Coexpression of GSK-3beta corrects phenotypic aberrations of dorsal root ganglion cells, cultured from adult transgenic mice overexpressing human protein tau

Interactions between Tau and α-synuclein augment neurotoxicity in a Drosophila model of Parkinson's disease

Clinical and pathological studies have suggested considerable overlap between tauopathies and synucleinopathies. Several genome-wide association studies have identified alpha-Synuclein (SNCA) and Tau (MAPT) polymorphisms as common risk factors for sporadic Parkinson's disease (PD). However, the mechanisms by which subtle variations in the expression of wild-type SNCA and MAPT influence risk for PD and the underlying cellular events that effect neurotoxicity remain unclear. To examine causes of neurotoxicity associated with the α-Syn/Tau interaction, we used the fruit fly as a model. We utilized misexpression paradigms in three different tissues to probe the α-Syn/Tau interaction: the retina, dopaminergic neurons and the larval neuromuscular junction. Misexpression of Tau and α-Syn enhanced a rough eye phenotype and loss of dopaminergic neurons in fly tauopathy and synucleinopathy models, respectively. Our findings suggest that interactions between α-Syn and Tau at the cellular level cause disruption of cytoskeletal organization, axonal transport defects and aberrant synaptic organization that contribute to neuronal dysfunction and death associated with sporadic PD. α-Syn did not alter levels of Tau phosphorylated at the AT8 epitope. However, α-Syn and Tau colocalized in ubiquitin-positive aggregates in eye imaginal discs. The presence of Tau also led to an increase in urea soluble α-Syn. Our findings have important implications in understanding the cellular and molecular mechanisms underlying α-Syn/Tau-mediated synaptic dysfunction, which likely arise in the early asymptomatic phase of sporadic PD.

Tau pathology is present in vivo and develops in vitro in sensory neurons from human P301S tau transgenic mice: a system for screening drugs against tauopathies

Intracellular tau aggregates are the neuropathological hallmark of several neurodegenerative diseases, including Alzheimer's disease, progressive supranuclear palsy, and cases of frontotemporal dementia, but the link between these aggregates and neurodegeneration remains unclear. Neuronal models recapitulating the main features of tau pathology are necessary to investigate the molecular mechanisms of tau malfunction, but current models show little and inconsistent spontaneous tau aggregation. We show that dorsal root ganglion (DRG) neurons in transgenic mice expressing human P301S tau (P301S-htau) develop tau pathology similar to that found in brain and spinal cord and a significant reduction in mechanosensation occurs before detectable fibrillar tau formation. DRG neuronal cultures established from adult P301S-htau mice at different ages retained the pattern of aberrant tau found in vivo. Moreover, htau became progressively hyperphosphorylated over 2 months in vitro beginning with nonsymptomatic neurons, while hyperphosphorylated P301S-htau-positive neurons from 5-month-old mice cultured for 2 months died preferentially. P301S-htau-positive neurons grew aberrant axons, including spheroids, typically found in human tauopathies. Neurons cultured at advanced stages of tau pathology showed a 60% decrease in the fraction of moving mitochondria. SEG28019, a novel O-GlcNAcase inhibitor, reduced steady-state pSer396/pSer404 phosphorylation over 7 weeks in a significant proportion of DRG neurons showing for the first time the possible beneficial effect of prolonged dosing of O-GlcNAcase inhibitor in vitro. Our system is unique in that fibrillar tau forms without external manipulation and provides an important new tool for understanding the mechanisms of tau dysfunction and for screening of compounds for treatment of tauopathies.

