Tau released from paired helical filaments with formic acid or guanidine is susceptible to calpain-mediated proteolysis

The Role of Extracellular Matrix Components in the Spreading of Pathological Protein Aggregates

Several neurodegenerative diseases are characterized by the accumulation of aggregated misfolded proteins. These pathological agents have been suggested to propagate in the brain via mechanisms similar to that observed for the prion protein, where a misfolded variant is transferred from an affected brain region to a healthy one, thereby inducing the misfolding and/or aggregation of correctly folded copies. This process has been characterized for several proteins, such as α-synuclein, tau, amyloid beta (Aβ) and less extensively for huntingtin and TDP-43. α-synuclein, tau, TDP-43 and huntingtin are intracellular proteins, and their aggregates are located in the cytosol or nucleus of neurons. They have been shown to spread between cells and this event occurs, at least partially, via secretion of these protein aggregates in the extracellular space followed by re-uptake. Conversely, Aβ aggregates are found mainly extracellularly, and their spreading occurs in the extracellular space between brain regions. Due to the inherent nature of their spreading modalities, these proteins are exposed to components of the extracellular matrix (ECM), including glycans, proteases and core matrix proteins. These ECM components can interact with or process pathological misfolded proteins, potentially changing their properties and thus regulating their spreading capabilities. Here, we present an overview of the documented roles of ECM components in the spreading of pathological protein aggregates in neurodegenerative diseases with the objective of identifying the current gaps in knowledge and stimulating further research in the field. This could potentially lead to the identification of druggable targets to slow down the spreading and/or progression of these pathologies.

First-in-Rat Study of Human Alzheimer's Disease Tau Propagation

One of the key features of misfolded tau in human neurodegenerative disorders is its propagation from one brain area into many others. In the last decade, in vivo tau spreading has been replicated in several mouse transgenic models expressing mutated human tau as well as in normal non-transgenic mice. In this study, we demonstrate for the first time that insoluble tau isolated from human AD brain induces full-blown neurofibrillary pathology in a sporadic rat model of tauopathy expressing non-mutated truncated tau protein. By using specific monoclonal antibodies, we were able to monitor the spreading of tau isolated from human brain directly in the rat hippocampus. We found that exogenous human AD tau was able to spread from the area of injection and induce tau pathology. Interestingly, solubilisation of insoluble AD tau completely abolished the capability of tau protein to induce and spread of neurofibrillary pathology in the rat brain. Our results show that exogenous tau is able to induce and drive neurofibrillary pathology in rat model for human tauopathy in a similar way as it was described in various mouse transgenic models. Rat tau spreading model has many advantages over mouse and other organisms including size and complexity, and thus is highly suitable for identification of pathogenic mechanism of tau spreading.

Enrichment of Detergent-insoluble Protein Aggregates from Human Postmortem Brain

In this study, we describe an abbreviated single-step fractionation protocol for the enrichment of detergent-insoluble protein aggregates from human postmortem brain. The ionic detergent N-lauryl-sarcosine (sarkosyl) effectively solubilizes natively folded proteins in brain tissue allowing the enrichment of detergent-insoluble protein aggregates from a wide range of neurodegenerative proteinopathies, such as Alzheimer's disease (AD), Parkinson's disease and amyotrophic lateral sclerosis, and prion diseases. Human control and AD postmortem brain tissues were homogenized and sedimented by ultracentrifugation in the presence of sarkosyl to enrich detergent-insoluble protein aggregates including pathologic phosphorylated tau, the core component of neurofibrillary tangles in AD. Western blotting demonstrated the differential solubility of aggregated phosphorylated-tau and the detergent-soluble protein, Early Endosome Antigen 1 (EEA1) in control and AD brain. Proteomic analysis also revealed enrichment of β-amyloid (Aβ), tau, snRNP70 (U1-70K), and apolipoprotein E (APOE) in the sarkosyl-insoluble fractions of AD brain compared to those of control, consistent with previous tissue fractionation strategies. Thus, this simple enrichment protocol is ideal for a wide range of experimental applications ranging from Western blotting and functional protein co-aggregation assays to mass spectrometry-based proteomics.

Cellular factors modulating the mechanism of tau protein aggregation

Pathological accumulation of the microtubule-associated protein tau, in the form of neurofibrillary tangles, is a major hallmark of Alzheimer's disease, the most prevalent neurodegenerative condition worldwide. In addition to Alzheimer's disease, a number of neurodegenerative diseases, called tauopathies, are characterized by the accumulation of aggregated tau in a variety of brain regions. While tau normally plays an important role in stabilizing the microtubule network of the cytoskeleton, its dissociation from microtubules and eventual aggregation into pathological deposits is an area of intense focus for therapeutic development. Here we discuss the known cellular factors that affect tau aggregation, from post-translational modifications to molecular chaperones.

