α-Synuclein: A Promising Biomarker for Parkinson's Disease and Related Disorders.

Mutations in the SNCA gene, which encodes α-synuclein (α-syn), play a key role in the development of genetic Parkinson's disease (PD). α-Syn is a major component of Lewy bodies in PD and glial cytoplasmic inclusions in multiple system atrophy (MSA). Rapid eye movement sleep behavior disorder patients often progress to PD, dementia with Lewy bodies, or MSA, which are collectively known as α-synucleinopathies. The loss of dopaminergic neurons with Lewy bodies precedes motor dysfunction in these diseases, but the mechanisms of neurodegeneration due to α-syn aggregation are poorly understood. Monitoring α-syn aggregation in vivo could serve as a diagnostic biomarker and help elucidate pathogenesis, necessitating a simple and accurate detection method. Seed amplification assays (SAAs), such as real-time quaking-induced conversion and protein misfolding cyclic amplification, are used to detect small amounts of abnormally structured α-syn protofibrils, which are central to aggregation. These methods are promising for the early diagnosis of α-synucleinopathy. Differences in α-syn filament structures between α-synucleinopathies, as observed through transmission electron microscopy and cryo-electron microscopy, suggest their role in the pathogenesis of neurodegeneration. SAAs may differentiate between subtypes of α-synucleinopathy and other diseases. Efforts are also being made to identify α-syn from blood using various methods. This review introduces body fluid α-syn biomarkers based on pathogenic α-syn seeds, which are expected to redefine α-synucleinopathy diagnosis and staging, improving clinical research accuracy and facilitating biomarker development.

The transcription factor NRF1 (NFE2L1) activates aggrephagy by inducing p62 and GABARAPL1 after proteasome inhibition to maintain proteostasis.

The ubiquitin‒proteasome system (UPS) and autophagy are the two primary cellular pathways of misfolded or damaged protein degradation that maintain cellular proteostasis. When the proteasome is dysfunctional, cells compensate for impaired protein clearance by activating aggrephagy, a type of selective autophagy, to eliminate ubiquitinated protein aggregates; however, the molecular mechanisms by which impaired proteasome function activates aggrephagy remain poorly understood. Here, we demonstrate that activation of aggrephagy is transcriptionally induced by the transcription factor NRF1 (NFE2L1) in response to proteasome dysfunction. Although NRF1 has been previously shown to induce the expression of proteasome genes after proteasome inhibition (i.e., the proteasome bounce-back response), our genome-wide transcriptome analyses identified autophagy-related p62/SQSTM1 and GABARAPL1 as genes directly targeted by NRF1. Intriguingly, NRF1 was also found to be indispensable for the formation of p62-positive puncta and their colocalization with ULK1 and TBK1, which play roles in p62 activation via phosphorylation. Consistently, NRF1 knockdown substantially reduced the phosphorylation rate of Ser403 in p62. Finally, NRF1 selectively upregulated the expression of GABARAPL1, an ATG8 family gene, to induce the clearance of ubiquitinated proteins. Our findings highlight the discovery of an activation mechanism underlying NRF1-mediated aggrephagy through gene regulation when proteasome activity is impaired.

Propagative α-synuclein seeds as serum biomarkers for synucleinopathies.

Abnormal α-synuclein aggregation is a key pathological feature of a group of neurodegenerative diseases known as synucleinopathies, which include Parkinson's disease (PD), dementia with Lewy bodies and multiple system atrophy (MSA). The pathogenic ß-sheet seed conformation of α-synuclein is found in various tissues, suggesting potential as a biomarker, but few studies have been able to reliably detect these seeds in serum samples. In this study, we developed a modified assay system, called immunoprecipitation-based real-time quaking-induced conversion (IP/RT-QuIC), which enables the detection of pathogenic α-synuclein seeds in the serum of individuals with synucleinopathies. In our internal first and second cohorts, IP/RT-QuIC showed high diagnostic performance for differentiating PD versus controls (area under the curve (AUC): 0.96 (95% confidence interval (CI) 0.95-0.99)/AUC: 0.93 (95% CI 0.84-1.00)) and MSA versus controls (AUC: 0.64 (95% CI 0.49-0.79)/AUC: 0.73 (95% CI 0.49-0.98)). IP/RT-QuIC also showed high diagnostic performance in differentiating individuals with PD (AUC: 0.86 (95% CI 0.74-0.99)) and MSA (AUC: 0.80 (95% CI 0.65-0.97)) from controls in a blinded external cohort. Notably, amplified seeds maintained disease-specific properties, allowing the differentiation of samples from individuals with PD versus MSA. In summary, here we present a novel platform that may allow the detection of individuals with synucleinopathies using serum samples.

