Biomedical subjects

Naruhiko Sahara

Publications and source records attributed to Naruhiko Sahara.

14 recordsLinked to original sources

PubMed2007

Decreased Abeta secretion by cells expressing familial Alzheimer's disease-linked mutant presenilin 1.

Presenilin (PS) is a catalytic subunit of the gamma-secretase complex that cleaves the intramembranous region of amyloid precursor protein (APP), producing amyloid beta (Abeta) peptide. Familial Alzheimer's disease (FAD) results from PS mutations, which may alter gamma-secretase activity to enhance the production of highly aggregable Abeta42. The precise pathogenic effects of mutant PS remain unclear. To exclude the effects of endogenous PS, we established cell lines from PS1/PS2-deficient (PS-/-) fibroblasts capable of stably expressing either wild-type (wt) PS1 or different mutant PS1s. Although both wt PS1 and mutant PS1 formed gamma-secretase complexes of the same size and containing the same components, the amount of Abeta secreted by FAD mutant PS1-expressing cells was significantly reduced. The ratio of Abeta42 to Abeta40 (Abeta42/Abeta40) secreted by these cells, however, was significantly higher than that secreted by cells expressing wt PS1, which corroborated findings from a previous report. The elevated Abeta42/Abeta40 ratio observed with mutant PS1-expressing cells may be due to reduced Abeta40 production not increased Abeta42 production.

Adult↗

PubMed2006

c-jun N-terminal kinase hyperphosphorylates R406W tau at the PHF-1 site during mitosis.

Tauopathies such as Alzheimer disease (AD) probably involve a type of phosphorylation imbalance causing the accumulation of abnormally hyperphosphorylated tau in neurons and/or glias. Investigation of R406W tau mutation may provide insight into such abnormal tau hyperphosphorylation, since this mutation causes AD-like dementia and tauopathy in humans and because it has the unique ability to reduce tau phosphorylation in vitro and in cultured cells. Here we show that R406W mutation primarily disrupts tau phosphorylation at Ser404, a priming phosphorylation site of glycogen synthase kinase-3beta (GSK-3beta), thereby reducing subsequent GSK-3beta-mediated phosphorylation at the PHF-1 site (mostly Ser396). In contrast, c-jun N-terminal kinase (JNK) as activated in the mitotic phase directly hyperphosphorylates R406W tau at the PHF-1 site. This was confirmed by PHF-1 hyperphosphorylation of R406W tau in mitotic cells, its association with cytoplasmic JNK activation, and its inhibition by a JNK inhibitor, SP600125. These data unveil the unknown mechanisms of physiological tau phosphorylation at the PHF-1 site and suggest that cytoplasmic JNK activation may play an important role in the abnormal tau hyperphosphorylation associated with R406W tau mutation and in AD.

Animals↗

PubMed2006

Increased levels of granular tau oligomers: an early sign of brain aging and Alzheimer's disease.

Development of neurofibrillary tangles (NFTs) is a pathological hallmark in various neurodegenerative disorders including Alzheimer's disease (AD). Recently, we identified a granular tau oligomer having a pre-filamentous structure. To determine the role of this oligomer in NFT formation, we quantified the amount of granular tau oligomer in 21 frontal cortex samples, each displaying varying degrees of Braak-staged NFT pathology. Here we report that granular tau oligomer levels in frontal cortex were significantly increased, even in brains displaying Braak-stage I neuropathology, a stage at which clinical symptoms of AD and NFTs in frontal cortex are believed to be absent. This suggests that increases in granular tau oligomer levels occur before NFTs form and before individuals manifest clinical symptoms of AD. Increased granular tau oligomer levels, therefore, may lead to NFT formation in frontal cortex, eventually leading to the development of AD. Thus, increases in granular tau oligomer levels may represent a very early sign of NFT formation and AD.

Aged↗

PubMed2005

Recent advances in experimental modeling of the assembly of tau filaments.

Intracellular assembly of microtubule-associated protein tau into filamentous inclusions is central to Alzheimer's disease and related disorders collectively known as tauopathies. Although tau mutations, posttranslational modifications and degradations have been the focus of investigations, the mechanism of tau fibrillogenesis in vivo still remains elusive. Different strategies have been undertaken to generate animal and cellular models for tauopathies. Some are used to study the molecular events leading to the assembly and accumulation of tau filaments, and others to identify potential therapeutic agents that are capable of impeding tauopathy. This review highlights the latest developments in new models and how their utility improves our understanding of the sequence of events leading to human tauopathy.