Addressing the complex etiology of Alzheimer’s disease: the role of p25/Cdk5

Alzheimer’s disease (AD) is an age-related neurodegenerative disorder characterized by the progressive loss of forebrain neurons and the deterioration of learning and memory. Therapies for AD have primarily focused upon either the inhibition of amyloid synthesis or its deposition in the brain, but clinical testing to date has not yet found an effective amelioration of cognitive symptoms. Synaptic loss closely correlates with the degree of dementia in AD patients. However, mouse AD models that target the amyloid-β pathway generally do not exhibit a profound loss of synapses, despite extensive synaptic dysfunction. The increased generation of p25, an activator of the cyclin-dependent kinase 5 (Cdk5) has been found in both human patients and mouse models of neurodegeneration. The current work reviews our knowledge, to date, on the role of p25/Cdk5 in Alzheimer’s disease, with a focus upon the interaction of amyloid-β and p25/Cdk5 in synaptic dysfunction and neuronal loss.

Yeast as a model system to study tau biology

Hyperphosphorylated and aggregated human protein tau constitutes a hallmark of a multitude of neurodegenerative diseases called tauopathies, exemplified by Alzheimer's disease. In spite of an enormous amount of research performed on tau biology, several crucial questions concerning the mechanisms of tau toxicity remain unanswered. In this paper we will highlight some of the processes involved in tau biology and pathology, focusing on tau phosphorylation and the interplay with oxidative stress. In addition, we will introduce the development of a human tau-expressing yeast model, and discuss some crucial results obtained in this model, highlighting its potential in the elucidation of cellular processes leading to tau toxicity.

Mechanism-based treatments for Alzheimer's disease

Treatment for Alzheimer's disease is entering a new and exciting phase, with several new drugs beginning clinical trials. Many of these new therapies are based on our best current understanding of the pathogenesis of Alzheimer's disease, and are designed to try to either slow or halt the progression of the disease. There are several different theories underlying the current efforts, and these are briefly reviewed. Therapies directed against some aspect of β-amyloid formation, against neurofibrillary tangle formation and against the inflammatory response are all considered, as are the problems associated with each area. It is as yet unclear which, if any, of these approaches will be successful, but the high level of activity in each of these three fields provides some hope that an effective treatment for Alzheimer's disease is on the horizon.

Tau--an inhibitor of deacetylase HDAC6 function

Analysis of brain microtubule protein from patients with Alzheimer's disease showed decreased alpha tubulin levels along with increased acetylation of the alpha tubulin subunit, mainly in those microtubules from neurons containing neurofibrillary tau pathology. To determine the relationship of tau protein and increased tubulin acetylation, we studied the effect of tau on the acetylation-deacetylation of tubulin. Our results indicate that tau binds to the tubulin-deacetylase, histone deacetylase 6 (HDAC6), decreasing its activity with a consequent increase in tubulin acetylation. As expected, increased acetylation was also found in tubulin from wild-type mice compared with tubulin from mice lacking tau because of the tau-mediated inhibition of the deacetylase. In addition, we found that an excess of tau protein, as a HDAC6 inhibitor, prevents induction of autophagy by inhibiting proteasome function.

Abeta, tau and ApoE4 in Alzheimer's disease: the axonal connection

Mutations in amyloid precursor protein (APP), tau and apolipoprotein E4 (ApoE4) lead to Alzheimer's disease (AD) or related pathologies. Pathogenesis and interactions between these pathways have been studied in mouse models. Here, we highlight the fact that axons are important sites of cellular pathology in each pathway and propose that pathway convergence at the molecular level might occur in axons. Recent developments suggest that axonal transport of APP influences beta-amyloid deposition and that tau regulates axonal transport. ApoE4 influences both axonal tau phosphorylation and amyloid-induced neurite pathology. Thus, a better understanding of axonal events in AD might help connect the pathogenic mechanisms of beta-amyloid, ApoE4 and tau, indicating the most important steps for therapeutic targeting.