Central role for p62/SQSTM1 in the elimination of toxic tau species in a mouse model of tauopathy.

Intracellular accumulation of filamentous tau aggregates with progressive neuronal loss is a common characteristic of tauopathies. Although the neurodegenerative mechanism of tau-associated pathology remains unclear, molecular elements capable of degrading and/or sequestering neurotoxic tau species may suppress neurodegenerative progression. Here, we provide evidence that p62/SQSTM1, a ubiquitinated cargo receptor for selective autophagy, acts protectively against neuronal death and neuroinflammation provoked by abnormal tau accumulation. P301S mutant tau transgenic mice (line PS19) exhibited accumulation of neurofibrillary tangles with localization of p62 mostly in the brainstem, but neuronal loss with few neurofibrillary tangles in the hippocampus. In the hippocampus of PS19 mice, the p62 level was lower compared to the brainstem, and punctate accumulation of phosphorylated tau unaccompanied by co-localization of p62 was observed. In PS19 mice deficient in p62 (PS19/p62-KO), increased accumulation of phosphorylated tau, acceleration of neuronal loss, and exacerbation of neuroinflammation were observed in the hippocampus as compared with PS19 mice. In addition, increase of abnormal tau and neuroinflammation were observed in the brainstem of PS19/p62-KO. Immunostaining and dot-blot analysis with an antibody selectively recognizing tau dimers and higher-order oligomers revealed that oligomeric tau species in PS19/p62-KO mice were significantly accumulated as compared to PS19 mice, suggesting the requirement of p62 to eliminate disease-related oligomeric tau species. Our findings indicated that p62 exerts neuroprotection against tau pathologies by eliminating neurotoxic tau species, suggesting that the manipulative p62 and selective autophagy may provide an intrinsic therapy for the treatment of tauopathy.

Loss of nuclear REST/NRSF in aged-dopaminergic neurons in Parkinson's disease patients.

Parkinson's disease (PD) is the second most common neurodegenerative disease. Lewy bodies and pale bodies in dopaminergic neurons in the substantia nigra are pathological hallmarks of PD. A number of neurodegenerative diseases demonstrate aggregate formation, but how these aggregates are associated with their pathogenesis remains unknown. It has been reported that repressor element-1 silencing transcription factor/neuron-restrictive silencer factor (REST/NRSF) is induced in the nuclei of aged neurons, preserves neuronal function, and protects against neurodegeneration during aging through the repression of cell death-inducing genes. The loss of REST is associated with Alzheimer's disease pathology. However, its function in dopaminergic neurons remains unknown. Here we demonstrated that REST enters the nucleus of aged dopaminergic neurons. On the other hand, REST is partially sequestrated in Lewy bodies and is mostly absent from the nucleus of neurons in brains with PD and dementia with Lewy bodies (DLB). Dopaminergic neuron-specific autophagy-deficient mice exhibit REST accumulation in aggregates. Defects in the protein quality control system induce REST mRNA expression; its gene product mainly appears in aggregates. Our results suggest that Lewy pathology disturbs normal aging processes in dopaminergic neurons by sequestering REST and the loss of REST may associate with the PD pathology.

N-Acyldopamine induces aggresome formation without proteasome inhibition and enhances protein aggregation via p62/SQSTM1 expression.

Accumulation of ubiquitinated protein aggregates is a common pathology associated with a number of neurodegenerative diseases and selective autophagy plays a critical role in their elimination. Although aging-related decreases in protein degradation properties may enhance protein aggregation, it remains unclear whether proteasome dysfunction is indispensable for ubiquitinated-protein aggregation in neurodegenerative diseases. Here, we show that N-oleoyl-dopamine and N-arachidonyl-dopamine, which are endogenous brain substances and belong to the N-acyldopamine (AcylDA) family, generate cellular inclusions through aggresome formation without proteasome inhibition. Although AcylDA itself does not inhibit proteasome activity in vitro, it activates the rearrangement of vimentin distribution to form a vimentin cage surrounding aggresomes and sequesters ubiquitinated proteins in aggresomes. The gene transcription of p62/SQSTM1 was significantly increased by AcylDAs, whereas the transcription of other ubiquitin-dependent autophagy receptors was unaffected. Genetic depletion of p62 resulted in the loss of ubiquitinated-protein sequestration in aggresomes, indicating that p62 is a critical component of aggresomes. Furthermore, AcylDAs accelerate the aggregation of mutant huntingtin exon 1 proteins. These results suggest that aggresome formation does not require proteasome dysfunction and AcylDA-induced aggresome formation may participate in forming cytoplasmic protein inclusions.

