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.

Phosphorylation of endogenous α-synuclein induced by extracellular seeds initiates at the pre-synaptic region and spreads to the cell body.

Accumulation of phosphorylated α-synuclein aggregates has been implicated in several diseases, such as Parkinson's disease (PD) and dementia with Lewy bodies (DLB), and is thought to spread in a prion-like manner. Elucidating the mechanisms of prion-like transmission of α-synuclein is important for the development of therapies for these diseases, but little is known about the details. Here, we injected α-synuclein fibrils into the brains of wild-type mice and examined the early phase of the induction of phosphorylated α-synuclein accumulation. We found that phosphorylated α-synuclein appeared within a few days after the intracerebral injection. It was observed initially in presynaptic regions and subsequently extended its localization to axons and cell bodies.　These results suggest that extracellular α-synuclein fibrils are taken up into the presynaptic region and seed-dependently convert the endogenous normal α-synuclein that is abundant there to an abnormal phosphorylated form, which is then transported through the axon to the cell body.

Development of a novel tau propagation mouse model endogenously expressing 3 and 4 repeat tau isoforms.

The phenomenon of 'prion-like propagation' in which aggregates of abnormal amyloid-fibrilized protein propagate between neurons and spread pathology, is attracting attention as a new mechanism in neurodegenerative diseases. There is a strong correlation between the accumulation or spread of abnormal tau aggregates and the clinical symptoms of tauopathies. Microtubule-associated protein tau (MAPT) contains a microtubule-binding domain that consists of three or four repeats (3R/4R) due to alternative mRNA splicing of transcripts for the MAPT gene. Although a number of models for tau propagation have been reported, most use 4R human tau transgenic mice or adult wild-type mice expressing only endogenous 4R tau and these models have not been able to reproduce the pathology of Alzheimer's disease in which 3R and 4R tau accumulate simultaneously, or that of Pick's disease in which only 3R tau is aggregated. These deficiencies may reflect differences between human and rodent tau isoforms in the brain. To overcome this problem, we used genome editing techniques to generate mice that express an equal ratio of endogenous 3R and 4R tau, even after they become adults. We injected these mice with sarkosyl-insoluble fractions derived from the brains of human tauopathy patients such as those afflicted with Alzheimer's disease (3R and 4R tauopathy), corticobasal degeneration (4R tauopathy) or Pick's disease (3R tauopathy). At 8-9 months following intracerebral injection of mice, histopathological and biochemical analyses revealed that the abnormal accumulation of tau was seed-dependent, with 3R and 4R tau in Alzheimer's disease-injected brains, 4R tau only in corticobasal degeneration-injected brains and 3R tau only in Pick disease-injected brains, all of which contained isoforms related to those found in the injected seeds. The injected abnormal tau was seeded, and accumulated at the site of injection and at neural connections, predominantly within the same site. The abnormal tau newly accumulated was found to be endogenous in these mice and to have crossed the species barrier. Of particular importance, Pick's body-like inclusions were observed in Pick's disease-injected mice, and accumulations characteristic of Pick's disease were reproduced, suggesting that we have developed the first model that recapitulates the pathology of Pick's disease. These models are not only useful for elucidating the mechanism of propagation of tau pathology involving both 3R and 4R isoforms, but can also reproduce the pathology of tauopathies, which should lead to the discovery of new therapeutic agents.

Dextran sulphate-induced tau assemblies cause endogenous tau aggregation and propagation in wild-type mice.

