme is proposed with TDP-43 immunohistochemistry on tissue from three brain areas, reflecting a hierarchical pattern of brain involvement: amygdala, hippocampus, and middle frontal gyrus. LATE-NC appears to affect the medial temporal lobe structures preferentially, but other areas also are impacted. Neuroimaging studies demonstrated that subjects with LATE-NC also had atrophy in the medial temporal lobes, frontal cortex, and other brain regions. Genetic studies have thus far indicated five genes with risk alleles for LATE-NC: GRN, TMEM106B, ABCC9, KCNMB2, and APOE. The discovery of these genetic risk variants indicate that LATE shares pathogenetic mechanisms with both frontotemporal lobar degeneration and Alzheimer's disease, but also suggests disease-specific underlying mechanisms. Large gaps remain in our understanding of LATE. For advances in prevention, diagnosis, and treatment, there is an urgent need for research focused on LATE, including in vitro and animal models. An obstacle to clinical progress is lack of diagnostic tools, such as biofluid or neuroimaging biomarkers, for ante-mortem detection of LATE. Development of a disease biomarker would augment observational studies seeking to further define the risk factors, natural history, and clinical features of LATE, as well as eventual subject recruitment for targeted therapies in clinical trials.

The Pathobiology of TDP-43 C-Terminal Fragments in ALS and FTLD

During neurodegenerative disease, the multifunctional RNA-binding protein TDP-43 undergoes a vast array of post-translational modifications, including phosphorylation, acetylation, and cleavage. Many of these alterations may directly contribute to the pathogenesis of TDP-43 proteinopathies, which include most forms of amyotrophic lateral sclerosis (ALS) and approximately half of all frontotemporal dementia, pathologically identified as frontotemporal lobar degeneration (FTLD) with TDP-43 pathology. However, the relative contributions of the various TDP-43 post-translational modifications to disease remain unclear, and indeed some may be secondary epiphenomena rather than disease-causative. It is therefore critical to determine the involvement of each modification in disease processes to allow the design of targeted treatments. In particular, TDP-43 C-terminal fragments (CTFs) accumulate in the brains of people with ALS and FTLD and are therefore described as a neuropathological signature of these diseases. Remarkably, these TDP-43 CTFs are rarely observed in the spinal cord, even in ALS which involves dramatic degeneration of spinal motor neurons. Therefore, TDP-43 CTFs are not produced non-specifically in the course of all forms of TDP-43-related neurodegeneration, but rather variably arise due to additional factors influenced by regional heterogeneity in the central nervous system. In this review, we summarize how TDP-43 CTFs are generated and degraded by cells, and critique evidence from studies of TDP-43 CTF pathology in human disease tissues, as well as cell and animal models, to analyze the pathophysiological relevance of TDP-43 CTFs to ALS and FTLD. Numerous studies now indicate that, although TDP-43 CTFs are prevalent in ALS and FTLD brains, disease-related pathology is only variably reproduced in TDP-43 CTF cell culture models. Furthermore, TDP-43 CTF expression in both transgenic and viral-mediated in vivo models largely fails to induce motor or behavioral dysfunction reminiscent of human disease. We therefore conclude that although TDP-43 CTFs are a hallmark of TDP-43-related neurodegeneration in the brain, they are not a primary cause of ALS or FTLD.

Znf179 E3 ligase-mediated TDP-43 polyubiquitination is involved in TDP-43- ubiquitinated inclusions (UBI) (+)-related neurodegenerative pathology

Background

The brain predominantly expressed RING finger protein, Znf179, is known to be important for embryonic neuronal differentiation during brain development. Downregulation of Znf179 has been observed in motor neurons of adult mouse models for amyotrophic lateral sclerosis (ALS), yet the molecular function of Znf179 in neurodegeneration has never been previously described. Znf179 contains the classical C3HC4 RING finger domain, and numerous proteins containing C3HC4 RING finger domain act as E3 ubiquitin ligases. Hence, we are interested to identify whether Znf179 possesses E3 ligase activity and its role in ALS neuropathy.

Methods

We used in vivo and in vitro ubiquitination assay to examine the E3 ligase autoubiquitination activity of Znf179 and its effect on 26S proteasome activity. To search for the candidate substrates of Znf179, we immunoprecipitated Znf179 and subjected to mass spectrometry (MS) analysis to identify its interacting proteins. We found that ALS/ FTLD-U (frontotemporal lobar degeneration (FTLD) with ubiquitin inclusions)-related neurodegenerative TDP-43 protein is the E3 ligase substrate of Znf179. To further clarify the role of E3 ubiquitin ligase Znf179 in neurodegenerative TDP-43-UBI (ubiquitinated inclusions) (+) proteinopathy, the effect of Znf179-mediated TDP-43 polyubiquitination on TDP-43 protein stability, aggregate formation and nucleus/cytoplasm mislocalization were evaluated in vitro cell culture system and in vivo animal model.

