Presenilin deficiency enhances tau phosphorylation and its secretion.

Alzheimer's disease (AD) is characterized by the accumulation of abnormally folded amyloid ß-protein (Aß) in the brain parenchyma and phosphorylated tau in neurons. Presenilin (PS, PSEN) 1 and PS2 are essential components of γ-secretase, which is responsible for the cleavage of amyloid precursor protein (APP) to generate Aß. PSEN mutations are associated with tau aggregation in frontotemporal dementia, regardless of the presence or absence of Aß pathology. However, the mechanism by which PS regulates tau aggregation is still unknown. Here, we found that tau phosphorylation and secretion were significantly increased in PS double-knock-out (PS1/2-/-) fibroblasts compared with wild-type fibroblasts. Tau-positive vesicles in the cytoplasm were significantly increased in PS1/2-/- fibroblasts. Active GSK-3ß was increased in PS1/2-/- fibroblasts, and inhibiting GSK3ß activity in PS1/2-/- fibroblasts resulted in decreased tau phosphorylation and secretion. Transfection of WT human PS1 and PS2 reduced the secretion of phosphorylated tau and active GSK-3ß in PS1/2-/- fibroblasts. However, PS1D257A without γ-secretase activity did not decrease the secretion of phosphorylated tau. Furthermore, nicastrin deficiency also increased tau phosphorylation and secretion. These results suggest that deficient PS complex maturation may increase tau phosphorylation and secretion. Thus, our studies discover a new pathway by which PS regulates tau phosphorylation/secretion and pathology independent of Aß and suggest that PS serves as a potential therapeutic target for treating neurodegenerative diseases involving tau aggregation.

Effects of bound nucleotides on the secondary structure, thermal stability, and phosphorylation of Rab3A.

Rab3A is a member of the Rab GTPase family involved in synaptic vesicle trafficking. Recent evidence has demonstrated that Rab3A is phosphorylated by leucine-rich repeat kinase 2 (LRRK2) that is implicated in both familial and sporadic forms of Parkinson's disease (PD), and an abnormal increase in Rab3A phosphorylation has been proposed as a cause of PD. Despite the potential importance of Rab3A in PD pathogenesis, its structural information is limited and the effects of bound nucleotides on its biophysical and biochemical properties remain unclear. Here, we show that GDP-bound Rab3A is preferentially phosphorylated by LRRK2 compared with GTP-bound Rab3A. The secondary structure of Rab3A, measured by circular dichroism (CD) spectroscopy, revealed that Rab3A is resistant to heat-induced denaturation at pH 7.4 or 9.0 regardless of the nucleotides bound. In contrast, Rab3A underwent heat-induced denaturation at pH 5.0 at a lower temperature in its GDP-bound form than in its GTP-bound form. The unfolding temperature of Rab3A was studied by differential scanning fluorimetry, which showed a significantly higher unfolding temperature in GTP-bound Rab3A than in GDP-bound Rab3A, with the highest at pH 7.4. These results suggest that Rab3A has unusual thermal stability under physiologically relevant conditions and that bound nucleotides influence both thermal stability and phosphorylation by LRRK2.

Leuco Ethyl Violet as Self-Activating Prodrug Photocatalyst for In Vivo Amyloid-Selective Oxygenation.

Aberrant aggregates of amyloid-ß (Aß) and tau protein (tau), called amyloid, are related to the etiology of Alzheimer disease (AD). Reducing amyloid levels in AD patients is a potentially effective approach to the treatment of AD. The selective degradation of amyloids via small molecule-catalyzed photooxygenation in vivo is a leading approach; however, moderate catalyst activity and the side effects of scalp injury are problematic in prior studies using AD model mice. Here, leuco ethyl violet (LEV) is identified as a highly active, amyloid-selective, and blood-brain barrier (BBB)-permeable photooxygenation catalyst that circumvents all of these problems. LEV is a redox-sensitive, self-activating prodrug catalyst; self-oxidation of LEV through a hydrogen atom transfer process under photoirradiation produces catalytically active ethyl violet (EV) in the presence of amyloid. LEV effectively oxygenates human Aß and tau, suggesting the feasibility for applications in humans. Furthermore, a concept of using a hydrogen atom as a caging group of a reactive catalyst functional in vivo is postulated. The minimal size of the hydrogen caging group is especially useful for catalyst delivery to the brain through BBB.