Regulation of tau proteolysis by phosphatases

One pathological hallmark of Alzheimer's disease is the accumulation of highly phosphorylated tau. Since tau phosphorylation inhibits its proteolysis, we examined the impact of endogenous phosphatase activities on tau proteolysis by homogenization of cultured cells and 3xTg-AD mouse brain followed by incubation with or without phosphatase inhibitors. Incubation without phosphatase inhibitors significantly increased tau immunoreactivity against antibody C3 (which reacts with tau truncated at D421), and increased the generation of tau breakdown products. These changes were augmented by lithium treatment and inhibited by constitutively active GSK3β. These findings underscore that tau proteolysis is regulated by a balance of kinase and phosphatase activities.

Calpain dysregulation in Alzheimer's disease

Alzheimer's disease (AD) is characterized by the presence of senile plaques and neurofibrillary tangles in the neocortex and hippocampus of AD patients. In addition, a marked decrease in synaptic contacts has been detected in these affected brain areas. Due to its prevalence in the aging population, this disease has been the focus of numerous studies. The data obtained from those studies suggest that the mechanisms leading to the formation of the hallmark lesions of AD might be linked. One of such mechanisms seems to be the dysregulation of calcium homeostasis that results in the abnormal activation of calpains. Calpains are a family of Ca(2+)-dependent cysteine proteases that play a key role in multiple cell functions including cell development, differentiation and proliferation, axonal guidance, growth cone motility, and cell death, among others. In this paper, we briefly reviewed data on the structure of these proteases and their regulation under normal conditions. We also summarized data underscoring the participation of calpains in the neurodegenerative mechanisms associated with AD.

Human high temperature requirement serine protease A1 (HTRA1) degrades tau protein aggregates

Protective proteases are key elements of protein quality control pathways that are up-regulated, for example, under various protein folding stresses. These proteases are employed to prevent the accumulation and aggregation of misfolded proteins that can impose severe damage to cells. The high temperature requirement A (HtrA) family of serine proteases has evolved to perform important aspects of ATP-independent protein quality control. So far, however, no HtrA protease is known that degrades protein aggregates. We show here that human HTRA1 degrades aggregated and fibrillar tau, a protein that is critically involved in various neurological disorders. Neuronal cells and patient brains accumulate less tau, neurofibrillary tangles, and neuritic plaques, respectively, when HTRA1 is expressed at elevated levels. Furthermore, HTRA1 mRNA and HTRA1 activity are up-regulated in response to elevated tau concentrations. These data suggest that HTRA1 is performing regulated proteolysis during protein quality control, the implications of which are discussed.

Calpain-mediated tau cleavage: a mechanism leading to neurodegeneration shared by multiple tauopathies

Tau dysfunction has been associated with a host of neurodegenerative diseases called tauopathies. These diseases share, as a common pathological hallmark, the presence of intracellular aggregates of hyperphosphorylated tau in affected brain areas. Aside from tau hyperphosphorylation, little is known about the role of other posttranslational modifications in tauopathies. Recently, we obtained data suggesting that calpain-mediated tau cleavage leading to the generation of a neurotoxic tau fragment might play an important role in Alzheimer's disease. In the current study, we assessed the presence of this tau fragment in several tauopathies. Our results show high levels of the 17-kDa tau fragment and enhanced calpain activity in the temporal cortex of AD patients and in brain samples obtained from patients with other tauopathies. In addition, our data suggest that this fragment could partially inhibit tau aggregation. Conversely, tau aggregation might prevent calpain-mediated cleavage, establishing a feedback circuit that might lead to the accumulation of this toxic tau fragment. Collectively, these data suggest that the mechanism underlying the generation of the 17-kDa neurotoxic tau fragment might be part of a conserved pathologic process shared by multiple tauopathies.

Proteolytic processing of tau

Tau aggregation is a hallmark of several neurodegenerative diseases, including AD (Alzheimer's disease), although the mechanism underlying tau aggregation remains unclear. Recent studies show that the proteolysis of tau plays an important role in both tau aggregation and neurodegeneration. On one hand, truncation of tau may generate amyloidogenic tau fragments that initiate the aggregation of tau, which in turn can cause toxicity. On the other hand, truncation of tau may result in tau fragments which induce neurodegeneration through unknown mechanisms, independently of tau aggregation. Blocking the truncation of tau thus may represent a promising therapeutic approach for AD or other tauopathies. In the present paper, we summarize our data on tau cleavage in a cell model of tauopathy and major results on tau cleavage reported in the literature.