Results

Here we report that Znf179 is a RING E3 ubiquitin ligase which possesses autoubiquitination feature and regulates 26S proteasome activity through modulating the protein expression levels of 19S/20S proteasome subunits. Our immunoprecipitation assay and MS analysis results revealed that the neuropathological TDP-43 protein is one of its E3 ligase substrate. Znf179 interactes with TDP-43 protein and mediates polyubiquitination of TDP-43 in vitro and in vivo. In neurodegenerative TDP-43 proteinopathy, we found that Znf179-mediated polyubiquitination of TDP-43 accelerates its protein turnover rate and attenuates insoluble pathologic TDP-43 aggregates, while knockout of Znf179 in mouse brain results in accumulation of insoluble TDP-43 and cytosolic TDP-43 inclusions in cortex, hippocampus and midbrain regions.

Conclusions

Here we unveil the important role for the novel E3 ligase Znf179 in TDP-43-mediated neuropathy, and provide a potential therapeutic strategy for combating ALS/ FTLD-U neurodegenerative pathologies.

Electronic supplementary material

The online version of this article (10.1186/s12929-018-0479-4) contains supplementary material, which is available to authorized users.

TDP-43 Prions

The most common neurodegenerative diseases, such as Alzheimer's, Parkinson's, and amyotrophic lateral sclerosis, are all protein-misfolding diseases and are characterized by the presence of disease-specific protein aggregates in affected neuronal cells. Recent studies have shown that, like tau and α-synuclein, TAR-DNA binding protein of 43 kDa (TDP-43) can form aggregates in vitro in a seed-dependent, self-templating, prion-like manner. Insoluble TDP-43 prepared from the brains of patients has been classified into several strains, which can be transferred from cell to cell in vitro, suggesting the involvement of mechanisms reminiscent of those by which prions spread through the nervous system. The idea that aberrant TDP-43 aggregates propagate in a prion-like manner between cells presents the possibility of novel therapeutic strategies to block spreading of these aggregates throughout the brain.

TDP-43 self-interaction is modulated by redox-active compounds Auranofin, Chelerythrine and Riluzole

Amyotrophic lateral sclerosis (ALS) represents a fatal neurodegenerative disease, which is characterized by a rapid loss of lower and upper motor neurons. As a major neuropathological hallmark, protein aggregates containing the Transactivating Response Region (TAR) DNA Binding Protein (TDP-43) are detectable in about 95% of sporadic ALS patients. TDP-43 interacts with itself physiologically to form liquid droplets, which may progress to pathological aggregates. In this study, we established the NanoBit luciferase complementation assay to measure TDP-43 self-interaction and found the fusion of the split luciferase subunits to the N-terminus of the protein as the strongest interacting partners. A screen of pharmacologically active compounds from the LOPAC®1280 library identified auranofin, chelerythrine and riluzole as dose-dependent inhibitors of TDP-43 self-interaction. Further analysis of drug action of the gold-containing thioredoxin reductase inhibitor auranofin revealed a redistribution from insoluble TDP-43 protein pool to PBS-soluble protein pool in N2a cells. In addition, auranofin treatment diminished reduced glutathione as a sign for oxidative modulation.

Prion-like mechanisms in amyotrophic lateral sclerosis

The prion hypothesis - a protein conformation capable of replicating without a nucleic acid genome - was heretical at the time of its discovery. However, the characteristics of the disease-misfolded prion protein and its ability to transmit disease, replicate, and spread are now widely accepted throughout the scientific community. In fact, in the last decade a wealth of evidence has emerged supporting similar properties observed for many of the misfolded proteins implicated in other neurodegenerative diseases, such as Alzheimer disease, Parkinson disease, tauopathies, and as described in this chapter, amyotrophic lateral sclerosis (ALS). Multiple studies have now demonstrated the ability for superoxide dismutase-1, 43-kDa transactive response (TAR) DNA-binding protein, fused-in sarcoma, and most recently, C9orf72-encoded polypeptides to display properties similar to those of prions. The majority of these are cell-free and in vitro assays, while superoxide dismutase-1 remains the only ALS-linked protein to demonstrate several prion-like properties in vivo. In this chapter, we provide an introduction to ALS and review the recent literature linking several proteins implicated in the familial forms of the disease to properties of the prion protein.