Molecular basis promoting centriole triplet microtubule assembly.

The triplet microtubule, a core structure of centrioles crucial for the organization of centrosomes, cilia, and flagella, consists of unclosed incomplete microtubules. The mechanisms of its assembly represent a fundamental open question in biology. Here, we discover that the ciliopathy protein HYLS1 and the ß-tubulin isotype TUBB promote centriole triplet microtubule assembly. HYLS1 or a C-terminal tail truncated version of TUBB generates tubulin-based superstructures composed of centriole-like incomplete microtubule chains when overexpressed in human cells. AlphaFold-based structural models and mutagenesis analyses further suggest that the ciliopathy-related residue D211 of HYLS1 physically traps the wobbling C-terminal tail of TUBB, thereby suppressing its inhibitory role in the initiation of the incomplete microtubule assembly. Overall, our findings provide molecular insights into the biogenesis of atypical microtubule architectures conserved for over a billion years.

Protocol for gene knockdown using siRNA in primary cultured neonatal murine microglia.

In this protocol, we describe the small interfering RNA (siRNA)-mediated gene knockdown in primary mouse microglia, providing an approach to investigate functions such as phagocytosis and chemotaxis. The approach includes siRNA design, establishment of mixed glial cultures, microglia isolation, and siRNA transfection. Validation of knockdown efficacy employs quantitative immunoblot analysis. This technique empowers the investigation of specific molecular and cellular functions within the intricate microenvironment of the brain, comprising diverse cell types. For complete details on the use and execution of this protocol, please refer to Iguchi et al. (2023).1.

Novel aspects of the phosphorylation and structure of pathological tau: implications for tauopathy biomarkers.

The deposition of highly phosphorylated and aggregated tau is a characteristic of tauopathies, including Alzheimer's disease. It has long been known that different isoforms of tau are aggregated in different cell types and brain regions in each tauopathy. Recent advances in analytical techniques revealed the details of the biochemical and structural biological differences of tau specific to each tauopathy. In this review, we explain recent advances in the analysis of post-translational modifications of tau, particularly phosphorylation, brought about by the development of mass-spectrometry and Phos-tag technology. We then discuss the structure of tau filaments in each tauopathy revealed by the advent of cryo-EM. Finally, we describe the progress in biofluid and imaging biomarkers for tauopathy. This review summarizes current efforts to elucidate the characteristics of pathological tau and the landscape of the use of tau as a biomarker to diagnose and determine the pathological stage of tauopathy.

TOLLIP acts as a cargo adaptor to promote lysosomal degradation of aberrant ER membrane proteins.

Endoplasmic reticulum (ER) proteostasis is maintained by various catabolic pathways. Lysosomes clear entire ER portions by ER-phagy, while proteasomes selectively clear misfolded or surplus aberrant proteins by ER-associated degradation (ERAD). Recently, lysosomes have also been implicated in the selective clearance of aberrant ER proteins, but the molecular basis remains unclear. Here, we show that the phosphatidylinositol-3-phosphate (PI3P)-binding protein TOLLIP promotes selective lysosomal degradation of aberrant membrane proteins, including an artificial substrate and motoneuron disease-causing mutants of VAPB and Seipin. These cargos are recognized by TOLLIP through its misfolding-sensing intrinsically disordered region (IDR) and ubiquitin-binding CUE domain. In contrast to ER-phagy receptors, which clear both native and aberrant proteins by ER-phagy, TOLLIP selectively clears aberrant cargos by coupling them with the PI3P-dependent lysosomal trafficking without promoting bulk ER turnover. Moreover, TOLLIP depletion augments ER stress after ERAD inhibition, indicating that TOLLIP and ERAD cooperatively safeguard ER proteostasis. Our study identifies TOLLIP as a unique type of cargo-specific adaptor dedicated to the clearance of aberrant ER cargos and provides insights into molecular mechanisms underlying lysosome-mediated quality control of membrane proteins.