Tau Fibril Formation in Cultured Cells Compatible with a Mouse Model of Tauopathy.

Neurofibrillary tangles composed of hyperphosphorylated tau protein are primarily neuropathological features of a number of neurodegenerative diseases collectively termed tauopathy. To understand the mechanisms underlying the cause of tauopathy, precise cellular and animal models are required. Recent data suggest that the transient introduction of exogenous tau can accelerate the development of tauopathy in the brains of non-transgenic and transgenic mice expressing wild-type human tau. However, the transmission mechanism leading to tauopathy is not fully understood. In this study, we developed cultured-cell models of tauopathy representing a human tauopathy. Neuro2a (N2a) cells containing propagative tau filaments were generated by introducing purified tau fibrils. These cell lines expressed full-length (2N4R) human tau and the green fluorescent protein (GFP)-fused repeat domain of tau with P301L mutation. Immunocytochemistry and super-resolution microscopic imaging revealed that tau inclusions exhibited filamentous morphology and were composed of both full-length and repeat domain fragment tau. Live-cell imaging analysis revealed that filamentous tau inclusions are transmitted to daughter cells, resulting in yeast-prion-like propagation. By a standard method of tau preparation, both full-length tau and repeat domain fragments were recovered in sarkosyl insoluble fraction. Hyperphosphorylation of full-length tau was confirmed by the immunoreactivity of phospho-Tau antibodies and mobility shifts by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). These properties were similar to the biochemical features of P301L mutated human tau in a mouse model of tauopathy. In addition, filamentous tau aggregates in cells barely co-localized with ubiquitins, suggesting that most tau aggregates were excluded from protein degradation systems, and thus propagated to daughter cells. The present cellular model of tauopathy will provide an advantage for dissecting the mechanisms of tau aggregation and degradation and be a powerful tool for drug screening to prevent tauopathy.

Serine 403-phosphorylated p62/SQSTM1 immunoreactivity in inclusions of neurodegenerative diseases.

Protein inclusions in neurodegenerative diseases are associated with p62, which has an important role in autophagic clearance of polyubiquitinated proteins. Selective autophagy is regulated by S403-phosphorylation of p62, and S403-phosphorylated p62 (S403-phos-p62) accumulates in Atg5 conditional knockout (Atg5CKO) mice in which autophagosome formation is impaired. We performed immunohistochemical tests for the presence of S403-phos-p62 in postmortem brain of neurodegenerative disease cases, and found accumulations in amyotrophic lateral sclerosis and Alzheimer's disease tissues. In Atg5CKO and HD190QG (Huntington's disease model) mice, however, we found a postmortem decrease in S403-phos-p62 immunoreactivity, suggesting that post-mortem changes should be considered when interpreting human data.

The Parkinson's Disease-Associated Protein Kinase LRRK2 Modulates Notch Signaling through the Endosomal Pathway.

Leucine-rich repeat kinase 2 (LRRK2) is a key molecule in the pathogenesis of familial and idiopathic Parkinson's disease (PD). We have identified two novel LRRK2-associated proteins, a HECT-type ubiquitin ligase, HERC2, and an adaptor-like protein with six repeated Neuralized domains, NEURL4. LRRK2 binds to NEURL4 and HERC2 via the LRRK2 Ras of complex proteins (ROC) domain and NEURL4, respectively. HERC2 and NEURL4 link LRRK2 to the cellular vesicle transport pathway and Notch signaling, through which the LRRK2 complex promotes the recycling of the Notch ligand Delta-like 1 (Dll1)/Delta (Dl) through the modulation of endosomal trafficking. This process negatively regulates Notch signaling through cis-inhibition by stabilizing Dll1/Dl, which accelerates neural stem cell differentiation and modulates the function and survival of differentiated dopaminergic neurons. These effects are strengthened by the R1441G ROC domain-mutant of LRRK2. These findings suggest that the alteration of Notch signaling in mature neurons is a component of PD etiology linked to LRRK2.