Accumulation of assembled tau protein in the central nervous system is characteristic of Alzheimer's disease and several other neurodegenerative diseases, called tauopathies. Recent studies have revealed that propagation of assembled tau is key to understanding the pathological mechanisms of these diseases. Mouse models of tau propagation are established by injecting human-derived tau seeds intracerebrally; nevertheless, these have a limitation in terms of regulation of availability. To date, no study has shown that synthetic assembled tau induce tau propagation in non-transgenic mice. Here we confirm that dextran sulphate, a sulphated glycosaminoglycan, induces the assembly of recombinant tau protein into filaments in vitro. As compared to tau filaments induced by heparin, those induced by dextran sulphate showed higher thioflavin T fluorescence and lower resistance to guanidine hydrochloride, which suggests that the two types of filaments have distinct conformational features. Unlike other synthetic filament seeds, intracerebral injection of dextran sulphate-induced assemblies of recombinant tau caused aggregation of endogenous murine tau in wild-type mice. AT8-positive tau was present at the injection site 1 month after injection, from where it spread to anatomically connected regions. Induced tau assemblies were also stained by anti-tau antibodies AT100, AT180, 12E8, PHF1, anti-pS396 and anti-pS422. They were thioflavin- and Gallyas-Braak silver-positive, indicative of amyloid. In biochemical analyses, accumulated sarkosyl-insoluble and hyperphosphorylated tau was observed in the injected mice. In conclusion, we revealed that intracerebral injection of synthetic full-length wild-type tau seeds prepared in the presence of dextran sulphate caused tau propagation in non-transgenic mice. These findings establish that propagation of tau assemblies does not require tau to be either mutant and/or overexpressed.

α-synuclein strains that cause distinct pathologies differentially inhibit proteasome.

Abnormal α-synuclein aggregation has been implicated in several diseases and is known to spread in a prion-like manner. There is a relationship between protein aggregate structure (strain) and clinical phenotype in prion diseases, however, whether differences in the strains of α-synuclein aggregates account for the different pathologies remained unclear. Here, we generated two types of α-synuclein fibrils from identical monomer and investigated their seeding and propagation ability in mice and primary-cultured neurons. One α-synuclein fibril induced marked accumulation of phosphorylated α-synuclein and ubiquitinated protein aggregates, while the other did not, indicating the formation of α-synuclein two strains. Notably, the former α-synuclein strain inhibited proteasome activity and co-precipitated with 26S proteasome complex. Further examination indicated that structural differences in the C-terminal region of α-synuclein strains lead to different effects on proteasome activity. These results provide a possible molecular mechanism to account for the different pathologies induced by different α-synuclein strains.

Effects of progranulin on the pathological conditions in experimental myocardial infarction model.

Progranulin is a secreted growth factor associated with multiple physiological functions in ischemic pathophysiology. However, it is still not fully understood how progranulin is involved in ischemic lesion and cardiac remodeling after myocardial infarction (MI). In this study, we investigated the effects of progranulin on myocardial ischemia and reperfusion injury. We investigated progranulin expression using Western blotting and immunostaining after permanent left coronary artery (LCA) occlusion in mice. Infarct size and the number of infiltrating neutrophils were measured 24 h after permanent LCA occlusion. Recombinant mouse progranulin was administered before LCA occlusion. In addition, we evaluated cardiac function using cardiac catheterization and echocardiography, and fibrosis size by Masson's trichrome staining after myocardial ischemia/reperfusion in rabbits. Recombinant human progranulin was administered immediately after induction of reperfusion. Progranulin expression increased in the myocardial ischemic area 1, 3, and 5 days after permanent LCA occlusion in mice. The administration of recombinant mouse progranulin significantly attenuated infarct size and infiltrating neutrophils 24 h after permanent LCA occlusion in mice. We also found that administration of recombinant human progranulin ameliorated the deterioration of cardiac dysfunction and fibrosis after myocardial ischemia/reperfusion in rabbits. These findings suggest that progranulin may protect myocardial ischemia/reperfusion injury.

The effect of truncation on prion-like properties of α-synuclein.

Increasing evidence suggests that α-synuclein (αS) aggregates in brains of individuals with Parkinson's disease and dementia with Lewy bodies can spread in a prion-like manner. Although the initial αS nuclei are pivotal in determining αS fibril polymorphs and resulting phenotypes, it is not clear how the initial fibril seeds are generated. Previous studies have shown that αS truncation might have an important role in αS aggregation. However, little is known about how this truncation influences αS's propagation properties. In the present study, we generated αS fibrils from a series of truncated human αS constructs, characterized their structures and conformational stabilities, and investigated their ability to convert the conformation of full-length αS in vitro, in cultured cells, and in WT mice. We show that both C- and N-terminal truncations of human αS induce fibril polymorphs and exhibit different cross-seeding activities. N-terminally 10- or 30-residue-truncated human αS fibrils induced more abundant αS pathologies than WT fibrils in mice, whereas other truncated fibrils induced less abundant pathologies. Biochemical analyses of these truncated fibrils revealed that N-terminal 10- or 30-residue truncations of human αS change the fibril conformation in a manner that increases their structural compatibility with WT mouse αS fibrils and reduces their stability. C-terminally 20-residue-truncated fibrils displayed enhanced seeding activity in vitro Our findings imply that truncation of αS can influence its prion-like pathogenicity, resulting in phenotypic diversity of α-synucleinopathies.

