RAS-MAPK-MSK1 pathway modulates ataxin 1 protein levels and toxicity in SCA1.

Many neurodegenerative disorders, such as Alzheimer's, Parkinson's and polyglutamine diseases, share a common pathogenic mechanism: the abnormal accumulation of disease-causing proteins, due to either the mutant protein's resistance to degradation or overexpression of the wild-type protein. We have developed a strategy to identify therapeutic entry points for such neurodegenerative disorders by screening for genetic networks that influence the levels of disease-driving proteins. We applied this approach, which integrates parallel cell-based and Drosophila genetic screens, to spinocerebellar ataxia type 1 (SCA1), a disease caused by expansion of a polyglutamine tract in ataxin 1 (ATXN1). Our approach revealed that downregulation of several components of the RAS-MAPK-MSK1 pathway decreases ATXN1 levels and suppresses neurodegeneration in Drosophila and mice. Importantly, pharmacological inhibitors of components of this pathway also decrease ATXN1 levels, suggesting that these components represent new therapeutic targets in mitigating SCA1. Collectively, these data reveal new therapeutic entry points for SCA1 and provide a proof-of-principle for tackling other classes of intractable neurodegenerative diseases.

Reduction of protein kinase A-mediated phosphorylation of ATXN1-S776 in Purkinje cells delays onset of Ataxia in a SCA1 mouse model.

Spinocerebellar ataxia type 1 (SCA1) is a polyglutamine (polyQ) repeat neurodegenerative disease in which a primary site of pathogenesis are cerebellar Purkinje cells. In addition to polyQ expansion of ataxin-1 protein (ATXN1), phosphorylation of ATXN1 at the serine 776 residue (ATXN1-pS776) plays a significant role in protein toxicity. Utilizing a biochemical approach, pharmacological agents and cell-based assays, including SCA1 patient iPSC-derived neurons, we examine the role of Protein Kinase A (PKA) as an effector of ATXN1-S776 phosphorylation. We further examine the implications of PKA-mediated phosphorylation at ATXN1-S776 on SCA1 through genetic manipulation of the PKA catalytic subunit Cα in Pcp2-ATXN1[82Q] mice. Here we show that pharmacologic inhibition of S776 phosphorylation in transfected cells and SCA1 patient iPSC-derived neuronal cells lead to a decrease in ATXN1. In vivo, reduction of PKA-mediated ATXN1-pS776 results in enhanced degradation of ATXN1 and improved cerebellar-dependent motor performance. These results provide evidence that PKA is a biologically important kinase for ATXN1-pS776 in cerebellar Purkinje cells.

Mechanisms of tunneling nanotube-based propagation of neurodegenerative disease proteins.

Tunneling nanotubes (TNTs), intercellular connections enriched with F-actin, were first identified as a viable means of cellular communication and organelle transport in animal cells at the early part of this century. Within the last 10 years, these microscopic and highly dynamic protrusions have been implicated in neurodegenerative disease propagation and pathogenesis. A host of aggregation-prone protein inclusions, including those containing alpha-synuclein, tau, prions and others, hijack this communication mechanism in both neurons and astrocytes. The exact cellular mechanisms underlying TNT-based propagation remain largely unknown, however, common practices can be identified. First, selective expression of the aggregation-prone form of proteins increases TNT density; next, endo-lysosomal pathways appear to support the loading and unloading of protein onto the TNT; and finally, TNT assembly results in the spontaneous formation of aggregation-prone protein inclusions in "acceptor" cells, indicating that TNTs are involved in not only the transport of inclusions but also in the seeding of new inclusions in naïve cells. These observations have implications for the spreading of neurodegenerative disease in the central nervous system and the consequent progression of symptoms. Here, I will summarize the empirical evidence of TNT-based aggregation-prone protein propagation to date, and propose an inclusive model of aggregate inclusion propagation along TNTs.

Intercellular Propagation and Aggregate Seeding of Mutant Ataxin-1.

Intercellular propagation of aggregated protein inclusions along actin-based tunneling nanotubes (TNTs) has been reported as a means of pathogenic spread in Alzheimer's, Parkinson's, and Huntington's diseases. Propagation of oligomeric-structured polyglutamine-expanded ataxin-1 (Atxn1[154Q]) has been reported in the cerebellum of a Spinocerebellar ataxia type 1 (SCA1) knock-in mouse to correlate with disease propagation. In this study, we investigated whether a physiologically relevant polyglutamine-expanded ATXN1 protein (ATXN1[82Q]) could propagate intercellularly. Using a cerebellar-derived live cell model, we observed ATXN1 aggregates form in the nucleus, subsequently form in the cytoplasm, and finally, propagate to neighboring cells along actin-based intercellular connections. Additionally, we observed the facilitation of aggregate-resistant proteins into aggregates given the presence of aggregation-prone proteins within cells. Taken together, our results support a pathogenic role of intercellular propagation of polyglutamine-expanded ATXN1 inclusions.