Molecular pathways that influence human tau-induced pathology in Caenorhabditis elegans

Mutations in the gene encoding tau cause frontotemporal dementia with parkinsonism--chromosome 17 type (FTDP-17). In FTDP-17, Alzheimer's disease, and other tauopathies, aggregated hyper-phosphorylated tau forms the neurofibrillary tangles characteristic of these disorders. We previously reported a Caenorhabditis elegans model for tauopathies using human normal and FTDP-17 mutant tau as transgenes. Neuronal transgene expression caused insoluble phosphorylated tau accumulation, neurodegeneration and uncoordinated (Unc) movement. Here we describe a genome-wide RNA-mediated interference (RNAi) screen for genes that modify the tau-induced Unc phenotype. We tested RNAi sequences for 16,757 genes and found 75 that enhanced the transgene-induced Unc phenotype. Forty-six of these genes have sequence similarity to known human genes and fall into a number of broad classes including kinases, chaperones, proteases and phosphatases. The remaining 29 modifiers have sequence similarity only with other nematode genes. To determine if the enhancers are specific for the tau-induced Unc behavior, we exposed several non-tau Unc mutants to tau RNAi enhancer clones. Fifteen enhancers modified phenotypes in multiple Unc mutants, whereas 60 modified only the Unc phenotype in the tau transgenic lines. We also introduced the tau transgene into the background of genetic loss-of-function mutations for a subset of the enhancer genes. Tau transgenic animals homozygous for loss of these enhancer genes exhibited increased impaired motility relative to the tau transgene line alone. This work uncovers novel candidate genes that prevent tau toxicity, as well as genes previously implicated in tau-mediated neurodegeneration.

Overexpression of tau results in defective synaptic transmission in Drosophila neuromuscular junctions

Synaptic dysfunction is believed to be an early pathological change in neurodegenerative diseases and may cause the earliest clinical symptoms. We have used Drosophila to model a tauopathy in order to analyse the earliest neuronal and synaptic dysfunction. Our work has shown that overexpression of human tau (0N3R) in larval motor neurons causes a disruption of axonal transport and a morphological and functional disruption of NMJs (neuromuscular junctions). Tau-expressing NMJs are smaller with an abnormal structure. Despite abnormal morphology, tau-expressing NMJs retain synaptotagmin expression and can form active zones. Tau-expressing NMJs are functionally abnormal and exhibit disrupted vesicle cycling and synaptic transmission. At low-frequency stimulation (1 Hz), ESPs (evoked synaptic potentials) produced by tau-expressing motor neurons were indistinguishable from wild-type; however, following high-frequency stimulation (50 Hz), ESPs from tau-expressing NMJs were significantly decreased in amplitude. To investigate the mechanism underlying the change in ESPs, we analysed the relative numbers and distribution of mitochondria. This revealed that motor neurons expressing tau had a significant reduction in the number of detectable mitochondria in the pre-synaptic terminal. Our results demonstrate that tau overexpression results in synaptic dysfunction, associated with a reduced complement of functional mitochondria. These findings suggest that disruption of axonal transport and synaptic transmission may be key components of the pathogenic mechanism that underlie neuronal dysfunction in the early stages of tauopathies.

Identification and isolation of a hyperphosphorylated, conformationally changed intermediate of human protein tau expressed in yeast

Hyperphosphorylation and aggregation of protein tau are typical for neurodegenerative tauopathies, including Alzheimer's disease (AD). We demonstrate here that human tau expressed in yeast acquired pathological phosphoepitopes, assumed a pathological conformation, and formed aggregates. These processes were modulated by yeast kinases Mds1 and Pho85, orthologues of GSK-3beta and cdk5, respectively. Surprisingly, inactivation of Pho85 increased phosphorylation of tau-4R, concomitant with increased conformational change defined by antibody MC1 and a 40-fold increase in aggregation. Soluble protein tau, purified from yeast lacking PHO85, spontaneously and rapidly formed tau filaments in vitro. Further fractionation of tau by anion-exchange chromatography yielded a hyperphosphorylated monomeric subfraction, termed hP-tau/MC1, with slow electrophoretic mobility and enriched with all major epitopes, including MC1. Isolated hP-tau/MC1 vastly accelerated in vitro aggregation of wild-type tau-4R, demonstrating its functional capacity to initiate aggregation, as well as its structural stability. Combined, this novel yeast model recapitulates hyperphosphorylation, conformation, and aggregation of protein tau, provides insight in molecular changes crucial in tauopathies, offers a source for isolation of modified protein tau, and has potential for identification of modulating compounds and genes.