Post-translational modifications clustering within proteolytic domains decrease mutant huntingtin toxicity

Huntington's disease (HD) is caused in large part by a polyglutamine expansion within the huntingtin (Htt) protein. Post-translational modifications (PTMs) control and regulate many protein functions and cellular pathways, and PTMs of mutant Htt are likely important modulators of HD pathogenesis. Alterations of selected numbers of PTMs of Htt fragments have been shown to modulate Htt cellular localization and toxicity. In this study, we systematically introduced site-directed alterations in individual phosphorylation and acetylation sites in full-length Htt constructs. The effects of each of these PTM alteration constructs were tested on cell toxicity using our nuclear condensation assay and on mitochondrial viability by measuring mitochondrial potential and size. Using these functional assays in primary neurons, we identified several PTMs whose alteration can block neuronal toxicity and prevent potential loss and swelling of the mitochondria caused by mutant Htt. These PTMs included previously described sites such as serine 116 and newly found sites such as serine 2652 throughout the protein. We found that these functionally relevant sites are clustered in protease-sensitive domains throughout full-length Htt. These findings advance our understanding of the Htt PTM code and its role in HD pathogenesis. Because PTMs are catalyzed by enzymes, the toxicity-modulating Htt PTMs identified here may be promising therapeutic targets for managing HD.

Post-Translational Modifications (PTMs), Identified on Endogenous Huntingtin, Cluster within Proteolytic Domains between HEAT Repeats

Post-translational modifications (PTMs) of proteins regulate various cellular processes. PTMs of polyglutamine-expanded huntingtin (Htt) protein, which causes Huntington's disease (HD), are likely modulators of HD pathogenesis. Previous studies have identified and characterized several PTMs on exogenously expressed Htt fragments, but none of them were designed to systematically characterize PTMs on the endogenous full-length Htt protein. We found that full-length endogenous Htt, which was immunoprecipitated from HD knock-in mouse and human post-mortem brain, is suitable for detection of PTMs by mass spectrometry. Using label-free and mass tag labeling-based approaches, we identified near 40 PTMs, of which half are novel (data are available via ProteomeXchange with identifier PXD005753). Most PTMs were located in clusters within predicted unstructured domains rather than within the predicted α-helical structured HEAT repeats. Using quantitative mass spectrometry, we detected significant differences in the stoichiometry of several PTMs between HD and WT mouse brain. The mass-spectrometry identification and quantitation were verified using phospho-specific antibodies for selected PTMs. To further validate our findings, we introduced individual PTM alterations within full-length Htt and identified several PTMs that can modulate its subcellular localization in striatal cells. These findings will be instrumental in further assembling the Htt PTM framework and highlight several PTMs as potential therapeutic targets for HD.

Prion-like propagation as a pathogenic principle in frontotemporal dementia

Frontotemporal dementia is a devastating neurodegenerative disease causing stark alterations in personality and language. Characterized by severe atrophy of the frontal and temporal brain lobes, frontotemporal dementia (FTD) shows extreme heterogeneity in clinical presentation, genetic causes, and pathological findings. Like most neurodegenerative diseases, the initial symptoms of FTD are subtle, but increase in severity over time, as the disease progresses. Clinical progression is paralleled by exacerbation of pathological findings and the involvement of broader brain regions, which currently lack mechanistic explanation. Yet, a flurry of studies indicate that protein aggregates accumulating in neurodegenerative diseases can act as propagating entities, amplifying their pathogenic conformation, in a way similar to infectious prions. In this prion‐centric view, FTD can be divided into three subtypes, TDP‐43 or FUS proteinopathy and tauopathy. Here, we review the current evidence that FTD‐linked pathology propagates in a prion‐like manner and discuss the implications of these findings for disease progression and heterogeneity.

Frontotemporal dementia (FTD) is a progressive neurodegenerative disease causing severe personality dysfunctions, characterized by profound heterogeneity. Accumulation of tau, TDP‐43 or FUS cytoplasmic aggregates characterize molecularly distinct and non‐overlapping FTD subtypes. Here, we discuss the current evidence suggesting that prion‐like propagation and cell‐to‐cell spread of each of these cytoplasmic aggregates may underlie disease progression and heterogeneity.

This article is part of the Frontotemporal Dementia special issue.

Molecular neuropathology of frontotemporal dementia: insights into disease mechanisms from postmortem studies

Frontotemporal dementia (FTD) is a clinical syndrome with a heterogeneous molecular basis. The past decade has seen the discovery of several new FTD-causing genetic mutations and the identification of many of the relevant pathological proteins. The current neuropathological classification is based on the predominant protein abnormality and allows most cases of FTD to be placed into one of three broad molecular subgroups; frontotemporal lobar degeneration with tau, TDP-43 or FET protein accumulation. This review will describe our current understanding of the molecular basis of FTD, focusing on insights gained from the study of human postmortem tissue, as well as some of the current controversies. Most cases of FTD can be subclassified into one of three broad molecular subgroups based on the predominant protein that accumulates as pathological cellular inclusions. Understanding the associated pathogenic mechanisms and recognizing these FTD molecular subtypes in vivo will likely be crucial for the development and use of targeted therapies. This article is part of the Frontotemporal Dementia special issue.