Degradation of Bone Quality in a Transgenic Mouse Model of Alzheimer's Disease.

Alzheimer's disease (AD) patients present with symptoms such as impairment of insulin signaling, chronic inflammation, and oxidative stress. Furthermore, there are comorbidities associated with AD progression. For example, osteoporosis is common with AD wherein patients exhibit reduced mineralization and a risk for fragility fractures. However, there is a lack of understanding on the effects of AD on bone beyond loss of bone density. To this end, we investigated the effects of AD on bone quality using the 5XFAD transgenic mouse model in which 12-month-old 5XFAD mice showed accumulation of amyloid-beta (Aß42) compared with wild-type (WT) littermates (n = 10/group; 50% female, 50% male). Here, we observed changes in cortical bone but not in cancellous bone quality. Both bone mass and bone quality, measured in femoral samples using imaging (micro-CT, confocal Raman spectroscopy, X-ray diffraction [XRD]), mechanical (fracture tests), and chemical analyses (biochemical assays), were altered in the 5XFAD mice compared with WT. Micro-CT results showed 5XFAD mice had lower volumetric bone mineral density (BMD) and increased endocortical bone loss. XRD results showed decreased mineralization with smaller mineral crystals. Bone matrix compositional properties, from Raman, showed decreased crystallinity along with higher accumulation of glycoxidation products and glycation products, measured biochemically. 5XFAD mice also demonstrated loss of initiation and maximum toughness. We observed that carboxymethyl-lysine (CML) and mineralization correlated with initiation toughness, whereas crystal size and pentosidine (PEN) correlated with maximum toughness, suggesting bone matrix changes predominated by advanced glycation end products (AGEs) and altered/poor mineral quality explained loss of fracture toughness. Our findings highlight two pathways to skeletal fragility in AD through alteration of bone quality: (i) accumulation of AGEs; and (ii) loss of crystallinity, decreased crystal size, and loss of mineralization. We observed that the accumulation of amyloidosis in brain correlated with an increase in several AGEs, consistent with a mechanistic link between elevated Aß42 levels in the brain and AGE accumulation in bone. © 2022 American Society for Bone and Mineral Research (ASBMR).

Treating Neurodegenerative Disease with Antioxidants: Efficacy of the Bioactive Phenol Resveratrol and Mitochondrial-Targeted MitoQ and SkQ.

Growing evidence from neurodegenerative disease research supports an early pathogenic role for mitochondrial dysfunction in affected neurons that precedes morphological and functional deficits. The resulting oxidative stress and respiratory malfunction contribute to neuronal toxicity and may enhance the vulnerability of neurons to continued assault by aggregation-prone proteins. Consequently, targeting mitochondria with antioxidant therapy may be a non-invasive, inexpensive, and viable means of strengthening neuronal health and slowing disease progression, thereby extending quality of life. We review the preclinical and clinical findings available to date of the natural bioactive phenol resveratrol and two synthetic mitochondrial-targeted antioxidants, MitoQ and SkQ.

Robust light-dark patterns and reduced amyloid load in an Alzheimer's disease transgenic mouse model.

Circadian disruption resulting from exposure to irregular light-dark patterns and sleep deprivation has been associated with beta amyloid peptide (Aß) aggregation, which is a major event in Alzheimer's disease (AD) pathology. We exposed 5XFAD mice and littermate controls to dim-light vs. bright-light photophases to investigate the effects of altering photophase strength on AD-associated differences in cortical Aß42 levels, wheel-running activity, and circadian free-running period (tauDD). We found that increasing light levels significantly reduced cortical Aß42 accumulation and activity levels during the light phase of the light:dark cycle, the latter being consistent with decreased sleep fragmentation and increased sleep duration for mice exposed to the more robust light-dark pattern. No significant changes were observed for tauDD. Our results are consistent with circadian pacemaker period being relatively unaffected by Aß pathology in AD, and with reductions in cortical Aß loads in AD through tailored lighting interventions.

The effect of succinic acid on the metabolic profile in high-fat diet-induced obesity and insulin resistance.