Photo-oxygenation of histidine residue inhibits α-synuclein aggregation.

Aggregation of α-synuclein (α-syn) into amyloid is the pathological hallmark of several neurodegenerative disorders, including Parkinson disease, dementia with Lewy bodies, and multiple system atrophy. It is widely accepted that α-syn aggregation is associated with neurodegeneration, although the mechanisms are not yet fully understood. Therefore, the inhibition of α-syn aggregation is a potential therapeutic approach against these diseases. This study used the photocatalyst for α-syn photo-oxygenation, which selectively adds oxygen atoms to fibrils. Our findings demonstrate that photo-oxygenation using this photocatalyst successfully inhibits α-syn aggregation, particularly by reducing its seeding ability. Notably, we also discovered that photo-oxygenation of the histidine at the 50th residue in α-syn aggregates is responsible for the inhibitory effect. These findings indicate that photo-oxygenation of the histidine residue in α-syn is a potential therapeutic strategy for synucleinopathies.

Distinct tau folds initiate templated seeding and alter the post-translational modification profile.

Pathological tau accumulates in the brain in tauopathies such as Alzheimer's disease, Pick's disease, progressive supranuclear palsy and corticobasal degeneration, and forms amyloid-like filaments incorporating various post-translational modifications (PTMs). Cryo-electron microscopic (cryo-EM) studies have demonstrated that tau filaments extracted from tauopathy brains are characteristic of the disease and share a common fold(s) in the same disease group. Furthermore, the tau PTM profile changes during tau pathology formation and disease progression, and disease-specific PTMs are detected in and around the filament core. In addition, templated seeding has been suggested to trigger pathological tau amplification and spreading in vitro and in vivo, although the molecular mechanisms are not fully understood. Recently, we reported that the cryo-EM structures of tau protofilaments in SH-SY5Y cells seeded with patient-derived tau filaments show a core structure(s) resembling that of the original seeds. Here, we investigated PTMs of tau filaments accumulated in the seeded cells by liquid chromatography/tandem mass spectrometry and compared them with the PTMs of patient-derived tau filaments. Examination of insoluble tau extracted from SH-SY5Y cells showed that numerous phosphorylation, deamidation and oxidation sites detected in the fuzzy coat in the original seeds were well reproduced in SH-SY5Y cells. Moreover, templated tau filament formation preceded both truncation of the N-/C-terminals of tau and PTMs in and around the filament core, indicating these PTMs may predominantly be introduced after the degradation of the fuzzy coat.

A Metalloproteinase Cocktail from the Venom of Protobothrops flavoviridis Cleaves Amyloid Beta Peptides at the α-Cleavage Site.

A disintegrin and metalloproteinase (ADAM) family proteins are a major class of membrane-anchored multidomain proteinases that are responsible for the shedding of cell surface protein ectodomains, including amyloid precursor protein (APP). Human ADAM 9, 10, and 17 proteolyze APPs and produce non-amyloid-genic p3 peptides, instead of neurotoxic amyloid-ß peptides (Aßs; Aß40 and Aß42), which form fibrils and accumulate in the brain of patients with Alzheimer's disease (AD). The ADAM family is closely related to snake venom metalloproteinases (SVMPs), which are derived from ancestral ADAMs but act as soluble proteinases. To test the therapeutic potential of SVMPs, we purified SVMPs from Protobothrops flavoviridis venom using metal ion affinity and pooled into a cocktail. Thus, 9 out of 11 SVMPs in the P. flavoviridis genome were identified in the cocktail. SVMPs inhibited Aß secretion when added to human cell culture medium without affecting APP proteolysis. SVMPs degraded synthetic Aß40 and Aß42 peptides at the same cleavage site (α-site of APP) as ADAM9, 10, and 17. SVMPs did not degrade Aß fibrils but interfered with their formation, assessed using thioflavin-T. Thus, SVMPs have therapeutic potential for AD as an Aß-degrading protease, and the finding adds to the discovery of bioactive peptides from venoms as novel therapeutics.