Gain-of-function profilin 1 mutations linked to familial amyotrophic lateral sclerosis cause seed-dependent intracellular TDP-43 aggregation

Profilin 1 (PFN1) is an actin monomer-binding protein essential for regulating cytoskeletal dynamics in all cell types. Recently, mutations in the PFN1 gene have been identified as a cause of familial amyotrophic lateral sclerosis (ALS). The co-aggregation of PFN1 bearing mutations that cause ALS with TDP-43 (a key molecule in both sporadic and some familial forms of ALS), together with the classical TDP-43 pathology detected in post-mortem tissues of patients with autosomal dominant PFN1 mutation, imply that gain-of-toxic-function of PFN1 mutants is associated with the onset of ALS. However, it remains unknown how PFN1 mutants cause ALS. We found mutant PFN1 that causes ALS formed cytoplasmic aggregates positive for ubiquitin and p62, and these aggregates sequestered endogenous TDP-43. In cells harboring PFN1 aggregates, formation of aggresome-like structures was inhibited in the presence of proteasome inhibitor, and conversion of LC3-I to LC3-II was suppressed in the presence of lysosome inhibitor. Further, insoluble TDP-43 was increased in both cases. Co-expression of ALS-linked mutant PFN1 and TDP-43 increased insoluble and phosphorylated TDP-43 levels. The C-terminal region of TDP-43, essential for aggregation of TDP-43, was also indispensable for the interaction with PFN1. Interestingly, insoluble fractions prepared from cells expressing ALS-linked mutant PFN1 functioned as a seed to induce accumulation and phosphorylation of TDP-43, indicating that TDP-43 accumulated in the presence of the PFN1 mutants is converted to prion-like species. These findings provide new insight into the mechanisms of neurodegeneration in ALS, suggesting that gain-of-toxic-function PFN1 gene mutation leads to conformational change of TDP-43.

ALS-Causing Mutations Significantly Perturb the Self-Assembly and Interaction with Nucleic Acid of the Intrinsically Disordered Prion-Like Domain of TDP-43

TAR-DNA-binding protein-43 (TDP-43) C-terminus encodes a prion-like domain widely presented in RNA-binding proteins, which functions to form dynamic oligomers and also, amazingly, hosts most amyotrophic lateral sclerosis (ALS)-causing mutations. Here, as facilitated by our previous discovery, by circular dichroism (CD), fluorescence and nuclear magnetic resonance (NMR) spectroscopy, we have successfully determined conformations, dynamics, and self-associations of the full-length prion-like domains of the wild type and three ALS-causing mutants (A315E, Q331K, and M337V) in both aqueous solutions and membrane environments. The study decodes the following: (1) The TDP-43 prion-like domain is intrinsically disordered only with some nascent secondary structures in aqueous solutions, but owns the capacity to assemble into dynamic oligomers rich in β-sheet structures. By contrast, despite having highly similar conformations, three mutants gained the ability to form amyloid oligomers. The wild type and three mutants all formed amyloid fibrils after incubation as imaged by electron microscopy. (2) The interaction with nucleic acid enhances the self-assembly for the wild type but triggers quick aggregation for three mutants. (3) A membrane-interacting subdomain has been identified over residues Met311-Gln343 indispensable for TDP-43 neurotoxicity, which transforms into a well-folded Ω-loop-helix structure in membrane environments. Furthermore, despite having very similar membrane-embedded conformations, three mutants will undergo further self-association in the membrane environment. Our study implies that the TDP-43 prion-like domain appears to have an energy landscape, which allows the assembly of the wild-type sequence into dynamic oligomers only under very limited condition sets, and ALS-causing point mutations are sufficient to remodel it to more favor the amyloid formation or irreversible aggregation, thus supporting the emerging view that the pathologic aggregation may occur via the exaggeration of functionally important assemblies. Furthermore, the coupled capacity of TDP-43 in aggregation and membrane interaction may critically account for its high neurotoxicity, and therefore its decoupling may represent a promising therapeutic strategy to treat TDP-43 causing neurodegenerative diseases.

The prion-like domain of TDP-43 appears to have an energy landscape that allows oligomerisation only under very limited conditions; however, TDP-43 mutations that cause amyotrophic lateral sclerosis are sufficient to remodel the protein in favor of amyloid formation.