Obesity, insulin resistance, and poor metabolic profile are hallmarks of a high-fat diet (HFD), highlighting the need to understand underlying mechanisms. Therefore, we sought to determine the effect of succinic acid (SA) on metabolism in high-fat diet (HFD)-induced obesity. Animals were randomly assigned to either low-fat diet (LFD) or a high-fat diet (HFD). Mice consumed their respective diets for 4.5 months and then assigned to the following groups: (LFD)+vehicle, LFD + SA (0.75 mg/ml), HFD + vehicle, or HFD + SA. Body weight (BW), food, and water intake, were tracked weekly. After 6 weeks, insulin, glucose, and pyruvate tolerance tests were completed, and spontaneous physical activity was assessed. Epididymal white adipose tissue (EWAT) mass and in vitro measurements of oxidative skeletal muscle (soleus) respiration were obtained. Expectedly, the HFD increased BW and EWAT mass, and reduced glucose and insulin tolerance. SA significantly reduced EWAT mass, more so in HFD (p < .05), but had no effect on any in vivo measurements (BW, insulin, glucose, or pyruvate tolerance, nor physical activity, all p > .05). A significant (p < .05) interaction was observed between mitochondrial respiration and treatment, where SA increased respiration, likely owed to greater mitochondrial content, as assessed by complex IV activity in both LFD and HFD. In HFD-induced obesity, coupled with insulin desensitization, we found no favorable effect of succinic acid on glucose regulation, though adiposity was attenuated. In oxidative skeletal muscle, there was a tendency for increased respiratory capacity, likely owed to greater mitochondrial content, suggestive of a succinic acid-induced mitochondrial biogenesis.

Active c-jun N-terminal kinase induces caspase cleavage of tau and additional phosphorylation by GSK-3beta is required for tau aggregation.

Neurofibrillary tangles (NFTs), comprising human intracellular microtubule-associated protein tau, are one of the hallmarks of tauopathies, including Alzheimer's disease. Recently, a report that caspase-cleaved tau is present in NFTs has led to the hypothesis that the mechanisms underlying NFT formation may involve the apoptosis cascade. Here, we show that adenoviral infection of tau into COS-7 cells induces activation of c-jun N-terminal kinase (JNK), followed by excessive phosphorylation of tau and its cleavage by caspase. However, JNK activation alone was insufficient to induce sodium dodecyl sulfate (SDS)-insoluble tau aggregation and additional phosphorylation by GSK-3beta was required. In SH-SY5Y neuroblastoma cells, overexpression of active JNK and GSK-3beta increased caspase-3 activation and cytotoxicity more than overexpression of tau alone. Taken together, these results indicate that, although JNK activation may be a primary inducing factor, further phosphorylation of tau is required for neuronal death and NFT formation in neurodegenerative diseases, including those characterized by tauopathy.

Treating SCA1 Mice with Water-Soluble Compounds to Non-Specifically Boost Mitochondrial Function.

Mitochondrial dysfunction plays a significant role in the aging process and in neurodegenerative diseases including several hereditary spinocerebellar ataxias and other movement disorders marked by progressive degeneration of the cerebellum. The goal of this protocol is to assess mitochondrial dysfunction in Spinocerebellar ataxia type 1 (SCA1) and assess the efficacy of pharmacological targeting of metabolic respiration via the water-soluble compound succinic acid to slow disease progression. This approach is applicable to other cerebellar diseases and can be adapted to a host of water-soluble therapies. Ex vivo analysis of mitochondrial respiration is used to detect and quantify disease-related changes in mitochondrial function. With genetic evidence (unpublished data) and proteomic evidence of mitochondrial dysfunction in the SCA1 mouse model, we evaluate the efficacy of treatment with the water-soluble metabolic booster succinic acid by dissolving this compound directly into the home cage drinking water. The ability of the drug to pass the blood brain barrier can be deduced using high performance liquid chromatography (HPLC). The efficacy of these compounds can then be tested using multiple behavioral paradigms including the accelerating rotarod, balance beam test and footprint analysis. Cytoarchitectural integrity of the cerebellum can be assessed using immunofluorescence assays that detect Purkinje cell nuclei and Purkinje cell dendrites and soma. These methods are robust techniques for determining mitochondrial dysfunction and the efficacy of treatment with water-soluble compounds in cerebellar neurodegenerative disease.

Short-term succinic acid treatment mitigates cerebellar mitochondrial OXPHOS dysfunction, neurodegeneration and ataxia in a Purkinje-specific spinocerebellar ataxia type 1 (SCA1) mouse model.