TBK1 controls autophagosomal engulfment of polyubiquitinated mitochondria through p62/SQSTM1 phosphorylation.

Selective autophagy adaptor proteins, including p62/SQSTM1, play pivotal roles in the targeted degradation of ubiquitinated proteins or organelles through the autophagy-lysosome system. However, how autophagy adaptors promote the autophagosomal engulfment of selected substrates is poorly understood. Here, we show that p62 phosphorylation at S403 is required for the efficient autophagosomal engulfment of polyubiquitinated mitochondria during Parkin-dependent mitophagy. p62 is able to interact with Parkin-recruited mitochondria without S403 phosphorylation under mitophagy-inducing conditions, but those mitochondria are not enclosed by autophagosomes. Intriguingly, the S403 phosphorylation occurs only in the early period of mitochondrial depolarization. Optineurin and TANK-binding kinase 1 (TBK1) are transiently recruited to the polyubiquitinated mitochondria, and the activated TBK1 phosphorylates p62 at S403. TBK1 inhibitor, BX795, prevents the p62-mediated autophagosomal engulfment of Parkin-recruited mitochondria. Our results suggest that TBK1-mediated S403 phosphorylation regulates the efficient autophagosomal engulfment of ubiquitinated mitochondria as an immediate response to the mitochondrial depolarization.

Depletion of p62 reduces nuclear inclusions and paradoxically ameliorates disease phenotypes in Huntington's model mice.

Huntington's disease (HD) is a dominantly inherited genetic disease caused by mutant huntingtin (htt) protein with expanded polyglutamine (polyQ) tracts. A neuropathological hallmark of HD is the presence of neuronal inclusions of mutant htt. p62 is an important regulatory protein in selective autophagy, a process by which aggregated proteins are degraded, and it is associated with several neurodegenerative disorders including HD. Here, we investigated the effect of p62 depletion in three HD model mice: R6/2, HD190QG and HD120QG mice. We found that loss of p62 in these models led to longer life spans and reduced nuclear inclusions, although cytoplasmic inclusions increased with polyQ length. In mouse embryonic fibroblasts (MEFs) with or without p62, mutant htt with a nuclear localization signal (NLS) showed no difference in nuclear inclusion between the two MEF types. In the case of mutant htt without NLS, however, p62 depletion increased cytoplasmic inclusions. Furthermore, to examine the effect of impaired autophagy in HD model mice, we crossed R6/2 mice with Atg5 conditional knockout mice. These mice also showed decreased nuclear inclusions and increased cytoplasmic inclusions, similar to HD mice lacking p62. These data suggest that the genetic ablation of p62 in HD model mice enhances cytoplasmic inclusion formation by interrupting autophagic clearance of polyQ inclusions. This reduces polyQ nuclear influx and paradoxically ameliorates disease phenotypes by decreasing toxic nuclear inclusions.

Phosphorylation of mitochondrial polyubiquitin by PINK1 promotes Parkin mitochondrial tethering.

The kinase PINK1 and the E3 ubiquitin (Ub) ligase Parkin participate in mitochondrial quality control. The phosphorylation of Ser65 in Parkin's ubiquitin-like (UBl) domain by PINK1 stimulates Parkin activation and translocation to damaged mitochondria, which induces mitophagy generating polyUb chain. However, Parkin Ser65 phosphorylation is insufficient for Parkin mitochondrial translocation. Here we report that Ser65 in polyUb chain is also phosphorylated by PINK1, and that phosphorylated polyUb chain on mitochondria tethers Parkin at mitochondria. The expression of Tom70MTS-4xUb SE, which mimics phospho-Ser65 polyUb chains on the mitochondria, activated Parkin E3 activity and its mitochondrial translocation. An E3-dead form of Parkin translocated to mitochondria with reduced membrane potential in the presence of Tom70(MTS)-4xUb SE, whereas non-phospho-polyUb mutant Tom70(MTS)-4xUb SA abrogated Parkin translocation. Parkin binds to the phospho-polyUb chain through its RING1-In-Between-RING (IBR) domains, but its RING0-linker is also required for mitochondrial translocation. Moreover, the expression of Tom70(MTS)-4xUb SE improved mitochondrial degeneration in PINK1-deficient, but not Parkin-deficient, Drosophila. Our study suggests that the phosphorylation of mitochondrial polyUb by PINK1 is implicated in both Parkin activation and mitochondrial translocation, predicting a chain reaction mechanism of mitochondrial phospho-polyUb production by which rapid translocation of Parkin is achieved.