Cleavage of Tau by calpain in Alzheimer's disease: the quest for the toxic 17 kD fragment

The amyloid cascade hypothesis of Alzheimer's disease (AD) posits that the generation of β-amyloid (Aβ) triggers Tau neurofibrillary pathology. Recently a "17 kD" calpain-induced Tau fragment, comprising residues 45-230 (molecular weight [MW], 18.7 kD), was proposed to mediate Aβ-induced toxicity. Here, we demonstrate that the "17 kD" fragment is actually much smaller, containing residues 125-230 (molecular weight, 10.7 kD). Inducing Tau phosphorylation by okadaic acid or mimicking phosphorylation by Glu mutations at the epitopes of Alzheimer-diagnostic antibodies AT100/AT8/PHF1 could not prevent the generation of this fragment. The fragment can be induced not only by Aβ oligomers, but also by other cell stressors, e.g., thapsigargin (a Ca(2+)-ATPase inhibitor) or glutamate (an excitatory neurotransmitter). However, overexpression of neither Tau(45-230) nor Tau(125-230) fragment is toxic to Chinese hamster ovary (CHO) cells, neuroblastoma cells (N2a) or primary hippocampal neurons. Finally, the calpain-induced fragment can be observed both in Alzheimer's disease brains and in control normal human brains. We conclude that the 17 kD Tau fragment is not a mediator of Aβ-induced toxicity, leaving open the possibility that upstream calpain activation might cause both Tau fragmentation and toxicity.

Involvement of puromycin-sensitive aminopeptidase in proteolysis of tau protein in cultured cells, and attenuated proteolysis of frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) mutant tau

In tauopathies, tau protein is hyperphosphorylated, ubiquitinated, and accumulated in the brain; however, the mechanisms underlying this accumulation remain unclear. To gain an understanding of the role of proteases in the metabolism of tau protein, in the present study we evaluated the effects of protease inhibitors in SH-SY5Y human neuroblastoma cells and COS-7 cells transfected with the tau gene. When cells were treated with 0.1-10 micromol/L of lactacystin and 1.0-20 micromol/L of MG-132 (inhibitors of proteasome), 0.1-10 micromol/L of CA-074Me (a cathepsin inhibitor), and 0.1-2 micromol/L of puromycin (a puromycin-sensitive aminopeptidase (PSA) inhibitor) for up to 24 h, there were no significant changes in tau protein levels. However, pulse-chase experiments demonstrated that the proteolysis of tau protein in SH-SY5Y cells was attenuated following treatment of cells with 200 nmol/L puromycin. Increased tau protein levels were also observed in SH-SY5Y cells treated with short interference (si) RNA to PSA to inhibit the expression of PSA. These data suggest that PSA is a protease that catalyses tau protein predominantly in SH-SY5Y cells. The protein metabolism of tau-containing mutations of frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) was also investigated using pulse-chase experiments. The results indicate attenuated proteolysis of tau in cells transfected with mutant tau genes after 48 h. Further immunocytochemical analysis and subcellular fractionation experiments revealed that the mutations did not alter the intracellular distribution of tau and suggested that impaired accessibility of tau to PSA is unlikely to account for the attenuated proteolysis of tau protein. Western blotting with phosphorylation-dependent antibodies revealed that phosphorylation levels of tau at Thr(231), Ser(396), and Ser(409) were increased in cells transfected with V337M, R406W, and R406W mutant tau genes, respectively. Together, the data suggest that attenuated proteolysis of FTDP-17 mutant tau may be explained by increased phosphorylation levels, resulting in resistance to proteolysis.

Familial FTDP-17 missense mutations inhibit microtubule assembly-promoting activity of tau by increasing phosphorylation at Ser202 in vitro