Mitochondrial dysfunction plays a significant role in neurodegenerative disease including ataxias and other movement disorders, particularly those marked by progressive degeneration in the cerebellum. In this study, we investigate the role of mitochondrial oxidative phosphorylation (OXPHOS) deficits in cerebellar tissue of a Purkinje cell-driven spinocerebellar ataxia type 1 (SCA1) mouse. Using RNA sequencing transcriptomics, OXPHOS complex assembly analysis and oxygen consumption assays, we report that in the presence of mutant polyglutamine-expanded ataxin-1, SCA1 mice display deficits in cerebellar OXPHOS complex I (NADH-coenzyme Q oxidoreductase). Complex I genes are upregulated at the time of symptom onset and upregulation persists into late stage disease; yet, functional assembly of complex I macromolecules are diminished and oxygen respiration through complex I is reduced. Acute treatment of postsymptomatic SCA1 mice with succinic acid, a complex II (succinate dehydrogenase) electron donor to bypass complex I dysfunction, ameliorated cerebellar OXPHOS dysfunction, reduced cerebellar pathology and improved motor behavior. Thus, exploration of mitochondrial dysfunction and its role in neurodegenerative ataxias, and warrants further investigation.

Formation of phospho-SAPK/JNK granules in the hippocampus is an early event in Alzheimer disease.

The mitogen-activated protein (MAP) kinase SAPK/JNK phosphorylates tau protein at many of its proline-directed serine/threonine residues in vitro and is a likely candidate kinase to phosphorylate the pathologically relevant S422 site on tau. Since phosphorylation of tau, particularly at S422, is a relatively early marker of AD and seems to precede tangle formation, it appears likely that an early form of activated SAPK/JNK might be detected by immunohistochemical means around the time that tau begins to aggregate into tangles. We report here that an antibody to phospho-SAPK/JNK (p-SAPK/JNK) reacts with several types of lesions including granular bodies in limbic areas; NFTs in limbic cortex and temporal neocortex; occasional neuritic plaques in temporal neocortex; and select axons in the hippocampus, entorhinal cortex, and inferior temporal cortex. In order to characterize the appearance of granular p-SAPK/JNK and determine if it appears early in disease, we employed an immunohistochemical study of postmortem limbic tissue from 20 cases ranging from Braak stages I-VI. By co-staining with anti-tau antibodies specific to different molecular events that occur during tangle evolution, we were able to identify the appearance of p-SAPK/JNK in early Braak stages with an increased elevation during the limbic stages of AD and during the early stages of the formation of individual hippocampal tangles.

Expansion, mosaicism and interruption: mechanisms of the CAG repeat mutation in spinocerebellar ataxia type 1.

Spinocerebellar ataxia type 1 (SCA1) is an autosomal dominant neurodegenerative disorder that primarily affects the cerebellum and brainstem. The genetic mutation is an expansion of CAG trinucleotide repeats within the coding region of the ataxin-1 gene, characterizing SCA1 as a polyglutamine expansion disease like Huntington's. As with most polyglutamine expansion diseases, SCA1 follows the rules of genetic anticipation: the larger the expansion, the earlier and more rapid the symptoms. Unlike the majority of polyglutamine expansion diseases, the presence of histidine interruptions within the polyglutamine tract of ataxin-1 protein can prevent or mitigate disease. The present review aims to synthesize three decades of research on the ataxin-1 polyglutamine expansion mutation that causes SCA1. Data from genetic population studies and case studies is gathered along with data from manipulation studies in animal models. Specifically, we examine the molecular mechanisms that cause tract expansions and contractions, the molecular pathways that confer instability of tract length in gametic and somatic cells resulting in gametic and somatic mosaicism, the influence of maternal or paternal factors in inheritance of the expanded allele, and the effects of CAT/histidine interruptions to the ataxin-1 allele and protein product. Our review of existing data supports the following conclusions. First, polyCAG expansion of gametic alleles occur due to the failure of gap repair mechanisms for single or double strand breaks during the transition from an immature haploid spermatid to a mature haploid sperm cell. Equivalent failures were not detected in female gametic cells. Second, polyCAG expansion of somatic alleles occur due to hairpins formed on Okazaki fragments and slipped strand structures due to failures in mismatch repair and transcription-coupled nucleotide excision repair mechanisms. Third, CAT trinucleotide interruptions, which code for histidines in the translated protein, attenuate the formation of slipped strand structures which may protect the allele from the occurrence of large expansions. Many of the mechanisms of expansion identified in this review differ from those noted in Huntington's disease indicating that gene -or sequence-specific factors may affect the behavior of the polyCAG/glutamine tract. Therefore, synthesis and review of research from the SCA1 field is valuable for future clinical and diagnostic work in the treatment and prevention of SCA1.