Contribution of nucleus accumbens core (AcbC) to behavior control during a learned resting period: introduction of a novel task and lesion experiments.

In recent years, the study of resting state neural activity has received much attention. To better understand the roles of different brain regions in the regulation of behavioral activity in an arousing or a resting period, we developed a novel behavioral paradigm (8-arm food-foraging task; 8-arm FFT) using the radial 8-arm maze and examined how AcbC lesions affect behavioral execution and learning. Repetitive training on the 8-arm FFT facilitated motivation of normal rats to run quickly to the arm tips and to the center platform before the last-reward collection. Importantly, just after this point and before confirmation of no reward at the next arm traverse, locomotor activity decreased. This indicates that well-trained rats can predict the absence of the reward at the end of food seeking and then start another behavior, namely planned resting. Lesions of the AcbC after training selectively impaired this reduction of locomotor activity after the last-reward collection without changing activity levels before the last-reward collection. Analysis of arm-selection patterns in the lesioned animals suggests little influence of the lesion in the ability to predict the reward absence. AcbC lesions did not change exploratory locomotor activity in an open-field test in which there were no rewards. This suggests that the AcbC controls the activity level of planned resting behavior shaped by the 8-arm FFT. Rats receiving training after AcbC lesioning showed a reduction in motivation for reward seeking. Thus, the AcbC also plays important roles not only in controlling the activity level after the last-reward collection but also in motivational learning for setting the activity level of reward-seeking behavior.

NF-Y inactivation causes atypical neurodegeneration characterized by ubiquitin and p62 accumulation and endoplasmic reticulum disorganization.

Nuclear transcription factor-Y (NF-Y), a key regulator of cell-cycle progression, often loses its activity during differentiation into nonproliferative cells. In contrast, NF-Y is still active in mature, differentiated neurons, although its neuronal significance remains obscure. Here we show that conditional deletion of the subunit NF-YA in postmitotic mouse neurons induces progressive neurodegeneration with distinctive ubiquitin/p62 pathology; these proteins are not incorporated into filamentous inclusion but co-accumulated with insoluble membrane proteins broadly on endoplasmic reticulum (ER). The degeneration also accompanies drastic ER disorganization, that is, an aberrant increase in ribosome-free ER in the perinuclear region, without inducing ER stress response. We further perform chromatin immunoprecipitation and identify several NF-Y physiological targets including Grp94 potentially involved in ER disorganization. We propose that NF-Y is involved in a unique regulation mechanism of ER organization in mature neurons and its disruption causes previously undescribed novel neuropathology accompanying abnormal ubiquitin/p62 accumulation.

Dysregulation of the proteasome increases the toxicity of ALS-linked mutant SOD1.

A hallmark of protein conformational disease, exemplified by neurodegenerative disorders, is the expression of misfolded and aggregated proteins. The relationship between protein aggregation and cellular toxicity is complex, and various models of experimental pathophysiology have often yielded conflicting or controversial results. In this study, we examined the biophysical properties of amyotrophic lateral sclerosis (ALS)-linked mutations of Cu/Zn superoxide dismutase 1 (SOD1) expressed in human tissue culture cells. Fluorescence correlation spectroscopy (FCS) and Förster resonance energy transfer (FRET) analyses revealed that changes in proteasome activity affected both the expression of FCS- and FRET-detected oligomers and cellular toxicity. Under normal conditions, highly aggregation-prone mutant SOD1 exhibited very little toxicity. However, when the activity of the proteasome was transiently inhibited, only upon recovery did we observe the appearance of ordered soluble oligomers, which were closely correlated with cellular toxicity. These results shed light on the importance of balance in proteostasis and suggest that transient shifts of activity in the cellular machinery can alter the course of protein conformational transitions and dysregulate modulation of proteasome activity. In neurodegenerative disorders including ALS, such changes may be a risk factor for pathogenesis.

ROCK-phosphorylated vimentin modifies mutant huntingtin aggregation via sequestration of IRBIT.