Amyotrophic lateral sclerosis (ALS) is the most prevalent fatal motor neuron disease. It was identified ~140 years ago, but the exact mechanism underlying the disease has still not been well defined. TAR-DNA-binding protein-43 (TDP-43) was identified as the major component of the proteinaceous inclusions present in ~97% ALS and ~45% frontotemporal dementia (FTD) patients, and has also been observed in an increasing spectrum of other neurodegenerative disorders, including Alzheimer disease. The TDP-43 C-terminus is a key domain—it encodes a prion-like domain and, crucially, hosts almost all ALS-causing mutations. Here we have successfully determined the conformations, dynamics, and self-associations of the prion-like domains of both wild type and three ALS-causing mutants in both aqueous solutions and membrane environments. The study suggests that the TDP-43 prion-like domain appears to have a unique energy landscape, which allows the assembly of the wild-type sequence into specific oligomers only under very limited conditions. Intriguingly, ALS-causing point mutations remodel the energy landscape to favor amyloid formation or irreversible aggregation, thus supporting the emerging view that pathologic aggregation may occur via the exaggeration of functionally important assemblies. Furthermore, the coupled capacity of TDP-43 in aggregation and membrane interaction may partly account for its high neurotoxicity; decoupling these may therefore represent a promising therapeutic strategy to treat TDP-43-mediated neurodegenerative diseases.

Pattern of Respiratory Deterioration in Sporadic Amyotrophic Lateral Sclerosis According to Onset Lesion by Using Respiratory Function Tests

Most amyotrophic lateral sclerosis (ALS) patients show focal onset of upper and lower motor neuron signs and spread of symptoms to other regions or the other side clinically. Progression patterns of sporadic ALS are unclear. The aim of this study was to evaluate the pattern of respiratory deterioration in sporadic ALS according to the onset site by using respiratory function tests. Study participants included 63 (42 cervical-onset [C-ALS] and 21 lumbosacral-onset [L-ALS]) ALS patients and 31 healthy controls. We compared respiratory function test parameters among the 3 groups. Age was 57.4±9.6 (mean±SD), 60.8±9, and 60.5±7 years, and there were 28, 15, and 20 male participants, in the C-ALS, L-ALS, and control groups, respectively. Disease duration did not differ between C-ALS and L-ALS patients. Sniff nasal inspiratory pressure (SNIP) was significantly low in C-ALS patients compared with controls. Maximal expiratory pressure (MEP) and forced vital capacity percent predicted (FVC% predicted) were significantly low in C-ALS and L-ALS patients compared with controls. Maximal inspiratory pressure to maximal expiratory pressure (MIP:MEP) ratio did not differ among the 3 groups. Eighteen C-ALS and 5 L-ALS patients were followed up. ΔMIP, ΔMEP, ΔSNIP, ΔPEF, and ΔFVC% predicted were higher in C-ALS than L-ALS patients without statistical significance. Fourteen C-ALS (77.8%) and 3 L-ALS (60%) patients showed a constant MIP:MEP ratio above or below 1 from the first to the last evaluation. Our results suggest that vulnerability of motor neurons in sporadic ALS might follow a topographic gradient.

Review: Prion-like mechanisms of transactive response DNA binding protein of 43 kDa (TDP-43) in amyotrophic lateral sclerosis (ALS)

Amyotrophic lateral sclerosis (ALS) is a fatal devastating neurodegenerative disorder which predominantly affects the motor neurons in the brain and spinal cord. The death of the motor neurons in ALS causes subsequent muscle atrophy, paralysis and eventual death. Clinical and biological evidence now demonstrates that ALS has many similarities to prion disease in terms of disease onset, phenotype variability and progressive spread. The pathognomonic ubiquitinated inclusions deposited in the neurons and glial cells in brains and spinal cords of patients with ALS and fronto-temporal lobar degeneration with ubiquitinated inclusions contain aggregated transactive response DNA binding protein of 43 kDa (TDP-43), and evidence now suggests that TDP-43 has cellular prion-like properties. The cellular mechanisms of prion protein misfolding and aggregation are thought to be responsible for the characteristics of prion disease. Therefore, there is a strong mechanistic basis for a prion-like behaviour of the TDP-43 protein being responsible for some characteristics of ALS. In this review, we compare the prion-like mechanisms of TDP-43 to the clinical and biological nature of ALS in order to investigate how this protein could be responsible for some of the characteristic properties of the disease.