Inhibition of ligand binding to G protein-coupled receptors by arachidonic acid.

Arachidonic acid (AA), released in response to muscarinic acetylcholine receptor (mAChR) stimulation, previously has been reported to function as a reversible feedback inhibitor of the mAChR. To determine if the effects of AA on binding to the mAChR are subtype specific and whether AA inhibits ligand binding to other G protein-coupled receptors (GPCRs), the effects of AA on ligand binding to the mAChR subtypes (M1, M2, M3, M4, and M5) and to the micro-opioid receptor, beta2-adrenergic receptor (beta2-AR), 5-hydroxytryptamine receptor (5-HTR), and nicotinic receptors were examined. AA was found to inhibit ligand binding to all mAChR subtypes, to the beta2-AR, the 5-HTR, and to the micro-opioid receptor. However, AA does not inhibit ligand binding to the nicotinic receptor, even at high concentrations of AA. Thus, AA inhibits several types of GPCRs, with 50% inhibition occurring at 3-25 MuM, whereas the nicotinic receptor, a non-GPCR, remains unaffected. Further research is needed to determine the mechanism by which AA inhibits GPCR function.

Stabilization and Degradation Mechanisms of Cytoplasmic Ataxin-1.

Aggregation-prone proteins in neurodegenerative disease disrupt cellular protein stabilization and degradation pathways. The neurodegenerative disease spinocerebellar ataxia type 1 (SCA1) is caused by a coding polyglutamine expansion in the Ataxin-1 gene (ATXN1), which gives rise to the aggregation-prone mutant form of ATXN1 protein. Cerebellar Purkinje neurons, preferentially vulnerable in SCA1, produce ATXN1 protein in both cytoplasmic and nuclear compartments. Cytoplasmic stabilization of ATXN1 by phosphorylation and 14-3-3-mediated mechanisms ultimately drive translocation of the protein to the nucleus where aggregation may occur. However, experimental inhibition of phosphorylation and 14-3-3 binding results in rapid degradation of ATXN1, thus preventing nuclear translocation and cellular toxicity. The exact mechanism of cytoplasmic ATXN1 degradation is currently unknown; further investigation of degradation may provide future therapeutic targets. This review examines the present understanding of cytoplasmic ATXN1 stabilization and potential degradation mechanisms during normal and pathogenic states.

Working memory and executive function decline across normal aging, mild cognitive impairment, and Alzheimer's disease.

Alzheimer's disease (AD) is a progressive neurodegenerative disease marked by deficits in episodic memory, working memory (WM), and executive function. Examples of executive dysfunction in AD include poor selective and divided attention, failed inhibition of interfering stimuli, and poor manipulation skills. Although episodic deficits during disease progression have been widely studied and are the benchmark of a probable AD diagnosis, more recent research has investigated WM and executive function decline during mild cognitive impairment (MCI), also referred to as the preclinical stage of AD. MCI is a critical period during which cognitive restructuring and neuroplasticity such as compensation still occur; therefore, cognitive therapies could have a beneficial effect on decreasing the likelihood of AD progression during MCI. Monitoring performance on working memory and executive function tasks to track cognitive function may signal progression from normal cognition to MCI to AD. The present review tracks WM decline through normal aging, MCI, and AD to highlight the behavioral and neurological differences that distinguish these three stages in an effort to guide future research on MCI diagnosis, cognitive therapy, and AD prevention.

Regulation of ataxin-1 phosphorylation and its impact on biology.

Ataxin-1 protein expression is found in the cytoplasm and nucleus of Purkinje cells, the primary site of spinocerebellar ataxia type 1 (SCA1). Phosphorylation at S776 occurs in the cytoplasm and stabilizes the protein through interaction with 14-3-3, allowing it to translocate into the nucleus where disease is initiated. Phosphorylation and stabilization are enhanced when the polyglutamine expansion is present. In this chapter, we present a model of neurodegeneration in SCA1 initiated through phosphorylation at S776 by cAMP-dependent protein kinase (PKA) and enhanced by the presence of the polyglutamine expansion. The biological methods used to uncover SCA1 pathogenesis and phosphorylation at S776 are described.