Cdk5 inhibits anterograde axonal transport of neurofilaments but not that of tau by inhibition of mitogen-activated protein kinase activity

Cyclin-dependent kinase 5 (cdk5) inhibits neurofilament (NF) anterograde axonal transport while p42/44 mitogen-activated protein kinase (MAPk) promotes it. Since cdk5 is known to inhibit MAP kinase activity, we examined whether or not cdk5 inhibits anterograde NF transport via inhibition of MAPk activity. To accomplish this, we manipulated the activity of these kinases in differentiated NB2a/d1 cells, and monitored anterograde axonal transport of green fluorescent protein-conjugated-NF-M (GFP-M) and cyan fluorescent protein-conjugated (CFP)-tau. The cdk5 inhibitor roscovitine increased anterograde axonal transport of GFP-M and CFP-tau; transfection with cdk5/p25 inhibited transport of both. Inhibition of MAPk activity by PD98059 or expression of dominant-negative MAPk inhibited anterograde GFP-M transport, while expression of constitutively active MAPk enhanced it; these treatments did not affect CFP-tau transport. PD98059 prevented roscovitine-mediated enhancement of GFP-M transport, but did not prevent enhancement of CFP-tau transport. Co-transfection with constitutively activated MAPk prevented the inhibition of GFP-M transport that normally accompanied transfection with cdk5/p25, but did not prevent inhibition of tau transport by cdk5/p25. Finally, the extent of inhibition of GFP-M axonal transport by PD98059 was not additive to that derived from transfection with cdk5/p35, and the increase in NF transport that accompanies roscovitine treatment was not additive to that derived from transfection with constitutively activated MAPk, suggesting that the influence of these kinases on NF transport was within the same, rather than distinct, pathways. These findings suggest that axonal transport of tau and NFs is under the control of distinct kinase cascades, and that cdk5 inhibits NF transport at least in part by inhibiting MAPk.

Tau phosphorylation: physiological and pathological consequences

The microtubule-associated protein tau, abundant in neurons, has gained notoriety due to the fact that it is deposited in cells as fibrillar lesions in numerous neurodegenerative diseases, and most notably Alzheimer's disease. Regulation of microtubule dynamics is the most well-recognized function of tau, but it is becoming increasingly evident that tau plays additional roles in the cell. The functions of tau are regulated by site-specific phosphorylation events, which if dysregulated, as they are in the disease state, result in tau dysfunction and mislocalization, which is potentially followed by tau polymerization, neuronal dysfunction and death. Given the increasing evidence that a disruption in the normal phosphorylation state of tau plays a key role in the pathogenic events that occur in Alzheimer's disease and other neurodegenerative conditions, it is of crucial importance that the protein kinases and phosphatases that regulate tau phosphorylation in vivo as well as the signaling cascades that regulate them be identified. This review focuses on recent literature pertaining to the regulation of tau phosphorylation and function in cell culture and animal model systems, and the role that a dysregulation of tau phosphorylation may play in the neuronal dysfunction and death that occur in neurodegenerative diseases that have tau pathology.