Animals↗

PubMed2005

In vivo evidence of CHIP up-regulation attenuating tau aggregation.

The carboxyl terminus of heat-shock cognate (Hsc)70-interacting protein (CHIP) is a ubiquitin E3 ligase that can collaborate with molecular chaperones to facilitate protein folding and prevent protein aggregation. Previous studies showed that, together with heat-shock protein (Hsp)70, CHIP can regulate tau ubiquitination and degradation in a cell culture system. Ubiquitinated tau is one component in neurofibrillary tangles (NFTs), which are a major histopathological feature of Alzheimer's disease (AD). However, the precise sequence of events leading to NFT formation and the mechanisms involved remain unclear. To confirm CHIP's role in suppressing NFT formation in vivo, we performed a quantitative analysis of CHIP in human and mouse brains. We found increased levels of CHIP and Hsp70 in AD compared with normal controls. CHIP levels in both AD and controls corresponded directly to Hsp90 levels, but not to Hsp70 or Hsc70 levels. In AD samples, CHIP was inversely proportional to sarkosyl-insoluble tau accumulation. In a JNPL3 mouse brain tauopathy model, CHIP was widely distributed but weakly expressed in spinal cord, which was the most prominent region for tau inclusions and neuronal loss. Protein levels of CHIP in cerebellar regions of JNPL3 mice were significantly higher than in non-transgenic littermates. Human tau was more highly expressed in this region of mouse brains, but only moderate levels of sarkosyl-insoluble tau were detected. This was confirmed when increased insoluble tau accumulation was found in mice lacking CHIP. These findings suggest that increases in CHIP may protect against NFT formation in the early stages of AD. If confirmed, this would indicate that the quality-control machinery in a neuron might play an important role in retarding the pathogenesis of tauopathies.

Aged↗

PubMed2004

Phosphorylated p38MAPK specific antibodies cross-react with sarkosyl-insoluble hyperphosphorylated tau proteins.

Neurofibrillary tangles (NFT) accumulated in Alzheimer's diseases and related disorders contain hyperphosphorylated tau and display immunoreactivity for active forms of various kinases. To understand the role of p38MAPK (mitogen-activated protein kinase) in NFT formation, we have studied a transgenic (Tg) mouse model of tauopathy, JNPL3, that expresses P301L mutant tau, and bigenic mice, TAPP, generated by cross-breeding of JNPL3 with Tg2576 mice. Age-matched non-Tg mice (NTg), wild-type human tau Tg mice (JN25), and Tg2576 mice were used as controls. Phosphorylated p38MAPK (active form) immunoreactivity was consistently located in NFT and granulovaculolar degeneration in JNPL3 and TAPP mice older than 5 months of age. Unphosphorylated/total-p38MAPK was not detectable in spinal cord and brain sections from 2- to 11-month-old mice, even though JNPL3 mice, but not controls had an age-dependent increase of total-p38MAPK by western blotting. Spinal cord/brain extracts from mice and human with tauopathy were demonstrated to have insignificant amount of active-p38MAPK. However, they contained antiactive-p38MAPK cross-reactive proteins insoluble in sarkosyl and similar to phosphorylated tau in size. Consistently, antiactive-p38MAPK immunoprecipitates displayed tau immunoreactivity, but not total-p38MAPK, and antitau immunoprecipitates displayed active-p38MAPK immunoreactivity. Together, the results indicate that the cross-reactivity of antiactive-p38MAPK antibody with phosphorylated tau is responsible for the immunolabeling of tau-positive inclusion.

Age Factors↗

PubMed2004

Assembly of filamentous tau aggregates in human neuronal cells.

Intraneuronal deposition of microtubule-associated protein tau in filamentous aggregates constitutes a pathological hallmark of neurofibrillary degeneration that is characteristic of Alzheimer's disease (AD) and related disorders known collectively as tauopathies. Formation of such fibril inclusions, consisting of hyperphosphorylated tau in multiple isoforms, correlates with the severity of cognitive decline in AD. How neurofibrillary pathology evolves in tauopathy remains unclear at present, but availability of a cellular model with robust tau aggregation will permit experimental scrutiny of the mechanistic process leading to such neurodegeneration. Through the use of a serial transfection strategy in conjunction with a tau minigene construct, we succeeded in generating conditional transfectants of human neuronal lineage that overproduce wild-type human brain tau in isoforms 4R0N, 3R1N and 4R1N via TetOff and ecdysone inducible expression mechanisms. Such transgenic overexpression of tau in multiple isoforms facilitated the assembly of filamentous tau aggregates that exhibit immunoreactivities, physicochemical properties, and ultrastructural attributes reminiscent of those found in human tauopathies. The conditional tau transfectants thus provide us with a useful tool to elucidate the molecular and cellular events leading to neurofibrillary degeneration and a convenient means to test hypothetical mechanisms implicated in the etiopathogenesis of AD and related tauopathies.