Staging of Alzheimer's pathology in triple transgenic mice: a light and electron microscopic analysis.

The age-related pathological cascade underlying intraneuronal tau formation in 3xTg-AD mice, which harbor the human APP(Swe), PS1(M126V) , and Tau(P301L) gene mutations, remains unclear. At 3 weeks of age, AT180, Alz50, MC1, AT8, and PHF-1 intraneuronal immunoreactivity appeared in the amygdala and hippocampus and at later ages in the cortex of 3xTg-AD mice. AT8 and PHF-1 staining was fixation dependent in young mutant mice. 6E10 staining was seen at all ages. Fluorescent immunomicroscopy revealed CA1 neurons dual stained for 6E10 and Alz50 and single Alz50 immunoreactive neurons in the subiculum at 3 weeks and continuing to 20 months. Although electron microscopy confirmed intraneuronal cytoplasmic Alz50, AT8, and 6E10 reaction product in younger 3xTg-AD mice, straight filaments appeared at 23 months of age in female mice. The present data suggest that other age-related biochemical mechanisms in addition to early intraneuronal accumulation of 6E10 and tau underlie the formation of tau filaments in 3xTg-AD mice.

Relation of hippocampal phospho-SAPK/JNK granules in Alzheimer's disease and tauopathies to granulovacuolar degeneration bodies.

Protein misfolding is a distinguishing feature of a number of neurodegenerative diseases. Accumulation of misfolded protein often results in cellular lesions, the location of lesions correlating with the nature of symptoms. Alzheimer's disease (AD), Progressive Supranuclear Palsy (PSP), Corticobasal Degeneration (CBD) and Pick's Disease (PiD) all present with pathological lesions containing hyperphosphorylated filamentous tau protein; however, the location and type of lesion varies. In addition, granulovacuolar degeneration (GVD) bodies have been reported within hippocampal pyramidal neurons in AD, PSP, CBD and PiD tissue. GVDs are defined as electron-dense granules within double membrane-bound cytoplasmic vacuoles. We have previously reported that the phosphorylated form of stress-activated protein kinase/c-Jun N-terminal kinase (p-SAPK/JNK) accumulates in granules within hippocampal pyramidal cell bodies in AD tissue at the time that hyperphosphorylated tau begins to aggregate into early-stage NFTs. We now report that p-SAPK/JNK granules are found within the hippocampal CA1 region of PSP, CBD and PiD cases as well and that these granules are likely GVD bodies. Quantitatively, p-SAPK/JNK granules and GVDs are found in comparable numbers of CA1 cells. Within cells, p-SAPK/JNK granules are distributed throughout the cytoplasm in a manner similar to the distribution of GVDs and a subset of granules co-localize with GVD markers. Ultrastructurally, p-SAPK/JNK granules are located in large cytoplasmic vacuoles, thereby fitting the definition of a GVD body. With the implication of granular p-SAPK/JNK as a marker of GVDs, our study strongly suggests that a heterogeneous group of proteins form GVDs. The mechanism of GVD formation is therefore an interesting one, and is likely separate and distinct from the mechanism of tau inclusion formation.

Caspase cleavage of tau: linking amyloid and neurofibrillary tangles in Alzheimer's disease.

The principal pathological features of Alzheimer's disease (AD) are extracellular amyloid plaques and intracellular neurofibrillary tangles, the latter composed of the microtubule-binding protein tau assembled into paired helical and straight filaments. Recent studies suggest that these pathological entities may be functionally linked, although the mechanisms by which amyloid deposition promotes pathological tau filament assembly are poorly understood. Here, we report that tau is proteolyzed by multiple caspases at a highly conserved aspartate residue (Asp421) in its C terminus in vitro and in neurons treated with amyloid-beta (Abeta) (1-42) peptide. Tau is rapidly cleaved at Asp421 in Abeta-treated neurons (within 2 h), and its proteolysis appears to precede the nuclear events of apoptosis. We also demonstrate that caspase cleavage of tau generates a truncated protein that lacks its C-terminal 20 amino acids and assembles more rapidly and more extensively into tau filaments in vitro than wild-type tau. Using a monoclonal antibody that specifically recognizes tau truncated at Asp421, we show that tau is proteolytically cleaved at this site in the fibrillar pathologies of AD brain. Taken together, our results suggest a novel mechanism linking amyloid deposition and neurofibrillary tangles in AD: Abeta peptides promote pathological tau filament assembly in neurons by triggering caspase cleavage of tau and generating a proteolytic product with enhanced polymerization kinetics.