Tau alteration and neuronal degeneration in tauopathies: mechanisms and models

Tau becomes characteristically altered both functionally and structurally in several neurodegenerative diseases now collectively called tauopathies. Although increasing evidence supports that alterations of tau may directly cause neuronal degeneration and cell death, the mechanisms, which render tau to become a toxic agent are still unclear. In addition, it is obscure, whether neurodegeneration in tauopathies occurs via a common mechanism or specific differences exist. The aim of this review is to provide an overview about the different experimental models that currently exist, how they are used to determine the role of tau during degeneration and what has been learnt from them concerning the mechanistic role of tau in the disease process. The review begins with a discussion about similarities and differences in tau alteration in paradigmatic tauopathies such as frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) and Alzheimer's disease (AD). The second part concentrates on major experimental models that have been used to address the mechanistic role of tau during degeneration. This will include a discussion of cell-free assays, culture models using cell lines or dissociated neurons, and animal models. How these models aid to understand (i) alterations in the function of tau as a microtubule-associated protein (MAP), (ii) direct cytotoxicity of altered tau protein, and (iii) the potential role of tau aggregation in neurodegenerative processes will be the central theme of this part. The review ends with concluding remarks about a general mechanistic model of the role of tau alteration and neuronal degeneration in tauopathies and future perspectives.

In vitro activation of murine DRG neurons by CGRP-mediated mucosal mast cell degranulation

Upregulation of CGRP-immunoreactive (IR) primary afferent nerve fibers accompanied by mastocytosis is characteristic for the Schistosoma mansoni-infected murine ileum. These mucosal mast cells (MMC) and CGRP-IR fibers, which originate from dorsal root (DRG) and nodose ganglia, are found in close apposition. We examined interactions between primary cultured MMC and CGRP-IR DRG neurons in vitro by confocal recording of intracellular Ca(2+) concentration ([Ca(2+)](i)). The degranulatory EC(50) for the mast cell secretagogue compound 48/80 (C48/80; 10 microg/ml) and the neuropeptides CGRP (2.10(-8) M) and substance P (SP; 3.10(-8) M) were determined by measurement of extracellular release of the granule chymase, mouse mast cell protease-1. Application of C48/80 (10 microg/ml) and CGRP and SP (both 10(-7) M) to Fluo-4-loaded MMC induced a transient rise in [Ca(2+)](i) after a lag time, indicative of mast cell degranulation and/or secretion. The CGRP response could be completely blocked by pertussis toxin (2 microg/ml), indicating involvement of G(i) proteins. Application of MMC juice, obtained by C48/80 degranulation of MMC, to Fluo-4-loaded DRG neurons induced in all neurons a rise in [Ca(2+)](i), indicative of activation. Degranulation of MMC by C48/80 in culture dishes containing Fluo-4-loaded DRG neurons also caused activation of the DRG neurons. In conclusion, these results demonstrate a bidirectional cross-talk between cultured MMC and CGRP-IR DRG neurons in vitro. This indicates that such a communication may be the functional relevance for the close apposition between MMC and CGRP-IR nerve fibers in vivo.

Lithium, a common drug for bipolar disorder treatment, regulates amyloid-beta precursor protein processing

Lithium is one of the most widely used mood-stabilizing agents for the treatment of bipolar disorder. Although the underlying mechanism(s) of this mood stabilizer remains controversial, recent evidence linking lithium to neurotrophic/neuroprotective effects (Choi and Sung (2000) 1475, 225-230; Davies et al. (2000) 351, 95-105) suggests novel benefits of this drug in addition to mood stabilization. Here, we report that both lithium as well as valproic acid (VPA) inhibit beta-amyloid peptide (Abeta) production in HEK293 cells stably transfected with Swedish amyloid precursor protein (APP)(751) and in the brains of the PDAPP (APP(V717F)) Alzheimer's disease transgenic mouse model at clinically relevant plasma concentrations. Both lithium and VPA are known to be glycogen synthase kinase-3 (GSK3) inhibitors. Our studies reveal that GSK3beta is a potential downstream kinase, which modulates APP processing because inhibition of GSK3 activity by either a dominant negative GSK3beta kinase-deficient construct or GSK3beta antisense oligonucleotide mimics lithium and VPA effects. Moreover, lithium treatment abolished GSK3beta-mediated Abeta increase in the brains of GSK3beta transgenics and reduced plaque burden in the brains of the PDAPP (APP(V717F)) transgenic mice.