Quantitative Assays for Catalytic Photo-Oxygenation of Alzheimer Disease-Related Tau Proteins.

Catalytic photo-oxygenation of tau amyloid is a potential therapeutic approach to tauopathies, including Alzheimer disease (AD). However, tau is a complex target containing great molecular size and heterogeneous isoforms/proteoforms. Although catalytic photo-oxygenation has been confirmed when using catalyst 1 and recombinant tau pretreated with heparin, its effects on tau from human patients have not yet been clarified. In this study, focusing on the histidine residues being oxygenated, we have constructed two assay systems capable of quantitatively evaluating the catalytic activity when used on human patient tau: (1) fluorescence labeling at oxygenated histidine sites and (2) LC-MS/MS analysis of histidine-containing fragments. Using these assays, we identified 2 as a promising catalyst for oxygenation of human tau. In addition, our results suggest that aggregated tau induced by heparin is different from actual AD patient tau in developing effective photo-oxygenation catalysts.

Pathological Roles of INPP5D in Alzheimer's Disease.

Current hypothesis of Alzheimer's disease (AD) postulates that amyloid ß (Aß) deposition in the brain causes tau inclusion in neurons and leads to cognitive decline. The discovery of the genetic association between triggering receptor expressed on myeloid cells 2 (TREM2) with increased AD risk points to a causal link between microglia and AD pathogenesis, and revealed a crucial role of TREM2-dependent clustering of microglia around amyloid plaques that prevents Aß toxicity to facilitate tau deposition near the plaques. Here we review the physiological and pathological roles of another AD risk gene expressed in microglia, inositol polyphosphate-5-polyphosphatase D (INPP5D), which encodes a phosphoinositide phosphatase. Evidence suggests that its risk polymorphisms alter the expression level and/or function of INPP5D, while concomitantly affecting tau levels in cerebrospinal fluids. In ß-amyloidosis mice, INPP5D was upregulated upon Aß deposition and negatively regulated the microglial clustering toward amyloid plaques. INPP5D seems to exert its function by acting antagonistically at downstream of the TREM2 signaling pathway, suggesting that it is a novel regulator of the protective barrier by microglia. Further studies to elucidate INPP5D's role in AD may help in developing new therapeutic targets for AD treatment.

Cryo-EM structures of tau filaments from SH-SY5Y cells seeded with brain extracts from cases of Alzheimer's disease and corticobasal degeneration.

The formation of amyloid filaments through templated seeding is believed to underlie the propagation of pathology in most human neurodegenerative diseases. A widely used model system to study this process is to seed amyloid filament formation in cultured cells using human brain extracts. Here, we report the electron cryo-microscopy structures of tau filaments from  undifferentiated seeded SH-SY5Y cells that transiently expressed N-terminally HA-tagged 1N3R or 1N4R human tau, using brain extracts from individuals with Alzheimer's disease or corticobasal degeneration. Although the resulting filament structures differed from those of the brain seeds, some degrees of structural templating were observed. Studying templated seeding in cultured cells, and determining the structures of the resulting filaments, can thus provide insights into the cellular aspects underlying neurodegenerative diseases.

LRRK2-mediated phosphorylation and thermal stability of Rab12 are regulated by bound nucleotides.