TDP-43 N terminus encodes a novel ubiquitin-like fold and its unfolded form in equilibrium that can be shifted by binding to ssDNA

Transactivation response element (TAR) DNA-binding protein 43 (TDP-43) is the principal component of ubiquitinated inclusions characteristic of most forms of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia-frontotemporal lobar degeneration with TDP-43-positive inclusions (FTLD-TDP), as well as an increasing spectrum of other neurodegenerative diseases. Previous structural and functional studies on TDP-43 have been mostly focused on its recognized domains. Very recently, however, its extreme N terminus was identified to be a double-edged sword indispensable for both physiology and proteinopathy, but thus far its structure remains unknown due to the severe aggregation. Here as facilitated by our previous discovery that protein aggregation can be significantly minimized by reducing salt concentrations, by circular dichroism and NMR spectroscopy we revealed that the TDP-43 N terminus encodes a well-folded structure in concentration-dependent equilibrium with its unfolded form. Despite previous failure in detecting any sequence homology to ubiquitin, the folded state was determined to adopt a novel ubiquitin-like fold by the CS-Rosetta program with NMR chemical shifts and 78 unambiguous long-range nuclear Overhauser effect (NOE) constraints. Remarkably, this ubiquitin-like fold could bind ssDNA, and the binding shifted the conformational equilibrium toward reducing the unfolded population. To the best of our knowledge, the TDP-43 N terminus represents the first ubiquitin-like fold capable of directly binding nucleic acid. Our results provide a molecular mechanism rationalizing the functional dichotomy of TDP-43 and might also shed light on the formation and dynamics of cellular ribonucleoprotein granules, which have been recently linked to ALS pathogenesis. As a consequence, one therapeutic strategy for TDP-43-causing diseases might be to stabilize its ubiquitin-like fold by ssDNA or designed molecules.

Significance and limitation of the pathological classification of TDP-43 proteinopathy

Based on the cerebral tans-activation response DNA protein 43 (TDP-43) immunohistochemistry, frontotemporal lobar degeneration with TDP-43 pathology (FTLD-TDP) is classified into four subtypes: type A has numerous neuronal cytoplasmic inclusions (NCIs) and dystrophic neurites (DNs); type B has numerous NCIs with few DNs; type C is characterized by DNs which are often longer and thicker than DNs in type A, with few NCIs; and type D has numerous neuronal intranuclear inclusions and DNs with few NCIs. The relevance of this classification system is supported by clinical, biochemical and genetic correlations, although there is still significant heterogeneity, especially in cases with type A pathology. The subtypes of TDP-43 pathology should be determined in cases with other neurodegenerative disorders, including Alzheimer's disease and dementia with Lewy bodies, to evaluate the pathological significance of TDP-43 abnormality in them. The results of the biochemical analyses of the diseased brains and the cellular models suggest that different strains of TDP-43 with different conformations may determine the clinicopathological phenotypes of TDP-43 proteinopathy, like prion disease. Clarifying the mechanism of the conformational changes of TDP-43 leading to the formation of multiple abnormal strains may be important for differential diagnosis and developing disease-modifying therapy for TDP-43 proteinopathy.

Distinct pathways leading to TDP-43-induced cellular dysfunctions

TAR DNA-binding protein of 43 kDa (TDP-43) is the major component protein of inclusions found in brains of patients with amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD-TDP). However, the molecular mechanisms by which TDP-43 causes neuronal dysfunction and death remain unknown. Here, we report distinct cytotoxic effects of full-length TDP-43 (FL-TDP) and its C-terminal fragment (CTF) in SH-SY5Y cells. When FL-TDP was overexpressed in the cells using a lentiviral system, exogenous TDP-43, like endogenous TDP-43, was expressed mainly in nuclei of cells without any intracellular inclusions. However, these cells showed striking cell death, caspase activation and growth arrest at G2/M phase, indicating that even simple overexpression of TDP-43 induces cellular dysfunctions leading to apoptosis. On the other hand, cells expressing TDP-43 CTF showed cytoplasmic aggregates but without significant cell death, compared with cells expressing FL-TDP. Confocal microscopic analyses revealed that RNA polymerase II (RNA pol II) and several transcription factors, such as specificity protein 1 and cAMP-response-element-binding protein, were co-localized with the aggregates of TDP-43 CTF, suggesting that sequestration of these factors into TDP-43 aggregates caused transcriptional dysregulation. Indeed, accumulation of RNA pol II at TDP-43 inclusions was detected in brains of patients with FTLD-TDP. Furthermore, apoptosis was not observed in affected neurons of FTLD-TDP brains containing phosphorylated and aggregated TDP-43 pathology. Our results suggest that different pathways of TDP-43-induced cellular dysfunction may contribute to the degeneration cascades involved in the onset of ALS and FTLD-TDP.

Extensive deamidation at asparagine residue 279 accounts for weak immunoreactivity of tau with RD4 antibody in Alzheimer's disease brain

Background

Intracytoplasmic inclusions composed of filamentous tau proteins are defining characteristics of neurodegenerative tauopathies, but it remains unclear why different tau isoforms accumulate in different diseases and how they induce abnormal filamentous structures and pathologies. Two tau isoform-specific antibodies, RD3 and RD4, are widely used for immunohistochemical and biochemical studies of tau species in diseased brains.