GSK-3beta inhibition reverses axonal transport defects and behavioural phenotypes in Drosophila

The tauopathies are a group of disorders characterised by aggregation of the microtubule-associated protein tau and include Alzheimer's disease (AD) and the fronto-temporal dementias (FTD). We have used Drosophila to analyse how tau abnormalities cause neurodegeneration. By selectively co-expressing wild-type human tau (0N3R isoform) and a GFP vesicle marker in motorneurons, we examined the consequences of tau overexpression on axonal transport in vivo. The results show that overexpression of tau disrupts axonal transport causing vesicle aggregation and this is associated with loss of locomotor function. All these effects occur without neuron death. Co-expression of constitutively active glycogen-synthase kinase-3beta (GSK-3beta) enhances and two GSK-3beta inhibitors, lithium and AR-A014418, reverse both the axon transport and locomotor phenotypes, suggesting that the pathological effects of tau are phosphorylation dependent. These data show that tau abnormalities significantly disrupt neuronal function, in a phosphorylation-dependent manner, before the classical pathological hallmarks are evident and also suggest that the inhibition of GSK-3beta might have potential therapeutic benefits in tauopathies.

Role of tau phosphorylation by glycogen synthase kinase-3beta in the regulation of organelle transport

Anterograde organelle transport is known to be inhibited by overexpression of the microtubule-associated protein tau in cultured cells. However, the molecular mechanism regulating this function of tau protein has not previously been understood. We found that in PC12 cells treated with NGF or fibroblast growth factor-2, glycogen synthase kinase-3beta and tau were upregulated simultaneously from around day 2 of differentiation, with increasing glycogen synthase kinase-3-mediated tau phosphorylation. This phosphorylation did not alter tau's ability to bind to microtubules but appeared to be required for the maintenance of the anterograde organelle transport in differentiated cells. Lithium, alsterpaullone or valproate, three independent glycogen synthase kinase-3 inhibitors, but not butyrolactone 1, an inhibitor of cyclin-dependent protein kinases, induced mitochondrial clustering in association with tau dephosphorylation. In CHO cells transfected with human tau(441), mitochondrial clustering was found in cells in which tau was unphosphorylated. These findings raise the possibility that the phosphorylation of tau by glycogen synthase kinase-3 might be involved in the regulation of organelle transport.

Primed phosphorylation of tau at Thr231 by glycogen synthase kinase 3beta (GSK3beta) plays a critical role in regulating tau's ability to bind and stabilize microtubules

Site-specific phosphorylation of tau negatively regulates its ability to bind and stabilize microtubule structure. Although tau is a substrate of glycogen synthase kinase 3beta (GSK3beta), the exact sites on tau that are phosphorylated by this kinase in situ have not yet been established, and the effect of these phosphorylation events on tau-microtubule interactions have not been fully elucidated. GSK3beta phosphorylates both primed and unprimed sites on tau, but only primed phosphorylation events significantly decrease the ability of tau to bind microtubules. The focus of the present study is on determining the importance of the GSK3beta-mediated phosphorylation of a specific primed site, Thr231, in regulating tau's function. Pre-phosphorylation of Ser235 primes tau for phosphorylation by GSK3beta at Thr231. Phosphorylation by GSK3beta of wild-type tau or tau with Ser235 mutated to Ala decreases tau-microtubule interactions. However, when Thr231 alone or Thr231 and Ser235 in tau were mutated to Ala, phosphorylation by GSK3beta did not decrease the association of tau with the cytoskeleton. Further, T231A tau was still able to efficiently bind microtubules after phosphorylation by GSK3beta. Expression of each tau construct alone increased tubulin acetylation, a marker of microtubule stability. However, when cells were cotransfected with wild-type tau and GSK3beta, the level of tubulin acetylation was decreased to vector-transfected levels. In contrast, coexpression of GSK3beta with mutated tau (T231A/S235A) did not significantly decrease the levels of acetylated tubulin. These results strongly indicate that phosphorylation of Thr231 in tau by GSK3beta plays a critical role in regulating tau's ability to bind and stabilize microtubules.