An abnormal increase in the phosphorylation of Rab12 by leucine-rich repeat kinase 2 (LRRK2), a serine/threonine kinase genetically linked to Parkinson's disease (PD), has been implicated in the pathogenesis of PD, although the underlying mechanism remains unclear. In this report, we show that LRRK2 phosphorylates Rab12 more efficiently in its GDP-bound form than in its GTP-bound form using an in vitro phosphorylation assay. This observation suggests that LRRK2 recognizes the structural difference of Rab12 caused by the bound nucleotide and that Rab12 phosphorylation inhibits its activation. Circular dichroism data revealed that Rab12, in its GDP-bound form, is more susceptible to heat-induced denaturation than its GTP-bound form, which was exacerbated at basic pH. Differential scanning fluorimetry showed that heat-induced denaturation of Rab12 in its GDP-bound form occurs at a lower temperature than in its GTP-bound form. These results suggest that the type of nucleotide bound to Rab12 determines the efficiency of LRRK2-mediated phosphorylation and the thermal stability of Rab12, and provide insights into elucidating the mechanism of the abnormal increase in Rab12 phosphorylation.

Attenuation of α-synuclein aggregation by catalytic photo-oxygenation.

We developed catalyst 11 to promote selective photo-oxygenation of α-synuclein amyloid and attenuate its aggregation. Catalyst 11 effectively oxygenated both small and large aggregates. The oxygenated α-synuclein exhibited lower seeding activity than intact α-synuclein. This study corroborates the feasibility of catalytic photo-oxygenation as an anti-synucleinopathy strategy.

INPP5D modulates TREM2 loss-of-function phenotypes in a ß-amyloidosis mouse model.

The genetic associations of TREM2 loss-of-function variants with Alzheimer disease (AD) indicate the protective roles of microglia in AD pathogenesis. Functional deficiencies of TREM2 disrupt microglial clustering around amyloid ß (Aß) plaques, impair their transcriptional response to Aß, and worsen neuritic dystrophy. However, the molecular mechanism underlying these phenotypes remains unclear. In this study, we investigated the pathological role of another AD risk gene, INPP5D, encoding a phosphoinositide PI(3,4,5)P3 phosphatase expressed in microglia. In a Tyrobp-deficient TREM2 loss-of-function mouse model, Inpp5d haplodeficiency restored the association of microglia with Aß plaques, partially restored plaque compaction, and astrogliosis, and reduced phosphorylated tau+ dystrophic neurites. Mechanistic analyses suggest that TREM2/TYROBP and INPP5D exert opposing effects on PI(3,4,5)P3 signaling pathways as well as on phosphoproteins involved in the actin assembly. Our results suggest that INPP5D acts downstream of TREM2/TYROBP to regulate the microglial barrier against Aß toxicity, thereby modulates Aß-dependent pathological conversion of tau.

Pathogenic LRRK2 compromises the subcellular distribution of lysosomes in a Rab12-RILPL1-dependent manner.

Mutations in leucine-rich repeat kinase 2 (LRRK2) cause familial Parkinson's disease (PD). Recent studies have shown that LRRK2 physiologically phosphorylates several Rab family proteins including Rab12 and that this phosphorylation is accelerated by the pathogenic mutations in LRRK2, although the significance in the PD pathogenesis remains unknown. Here we examined the effect of the overexpression of LRRK2 on the distribution of organelles in cultured cells and found that lysosomes become clustered in a perinuclear region upon the overexpression of pathogenic mutant LRRK2 in a manner dependent on its kinase activity. The perinuclear clustering of lysosomes was abolished by knocking out RAB12 as well as its effector protein RILPL1. Re-expression of Rab12 in RAB12 knockout cells suggested that the phosphorylation at Ser106 of Rab12 is required for the perinuclear clustering of lysosomes. Moreover, phosphorylated Rab12 was also accumulated on the clustered lysosomes, and the phosphorylation of Rab12 increased its interaction with RILPL1, leading us to conclude that the increase in the phosphorylation of Rab12 by pathogenic LRRK2 compromised intracellular lysosomal transport via the enhanced interaction of Rab12 with RILPL1. These data suggest the involvement of abnormal regulation of lysosomal transport in the LRRK2-mediated pathogenesis of PD.

Presenilin 1 deficiency impairs Aß42-to-Aß40- and angiotensin-converting activities of ACE.