Following the fate of endocytosed fibrils.

Cell-to-cell transmission of intracellular protein aggregates is considered a central event in many neurodegenerative diseases, but little is known about the underlying molecular mechanisms. A new study employs fluorescence quenching to examine the fate of α-synuclein, a key molecule in the pathology of Parkinson's disease and related disorders, in primary cultured neurons, finding that endocytosis and lysosomal processing of exogenous fibrils may explain the transmission of α-synuclein pathology.

Progranulin regulates lysosomal function and biogenesis through acidification of lysosomes.

Progranulin (PGRN) haploinsufficiency resulting from loss-of-function mutations in the PGRN gene causes frontotemporal lobar degeneration accompanied by TDP-43 accumulation, and patients with homozygous mutations in the PGRN gene present with neuronal ceroid lipofuscinosis. Although it remains unknown why PGRN deficiency causes neurodegenerative diseases, there is increasing evidence that PGRN is implicated in lysosomal functions. Here, we show PGRN is a secretory lysosomal protein that regulates lysosomal function and biogenesis by controlling the acidification of lysosomes. PGRN gene expression and protein levels increased concomitantly with the increase of lysosomal biogenesis induced by lysosome alkalizers or serum starvation. Down-regulation or insufficiency of PGRN led to the increased lysosomal gene expression and protein levels, while PGRN overexpression led to the decreased lysosomal gene expression and protein levels. In particular, the level of mature cathepsin D (CTSDmat) dramatically changed depending upon PGRN levels. The acidification of lysosomes was facilitated in cells transfected with PGRN. Then, this caused degradation of CTSDmat by cathepsin B. Secreted PGRN is incorporated into cells via sortilin or cation-independent mannose 6-phosphate receptor, and facilitated the acidification of lysosomes and degradation of CTSDmat. Moreover, the change of PGRN levels led to a cell-type-specific increase of insoluble TDP-43. In the brain tissue of FTLD-TDP patients with PGRN deficiency, CTSD and phosphorylated TDP-43 accumulated in neurons. Our study provides new insights into the physiological function of PGRN and the role of PGRN insufficiency in the pathogenesis of neurodegenerative diseases.

The Effect of Fragmented Pathogenic α-Synuclein Seeds on Prion-like Propagation.

Aggregates of abnormal proteins are widely observed in neuronal and glial cells of patients with various neurodegenerative diseases, and it has been proposed that prion-like behavior of these proteins can account for not only the onset but also the progression of these diseases. However, it is not yet clear which abnormal protein structures function most efficiently as seeds for prion-like propagation. In this study, we aimed to identify the most pathogenic species of α-synuclein (α-syn), the main component of the Lewy bodies and Lewy neurites that are observed in α-synucleinopathies. We prepared various forms of α-syn protein and examined their seeding properties in vitro in cells and in mouse experimental models. We also characterized these α-syn species by means of electron microscopy and thioflavin fluorescence assays and found that fragmented ß sheet-rich fibrous structures of α-syn with a length of 50 nm or less are the most efficient promoters of accumulation of phosphorylated α-syn, which is the hallmark of α-synucleinopathies. These results indicate that fragmented amyloid-like aggregates of short α-syn fibrils are the key pathogenic seeds that trigger prion-like conversion.

[The proceeding of drug development based on the propagation of tau protein].