Results

Here, we show that extensive irreversible post-translational deamidation takes place at asparagine residue 279 (N279) in the RD4 epitope of tau in Alzheimer’s disease (AD), but not corticobasal degeneration (CBD) or progressive supranuclear palsy (PSP), and this modification abrogates the immunoreactivity to RD4. An antiserum raised against deamidated RD4 peptide specifically recognized 4R tau isoforms, regardless of deamidation, and strongly stained tau in AD brain. We also found that mutant tau with N279D substitution showed reduced ability to bind to microtubules and to promote microtubule assembly.

Conclusion

The biochemical and structural differences of tau in AD from that in 4R tauopathies found in this study may therefore have implications for prion-like propagation of tau.

CDC7 inhibition blocks pathological TDP-43 phosphorylation and neurodegeneration

OBJECTIVE

Kinase hyperactivity occurs in both neurodegenerative disease and cancer. Lesions containing hyperphosphorylated aggregated TDP-43 characterize amyotrophic lateral sclerosis and frontotemporal lobar degeneration with TDP-43 inclusions. Dual phosphorylation of TDP-43 at serines 409/410 (S409/410) drives neurotoxicity in disease models; therefore, TDP-43-specific kinases are candidate targets for intervention.

METHODS

To find therapeutic targets for the prevention of TDP-43 phosphorylation, we assembled and screened a comprehensive RNA interference library targeting kinases in TDP-43 transgenic Caenorhabditis elegans.

RESULTS

We show CDC7 robustly phosphorylates TDP-43 at pathological residues S409/410 in C. elegans, in vitro, and in human cell culture. In frontotemporal lobar degeneration (FTLD)-TDP cases, CDC7 immunostaining overlaps with the phospho-TDP-43 pathology found in frontal cortex. Furthermore, PHA767491, a small molecule inhibitor of CDC7, reduces TDP-43 phosphorylation and prevents TDP-43-dependent neurodegeneration in TDP-43-transgenic animals.

INTERPRETATION

Taken together, these data support CDC7 as a novel therapeutic target for TDP-43 proteinopathies, including FTLD-TDP and amyotrophic lateral sclerosis.

Glucocorticoids exacerbate cognitive deficits in TDP-25 transgenic mice via a glutathione-mediated mechanism: implications for aging, stress and TDP-43 proteinopathies

The accumulation of TDP-43 (transactive response DNA-binding protein 43) and its 25 kDa C-terminal fragment (TDP-25) is a hallmark of several neurodegenerative disorders, including frontotemporal lobar degeneration (FTLD-TDP) and amyotrophic lateral sclerosis (ALS). The majority of FTLD-TDP cases are due to loss of function mutations in the gene encoding progranulin, a secreted growth factor. In ALS, specific mutations in the gene encoding TDP-43 have been linked to the disease pathogenesis. In both cases, however, the penetrance of the mutations greatly increases during aging, suggesting that other genetic or environmental factors may facilitate the development of the disease. Using transgenic mice that overexpress the 25 kDa C-terminal fragment of TDP-43, here we show that glucocorticoids, stress hormones known to increase the brain susceptibility to neurotoxic insults, increase the levels of soluble TDP-25 and exacerbate cognitive deficits, without altering full-length TDP-43 levels. Additionally, we show that the mechanism underlying the glucocorticoid-mediated increase in TDP-25 levels is coupled to changes in the glutathione redox state. Glutathione is an antioxidant involved in protecting cells from damage caused by reactive oxygen species; notably, alterations in the ratio of reduced to oxidized glutathione, which is the primary determinant of the cellular redox state, are associated with aging and neurodegeneration. We show that restoring the ratio of reduced to oxidized glutathione blocks the glucocorticoid effects on TDP-25. These data show that glucocorticoids potentiate the neurotoxic action of TDP-25 by increasing its levels and clearly indicate the role of cellular oxidative damage in this process.

[Intracellular seeded aggregation of TDP-43]

TAR-DNA binding protein of 43kDa(TDP-43) is the component protein of inclusions in brains of patients with frontotemporal lobar degeneration (FTLD-TDP) and amyotrophic lateral sclerosis (ALS). Here we report a seed-dependent TDP-43 aggregation model using SH-SY5Y cells into which detergent-insoluble TDP-43 from diseased brains is introduced to provide seeds for aggregation. When these seeds were introduced into cells expressing HA-tagged TDP-43, round aggregates composed of phosphorylated and ubiquitinated HA-tagged TDP-43 were formed. Biochemical fractionation revealed the presence of Sarkosyl-insoluble phosphorylated full-length TDP-43 as well as its C-terminal fragments. Cells bearing TDP-43 inclusions exhibited increased levels of cell death and proteasome dysfunction. This seeding model reproduces characteristic features of affected neurons in brains with TDP-43 proteinopathy.