Introduction: Alzheimer's disease (AD) is associated with amyloid ß-protein 1-42 (Aß42) accumulation in the brain. Aß42 and Aß40 are the major two species generated from amyloid precursor protein. We found that angiotensin-converting enzyme (ACE) converts neurotoxic Aß42 to neuroprotective Aß40 in an ACE domain- and glycosylation-dependent manner. Presenilin 1 (PS1) mutations account for most of cases of familial AD and lead to an increased Aß42/40 ratio. However, the mechanism by which PSEN1 mutations induce a higher Aß42/40 ratio is unclear. Methods: We over expressed human ACE in mouse wild-type and PS1-deficient fibroblasts. The purified ACE protein was used to analysis the Aß42-to-Aß40- and angiotensin-converting activities. The distribution of ACE was determined by Immunofluorescence staining. Result: We found that ACE purified from PS1-deficient fibroblasts exhibited altered glycosylation and significantly reduced Aß42-to-Aß40- and angiotensin-converting activities compared with ACE from wild-type fibroblasts. Overexpression of wild-type PS1 in PS1-deficient fibroblasts restored the Aß42-to-Aß40- and angiotensin-converting activities of ACE. Interestingly, PS1 mutants completely restored the angiotensin-converting activity in PS1-deficient fibroblasts, but some PS1 mutants did not restore the Aß42-to-Aß40-converting activity. We also found that the glycosylation of ACE in adult mouse brain differed from that of embryonic brain and that the Aß42-to-Aß40-converting activity in adult mouse brain was lower than that in embryonic brain. Conclusion: PS1 deficiency altered ACE glycosylation and impaired its Aß42-to-Aß40- and angiotensin-converting activities. Our findings suggest that PS1 deficiency and PSEN1 mutations increase the Aß42/40 ratio by reducing the Aß42-to-Aß40-converting activity of ACE.

Soluble form of the APP fragment, sAPPß, positively regulates tau secretion.

Extracellular tau has been highlighted in the pathogenesis of Alzheimer disease (AD), which is the most common neurodegenerative disease. Pathological analyses as well as model animal studies suggest that amyloid-ß peptide (Aß) deposition facilitates the spreading of tau aggregation pathology via extracellular tau. However, the precise mechanism of tau secretion remains unknown. Here, we show that the overexpression of amyloid precursor protein (APP) enhances the secretion of tau phosphorylated at threonine 181 in mouse neuroblastoma Neuro2a cells. Moreover, we found that soluble amyloid precursor protein ß (sAPPß), which is generated by ß-site APP cleaving enzyme 1 (BACE1), mediates tau secretion. Our results demonstrate that BACE1-mediated cleavage of APP plays pathological roles in AD pathogenesis by not only Aß production, but by the spreading of tau aggregation pathology via sAPPß in AD patients.

[Identification of novel regulators involved in AD pathogenesis using the CRISPR-Cas9 system].

The production of amyloid ß peptide (Aß) is an important process relating to the pathogenesis of Alzheimer disease (AD). It is widely known that the sequential cleavage of amyloid precursor protein (APP) by ß- and γ-secretases lead to the production of Aß. However, the precise regulatory mechanism for Aß production remains unclear. We have established a CRISPR-Cas9 based screening system to identify the novel regulators of Aß production. Calcium and integrin-binding protein 1 (CIB1) was identified as a novel potential negative regulator of Aß production. The knockdown and knockout of Cib1 significantly increased Aß levels. In addition, immunoprecipitation showed that CIB1 interacts with the γ-secretase complex but did not alter its enzymatic activity. Moreover, Cib1 disruption specifically reduced the cell-surface localization of the γ-secretase complex. Finally, the single-cell RNA-seq analysis in the human brain demonstrated that early-stage AD patients have lower neuronal CIB1 mRNA levels compared to healthy controls. Taken together, we have shown that CIB1 controls the subcellular localization of γ-secretase, resulting in the regulation of Aß production, suggesting the involvement of CIB1 in the development of AD pathogenesis.