Many neurodegenerative diseases accompanied by dementia are associated with the accumulation of protein aggregates. These aggregates are thought to be causes of progressive neurodegeneration and death of neurons. Recent studies indicated that these aggregates could propagate between cells in a prion like manner. Once a protein aggregate is formed it can transmit cell to cell and affect broad regions of brain. Some neurodegenerative diseases such as Alzheimer's disease are associated with the pathological aggregation of tau protein. Thus, these diseases are named as tauopathies. In the brain of these diseases, aggregated tau must also propagate in a prion like manner. Here, we discuss about the therapeutic approaches against dementia based on the prion like propagation of tau proteins.

Templated Aggregation of TAR DNA-binding Protein of 43 kDa (TDP-43) by Seeding with TDP-43 Peptide Fibrils.

TAR DNA-binding protein of 43 kDa (TDP-43) has been identified as the major component of ubiquitin-positive neuronal and glial inclusions in frontotemporal lobar degeneration and amyotrophic lateral sclerosis. Aggregation of TDP-43 to amyloid-like fibrils and spreading of the aggregates are suggested to account for the pathogenesis and progression of these diseases. To investigate the molecular mechanisms of TDP-43 aggregation, we attempted to identify the amino acid sequence required for the aggregation. By expressing a series of deletion mutants lacking 20 amino acid residues in the C-terminal region in SH-SY5Y cells, we established that residues 274-313 in the glycine-rich region are essential for aggregation. In vitro aggregation experiments using synthetic peptides of 40 amino acids from this sequence and adjacent regions showed that peptides 274-313 and 314-353 formed amyloid-like fibrils. Transduction of these fibrils induced seed-dependent aggregation of TDP-43 in cells expressing wild-type TDP-43 or TDP-43 lacking nuclear localization signal. These cells showed different phosphorylated C-terminal fragments of TDP-43 and different trypsin-resistant bands. These results suggest that residues 274-353 are responsible for the conversion of TDP-43 to amyloid-like fibrils and that templated aggregation of TDP-43 by seeding with different peptides induces various types of TDP-43 pathologies, i.e. the peptides appear to act like prion strains.

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.

Phosphorylation of TAR DNA-binding Protein of 43 kDa (TDP-43) by Truncated Casein Kinase 1δ Triggers Mislocalization and Accumulation of TDP-43.

Intracellular aggregates of phosphorylated TDP-43 are a major component of ubiquitin-positive inclusions in the brains of patients with frontotemporal lobar degeneration and ALS and are considered a pathological hallmark. Here, to gain insight into the mechanism of intracellular TDP-43 accumulation, we examined the relationship between phosphorylation and aggregation of TDP-43. We found that expression of a hyperactive form of casein kinase 1 δ (CK1δ1-317, a C-terminally truncated form) promotes mislocalization and cytoplasmic accumulation of phosphorylated TDP-43 (ubiquitin- and p62-positive) in cultured neuroblastoma SH-SY5Y cells. Insoluble phosphorylated TDP-43 prepared from cells co-expressing TDP-43 and CK1δ1-317 functioned as seeds for TDP-43 aggregation in cultured cells, indicating that CK1δ1-317-induced aggregated TDP-43 has prion-like properties. A striking toxicity and alterations of TDP-43 were also observed in yeast expressing TDP-43 and CK1δ1-317. Therefore, abnormal activation of CK1δ causes phosphorylation of TDP-43, leading to the formation of cytoplasmic TDP-43 aggregates, which, in turn, may trigger neurodegeneration.

[KIL-d] Protein Element Confers Antiviral Activity via Catastrophic Viral Mutagenesis.

Eukaryotic cells are targeted by pathogenic viruses and have developed cell defense mechanisms against viral infection. In yeast, the cellular extrachromosomal genetic element [KIL-d] alters killer activity of M double-stranded RNA killer virus and confers cell resistance against the killer virus. However, its underlying mechanism and the molecular nature of [KIL-d] are unknown. Here, we demonstrate that [KIL-d] is a proteinaceous prion-like aggregate with non-Mendelian cytoplasmic transmission. Deep sequencing analyses revealed that [KIL-d] selectively increases the rate of de novo mutation in the killer toxin gene of the viral genome, producing yeast harboring a defective mutant killer virus with a selective growth advantage over those with WT killer virus. These results suggest that a prion-like [KIL-d] element reprograms the viral replication machinery to induce mutagenesis and genomic inactivation via the long-hypothesized mechanism of "error catastrophe." The findings also support a role for prion-like protein aggregates in cellular defense and adaptation.