Alzheimer Disease↗

PubMed2003

The L266V tau mutation is associated with frontotemporal dementia and Pick-like 3R and 4R tauopathy.

We report a case of rapidly progressive frontotemporal dementia presenting at age 33 years. At autopsy there was severe atrophy of the frontal and temporal lobes. Tau-positive Pick bodies, which ultrastructurally were composed of straight filaments, were present, accompanied by severe neuronal loss and gliosis. RD3, a tau antibody specific for the three-repeat (3R) isoforms, labeled the Pick bodies. ET3, a four-repeat (4R) isoform-specific tau antibody, did not label Pick bodies, but highlighted rare astrocytes, and threads in white matter bundles in the corpus striatum. Analysis of the tau gene revealed an L266V mutation in exon 9. Analysis of brain tissue from this case revealed elevated levels of exon 10+ tau RNA and soluble 4R tau. However, both 3R and 4R isoforms were present in sarkosyl-insoluble tau fractions with a predominance of the shortest 3R isoform. The L266V mutation is associated with decreased rate and extent of tau-induced microtubule assembly, and a 3R isoform-specific increase in tau self assembly as measured by an in vitro assay. Combined, these data indicate that L266V is a pathogenic tau mutation that is associated with Pick-like pathology. In addition, the results of the RD3 and ET3 immunostains clearly explain for the first time the presence of both 3R and 4R tau isoforms in preparations of insoluble tau from some Pick's disease cases.

Adult↗

PubMed2003

A novel tau mutation in exon 9 (1260V) causes a four-repeat tauopathy.

A novel mutation in exon 9 of tau, I260V, is associated with a clinical syndrome consistent with frontotemporal dementia with extensive tau pathology; however, neurofibrillary tangles and Pick bodies are absent. Significantly, Sarkosyl-insoluble tau extracted from affected brain tissue consisted almost exclusively of four-repeat isoforms. Consistent with these findings, in vitro biochemical assays demonstrated that the I260V mutation causes a selective increase in tau aggregation and a decrease in tau-induced microtubule assembly with four-repeat isoforms only. The contrasting pathology and biochemical effects of this mutation suggest a different disease mechanism from the other exon 9 mutations and demonstrates the critical role for the first microtubule-binding domain in tau-promoted microtubule assembly and the pathogenic aggregation of tau.

Aged↗

PubMed2002

Chrysamine G and its derivative reduce amyloid beta-induced neurotoxicity in mice.

The neurotoxicity of amyloid beta (Abeta) is widely believed to play a seminal role in neurodegeneration in Alzheimer's disease. We examined the effect of Chrysamine G (CG) on such neurotoxicity using the specific measurement of surviving neurons. CG was found to reduce the neurodegeneration induced by both the active short fragment of Abeta(25-35) and full-sized Abeta(1-40). In this study, we synthesized a new chemical compound from a monovalent structure of CG (hCG), with a lower affinity for Abeta, and compared its activity with that of CG. Both CG and hCG were found to be equally efficacious in reducing Abeta-induced neuronal death at a concentration of 0.1-1 microM, indicating that the mechanism of action for CG was not due to its chelating activity, but rather due to its anti-oxidant activity.

Amyloid beta-Peptides↗

PubMed2002

Impaired cell cycle control of neuronal precursor cells in the neocortical primordium of presenilin-1-deficient mice.

Recent studies have implicated presenilin-1 (PS-1) in the processing of the amyloid precursor protein and Notch-1. We show that PS-1 has biological effects on differentiation and cell cycle control of neuronal precursor cells in vivo using PS-1-deficient mice. The expression of Class III beta-tubulin was upregulated throughout the neocortical primordia of PS-1-deficient E14 embryos, especially on the ventricular surface. The increased speed of migration of the immature neurons from the ventricular zone outward in the PS-1-deficient neocortical primordia was indicated by an in vivo bromodeoxyuridine (BrdU)-labeling assay and a DiI-labeling assay in slice culture. Furthermore, we investigated the cell cycle of neuronal precursor cells in the neocortical ventricular zone using an in vivo cumulative BrdU-labeling assay. The length of the cell cycle in the neocortical precursor cells of wild-type mice was 11.4 hr whereas that of the PS-1-deficient mice was 15.4 hr. Among all phases of the cell cycle, S-phase exhibited the most prominent change in length, increasing from 2.4 hr in the wild-type mice to 7.4 hr in the mutant mice. The distribution of beta-catenin was specifically affected in the ventricular zone of the PS-1-deficient mice. These findings suggest that PS-1 is involved in the differentiation and the cell cycle control of neuronal precursor cells in the ventricular proliferating zone of the neocortical primordium.