Can regional spreading of amyotrophic lateral sclerosis motor symptoms be explained by prion-like propagation?

Progressive accumulation of specific misfolded protein is a defining feature of amyotrophic lateral sclerosis (ALS), similarly seen in Alzheimer disease, Parkinson disease, Huntington disease and Creutzfeldt-Jakob disease. The intercellular transfer of inclusions made of tau, α-synuclein and huntingtin has been demonstrated, revealing the existence of mechanisms reminiscent of those by which prions spread through the nervous system. Evidence for such a prion-like propagation mechanism has now spread to the major misfolded proteins, superoxide dismutase 1 (SOD1) and the 43 kDa transactive response DNA binding protein (TDP-43), implicated in ALS. The focus in this review is on what is known about ALS progression in terms of clinical as well as molecular aspects. Furthermore, the concept of 'propagation' is dissected into contiguous and non-contiguous types, and this concept is expanded to the severity of the focal symptom as well as its regional spread which can be explained by cell to cell propagation in the local neuron pool.

Advances in understanding the molecular basis of frontotemporal dementia

Frontotemporal dementia (FTD) is a clinical syndrome with a heterogeneous molecular basis. Until recently, the underlying cause was known in only a minority of cases that were associated with abnormalities of the tau protein or gene. In 2006, however, mutations in the progranulin gene were discovered as another important cause of familial FTD. That same year, TAR DNA-binding protein 43 (TDP-43) was identified as the pathological protein in the most common subtypes of FTD and amyotrophic lateral sclerosis (ALS). Since then, substantial efforts have been made to understand the functions and regulation of progranulin and TDP-43, as well as their roles in neurodegeneration. More recently, other DNA/RNA binding proteins (FET family proteins) have been identified as the pathological proteins in most of the remaining cases of FTD. In 2011, abnormal expansion of a hexanucleotide repeat in the gene C9orf72 was found to be the most common genetic cause of both FTD and ALS. All common FTD-causing genes have seemingly now been discovered and the main pathological proteins identified. In this Review, we highlight recent advances in understanding the molecular aspects of FTD, which will provide the basis for improved patient care through the development of more-targeted diagnostic tests and therapies.

Epitope mapping of antibodies against TDP-43 and detection of protease-resistant fragments of pathological TDP-43 in amyotrophic lateral sclerosis and frontotemporal lobar degeneration

TAR DNA-binding protein of 43 kDa (TDP-43) is the major component of the intracellular inclusions in amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). Here, we show that both monoclonal (60019-2-Ig) and polyclonal (10782-2-AP) anti-TDP-43 antibodies recognize amino acids 203-209 of human TDP-43. The monoclonal antibody labeled human TDP-43 by recognizing Glu204, Asp205 and Arg208, but failed to react with mouse TDP-43. The antibodies stained the abnormally phosphorylated C-terminal fragments of 24-26 kDa in addition to normal TDP-43 in ALS and FTLD brains. Immunoblot analysis after protease treatment demonstrated that the epitope of the antibodies (residues 203-209) constitutes part of the protease-resistant domain of TDP-43 aggregates which determine a common characteristic of the pathological TDP-43 in both ALS and FTLD-TDP. The antibodies and methods used in this study will be useful for the characterization of abnormal TDP-43 in human materials, as well as in vitro and animal models for TDP-43 proteinopathies.

["Protein cancers" hypothesis for neurodegenerative diseases]

Intracellular filamentous inclusions composed of amyloid-like proteins are common neuropathological features of many neurodegenerative disorders. Although the extent of the abnormal protein pathologies is closely correlated with the disease progression, little attention has been given to the molecular mechanisms to explain how these pathological proteins spread. We developed a novel method for introducing amyloid seeds into cells, and presented experimental evidence of seed-dependent polymerization, leading to the formation of filamentous protein deposits and cell death. Overexpression of alpha-synuclein itself does not generate abnormal inclusions, but if fibril seeds are introduced, abundant alpha-synuclein inclusions positive for PSer129 and ubiquitin are developed, and the cells undergo cell death. This was also clearly demonstrated in cells expressing different tau isoforms by introducing the corresponding tau fibril seeds. These results obtained from biochemical analyses of abnormal proteins in patients strongly suggest that amyloid-like proteins, including tau, alpha-synuclein and TDP-43, propagate from cell to cell and this propagation is the cause of disease progression, analogously to metastasis of cancer cells to multiple different tissues in cancer progression. From this point of view, I have proposed as a hypothesis that neurodegenerative diseases with amyloid-like proteins can be regarded as "protein cancers".