Expanding the yeast prion world: Active prion conversion of non-glutamine/asparagine-rich Mod5 for cell survival.

Mammalian and fungal prion proteins form self-perpetuating ß-sheet-rich fibrillar aggregates called amyloid. Prion inheritance is based on propagation of the regularly oriented amyloid structures of the prion proteins. All yeast prion proteins identified thus far contain aggregation-prone glutamine/asparagine (Gln/Asn)-rich domains, although the mammalian prion protein and fungal prion protein HET-s do not contain such sequences. In order to fill this gap, we searched for novel yeast prion proteins lacking Gln/Asn-rich domains via a genome-wide screen based on cross-seeding between two heterologous proteins and identified Mod5, a yeast tRNA isopentenyltransferase, as a novel non-Gln/Asn-rich yeast prion protein. Mod5 formed self-propagating amyloid fibers in vitro and the introduction of Mod5 amyloids into non-prion yeast induced dominantly and cytoplasmically heritable prion state [MOD (+) ], which harbors aggregates of endogenous Mod5. [MOD (+) ] yeast showed an increased level of membrane lipid ergosterol and acquired resistance to antifungal agents. Importantly, enhanced de novo formation of [MOD (+) ] was observed when non-prion yeast was grown under selective pressures from antifungal drugs. Our findings expand the family of yeast prions to non-Gln/Asn-rich proteins and reveal the acquisition of a fitness advantage for cell survival through active prion conversion.

A yeast prion, Mod5, promotes acquired drug resistance and cell survival under environmental stress.

Prion conversion from a soluble protein to an aggregated state may be involved in the cellular adaptation of yeast to the environment. However, it remains unclear whether and how cells actively use prion conversion to acquire a fitness advantage in response to environmental stress. We identified Mod5, a yeast transfer RNA isopentenyltransferase lacking glutamine/asparagine-rich domains, as a yeast prion protein and found that its prion conversion in yeast regulated the sterol biosynthetic pathway for acquired cellular resistance against antifungal agents. Furthermore, selective pressure by antifungal drugs on yeast facilitated the de novo appearance of Mod5 prion states for cell survival. Thus, phenotypic changes caused by active prion conversion under environmental selection may contribute to cellular adaptation in living organisms.

Evaluation of image processing programs for accurate measurement of budding and fission yeast morphology.

To study the cellular functions of gene products, various yeast morphological mutants have been investigated. To describe yeast morphology objectively, we have developed image processing programs for budding and fission yeast. The programs, named CalMorph for budding yeast and F-CalMorph for fission yeast, directly process microscopic images and generate quantitative data about yeast cell shape, nuclear shape and location, and actin distribution. Using CalMorph, we can easily and quickly obtain various quantitative data reproducibly. To study the utility and reliability of CalMorph, we evaluated its data in three ways: (1) The programs extracted three-dimensional bud information from two-dimensional digital images with a low error rate (<1%). (2) The absolute values of the diameters of manufactured fluorescent beads calculated with CalMorph were very close to those given in the manufacturer's data sheet. (3) The programs generated reproducible data consistent with that obtained by hand. Based on these results, we determined that CalMorph could monitor yeast morphological changes accompanied by the progression of the cell cycle. We discuss the potential of the CalMorph series as a novel tool for the analysis of yeast cell morphology.

High-dimensional and large-scale phenotyping of yeast mutants.

One of the most powerful techniques for attributing functions to genes in uni- and multicellular organisms is comprehensive analysis of mutant traits. In this study, systematic and quantitative analyses of mutant traits are achieved in the budding yeast Saccharomyces cerevisiae by investigating morphological phenotypes. Analysis of fluorescent microscopic images of triple-stained cells makes it possible to treat morphological variations as quantitative traits. Deletion of nearly half of the yeast genes not essential for growth affects these morphological traits. Similar morphological phenotypes are caused by deletions of functionally related genes, enabling a functional assignment of a locus to a specific cellular pathway. The high-dimensional phenotypic analysis of defined yeast mutant strains provides another step toward attributing gene function to all of the genes in the yeast genome.