Alleles↗

PubMed2002

Assembly of tau in transgenic animals expressing P301L tau: alteration of phosphorylation and solubility.

Transgenic mice (JNPL3), which develop neurofibrillary degeneration and express four-repeat human tau with P301L missense mutation, were characterized biochemically to determine whether the development of aggregated tau from soluble tau involves an intermediate stage. Homogenates from mice of different ages were separated into buffer-soluble (S1), sarkosyl- and salt-extractable (S2) and sarkosyl-insoluble pellet (P3) fractions, and analyzed for human tau distribution, phosphorylation and filament formation. S1 and S2 fractions contained 50-60-kDa tau whereas the S2 fraction also had 64-kDa tau. The level of tau in the P3 fraction increased in an age-dependent manner and correlated positively with the soluble tau concentration. The P3 fraction from 2.5-6.5-month-old mice contained 64- and 50-60-kDa tau, whereas that from 8.5-month and older transgenic animals contained mostly 64-kDa and higher molecular weight tau. The S2 and P3 fractions contained comparable amounts of 64-kDa tau. The 64-kDa tau was predominantly human, and phosphorylated at multiple sites: Thr181, Ser202/Thr205, Thr212, Thr231, Ser262, Ser396/Ser404, Ser409 and Ser422. Most of these sites were phosphorylated to a lesser extent in S2 than in P3 fractions. Tau polymers were detected in P3 fractions from 3-month and older female JNPL3 mice, but not in non-transgenic controls. The results suggest that tau in S2 represents an intermediate from which insoluble tau is derived, and that phosphorylation may play a role in filament formation and/or stabilization.

Age Factors↗

PubMed2002

Argyrophilic grain disease is a sporadic 4-repeat tauopathy.

Argyrophilic grain disease (AGD) was first reported as an adult-onset dementia, but recent studies have emphasized personality change, emotional imbalance, and memory problems as clinical features of AGD. AGD is characterized by spindle- or comma-shaped argyrophilic grains in the neuropil of entorhinal cortex, hippocampus, and amygdala. Immunohistochemistry with monoclonal antibodies specific to tau isoforms with four (4R) or three (3R) repeats in the microtubule-binding domain showed immunostaining of grains with 4R, but not 3R, tau antibodies, suggesting that AGD was a 4R tauopathy. The tau isoform composition of AGD was confirmed with densitometric analysis of Western blots of sarkosyl-insoluble tau from the medial temporal lobe of AGD brains with a range of concurrent neurofibrillary pathology and compared with Alzheimer controls. The 4R/3R ratio was 1 or less for Alzheimer disease; the 4R/3R ratio was more than 1 for AGD, decreasing with increasing neurofibrillary pathology and demonstrating that insoluble tau in AGD was enriched in 4R tau. The frequency of the extended tau haplotype was not different in AGD compared to other sporadic 4R tauopathies, progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD). Furthermore, AGD occurred in PSP and CBD more frequently than in dementia controls, including Alzheimer disease. These results suggest that AGD, PSP and CBD are 4R tauopathies that share common pathologic, biochemical, and genetic characteristics.

Aged↗

PubMed2002

Cellular models for tau filament assembly.

Filamentous inclusions made of phosphorylated tau constitute a neuropathologic hallmark of Alzheimer's disease and related disorders. Because abnormal accumulation of tau correlates with the decline in cognitive function, it is conceivable that effective intervention at the early stage of tau filament assembly could impede the progression of these neurodegnerative diseases. Cellular models recapitulating the tau aberration are useful for screening and identifying compounds that are capable of interfering with tau aggregation in a cost-effective manner. To develop such cell culture models, we have established from human neuroglioma [H4] and neuronal [BE(2)-M17D] cells conditional transfectants whose transgenic expression of wild-type or mutant tau via inducible expression mechanisms leads to tau aggregation and filament assembly.

Alzheimer Disease↗