BACKGROUND: Huntington's Disease (HD) is a fatal hereditary neurodegenerative disease caused by the accumulation of mutant huntingtin protein (Htt) containing an expanded polyglutamine (polyQ) tract. Activation of the channel responsible for the inositol-induced Ca²âº release from ensoplasmic reticulum (ER), was found to contribute substantially to neurodegeneration in HD. Importantly, chemical and genetic inhibition of inositol 1,4,5-trisphosphate (IP3) receptor type 1 (IP3R1) has been shown to reduce mutant Htt aggregation. RESULTS: In this study, we propose a novel regulatory mechanism of IP3R1 activity by type III intermediate filament vimentin which sequesters the negative regulator of IP3R1, IRBIT, into perinuclear inclusions, and reduces its interaction with IP3R1 resulting in promotion of mutant Htt aggregation. Proteasome inhibitor MG132, which causes polyQ proteins accumulation and aggregation, enhanced the sequestration of IRBIT. Furthermore we found that IRBIT sequestration can be prevented by a rho kinase inhibitor, Y-27632. CONCLUSIONS: Our results suggest that vimentin represents a novel and additional target for the therapy of polyQ diseases.

Serine 403 phosphorylation of p62/SQSTM1 regulates selective autophagic clearance of ubiquitinated proteins.

Selective macroautophagy (autophagy) of ubiquitinated protein is implicated as a compensatory mechanism of the ubiquitin-proteasome system. p62/SQSTM1 is a key molecule managing autophagic clearance of polyubiquitinated proteins. However, little is known about mechanisms controlling autophagic degradation of polyubiquitinated proteins. Here, we show that the specific phosphorylation of p62 at serine 403 (S403) in its ubiquitin-associated (UBA) domain increases the affinity between UBA and polyubiquitin chain, resulting in efficiently targeting polyubiquitinated proteins in "sequestosomes" and stabilizing sequestosome structure as a cargo of ubiquitinated proteins for autophagosome entry. Casein kinase 2 (CK2) phosphorylates S403 of p62 directly. Furthermore, CK2 overexpression or phosphatase inhibition reduces the formation of inclusion bodies of the polyglutamine-expanded huntingtin exon1 fragment in a p62-dependent manner. We propose that phosphorylation of p62 at S403 regulates autophagic clearance of ubiquitinated proteins and protein aggregates that are poorly degraded by proteasomes.

Harnessing chaperone-mediated autophagy for the selective degradation of mutant huntingtin protein.

Huntington's Disease (HD) is a dominantly inherited pathology caused by the accumulation of mutant huntingtin protein (HTT) containing an expanded polyglutamine (polyQ) tract. As the polyglutamine binding peptide 1 (QBP1) is known to bind an expanded polyQ tract but not the polyQ motif found in normal HTT, we selectively targeted mutant HTT for degradation by expressing a fusion molecule comprising two copies of QBP1 and copies of two different heat shock cognate protein 70 (HSC70)-binding motifs in cellular and mouse models of HD. Chaperone-mediated autophagy contributed to the specific degradation of mutant HTT in cultured cells expressing the construct. Intrastriatal delivery of a virus expressing the fusion molecule ameliorated the disease phenotype in the R6/2 mouse model of HD. Similar adaptor molecules comprising HSC70-binding motifs fused to an appropriate structure-specific binding agent(s) may have therapeutic potential for treating diseases caused by misfolded proteins other than those with expanded polyQ tracts.

Cross-seeding fibrillation of Q/N-rich proteins offers new pathomechanism of polyglutamine diseases.

A pathological hallmark of the Huntington's disease (HD) is intracellular inclusions containing a huntingtin (Htt) protein with an elongated polyglutamine tract. Aggregation of mutant Htt causes abnormal protein-protein interactions, and the functional dysregulation of aggregate-interacting proteins (AIPs) has been proposed as a pathomechanism of HD. Despite this, a molecular mechanism remains unknown how Htt aggregates sequester AIPs. We note an RNA-binding protein, TIA-1, as a model of AIPs containing a Q/N-rich sequence and suggest that in vitro and in vivo Htt fibrillar aggregates function as a structural template for inducing insoluble fibrillation of TIA-1. It is also plausible that such a cross-seeding activity of Htt aggregates represses the physiological function of TIA-1. We thus propose that Htt aggregates act as an intracellular hub for the cross-seeded fibrillation of Q/N-rich AIPs and that a cross-seeding reaction is a molecular origin to cause diverse pathologies in a polyglutamine disease.