In Alzheimer disease (AD), frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) and other tauopathies, tau accumulates and forms paired helical filaments (PHFs) in the brain. Tau isolated from PHFs is phosphorylated at a number of sites, migrates as approximately 60-, 64-, and 68-kDa bands on SDS-gel, and does not promote microtubule assembly. Upon dephosphorylation, the PHF-tau migrates as approximately 50-60-kDa bands on SDS-gels in a manner similar to tau that is isolated from normal brain and promotes microtubule assembly. The site(s) that inhibits microtubule assembly-promoting activity when phosphorylated in the diseased brain is not known. In this study, when tau was phosphorylated by Cdk5 in vitro, its mobility shifted from approximately 60-kDa bands to approximately 64- and 68-kDa bands in a time-dependent manner. This mobility shift correlated with phosphorylation at Ser(202), and Ser(202) phosphorylation inhibited tau microtubule-assembly promoting activity. When several tau point mutants were analyzed, G272V, P301L, V337M, and R406W mutations associated with FTDP-17, but not nonspecific mutations S214A and S262A, promoted Ser(202) phosphorylation and mobility shift to a approximately 68-kDa band. Furthermore, Ser(202) phosphorylation inhibited the microtubule assembly-promoting activity of FTDP-17 mutants more than of WT. Our data indicate that FTDP-17 missense mutations, by promoting phosphorylation at Ser(202), inhibit the microtubule assembly-promoting activity of tau in vitro, suggesting that Ser(202) phosphorylation plays a major role in the development of NFT pathology in AD and related tauopathies.

Characterization of paired helical filaments by scanning transmission electron microscopy

Paired helical filaments (PHFs) are abnormal twisted filaments composed of hyperphosphorylated tau protein. They are found in Alzheimer's disease and other neurodegenerative disorders designated as tauopathies. They are a major component of intracellular inclusions known as neurofibrillary tangles (NFTs). The objective of this review is to summarize various structural studies of PHFs in which using scanning transmission electron microscopy (STEM) has been particularly informative. STEM provides shape and mass per unit length measurements important for studying ultrastructural aspects of filaments. These include quantitative comparisons between dispersed and aggregated populations of PHFs as well as comparative studies of PHFs in Alzheimer's disease and other neurodegenerative disorders. Other approaches are also discussed if relevant or complementary to studies using STEM, e.g., application of a novel staining reagent, Nanovan. Our understanding of the PHF structure and the development of PHFs into NFTs is presented from a historical perspective. Others goals are to describe the biochemical and ultrastructural complexity of authentic PHFs, to assess similarities between authentic and synthetic PHFs, and to discuss recent advances in PHF modeling.

A novel fluorescent method for direct visualization of neurofibrillary pathology in Alzheimer's disease

Methods currently available for detecting neurofibrillary pathology are indirect and depend on staining with exogenous chemicals or antibodies. In the present study, we report a novel method named intrinsic fluorescence induction (IFI), which allows direct visualization of neurofibrillary tangles (NFTs), neuropil threads (NTs), and neuritic plaques (NPs) in tissue sections of Alzheimer's disease (AD) brain. The IFI method is based on both induction of a red intrinsic fluorescence and quenching red background autofluorescence. The IFI procedure includes sustained hydrophobic treatment, protein secondary structure enhancement and incubation in high concentration of phosphate buffer. Following this procedure, a unique red fluorescence is generated from the structures of NFTs, NTs, and NPs in brain sections from AD patients. Sequential application of mild permanganate oxidation and 1% sodium borohydride selectively removes the red background autofluorescence, while the latter enhances the intrinsic fluorescence of neurofibrillary pathology. Comparative studies reveal that the IFI method is as sensitive as Gallyas silver staining, and more sensitive than Bielschowsky silver staining or PHF-1 immunostaining in detecting NFTs in the pre-alpha layer of entorhinal cortex and the pri-alpha layer of the entorhinal/transentorhinal cortex. Furthermore, the IFI method is sensitive in displaying plaque neurites and threads, but not NFTs in the hippocampus. This novel finding provides a direct method for detecting neurofibrillary pathology in particular regions of AD brain and a novel tool for AD research.

Structural analysis of Pick's disease-derived and in vitro-assembled tau filaments

Pick's and Alzheimer's diseases are distinct neurodegenerative disorders both characterized in part by the presence of intracellular filamentous tau protein inclusions. The tight bundles of paired helical filaments (PHFs) of tau protein found in Alzheimer's disease (AD) differ from the tau filaments of Pick's disease in their morphology, distribution, and pathological structure as identified by silver impregnation. The filaments of Pick's disease are loosely arranged in pathognomonic spherical inclusions found in ballooned neurons, whereas the tau pathology of AD is classically described as a triad of neuropil threads, neurofibrillary tangles, and dystrophic neurites surrounding and invading plaques. In this study we used the high-resolution technique of scanning transmission electron microscopy to characterize and compare the filaments found in Pick's disease with those found in AD. In addition, we determined the mass/nm length and density of arachidonic acid-induced in vitro-assembled filaments. Three morphologically distinct populations of Pick's filaments were identified but each was indistinguishable from AD-PHFs in mass/nm length and density. Filaments assembled in vitro from single isoforms were similar in mass/nm length, but less dense than AD-PHFs and Pick's disease filaments. Finally, we provide clear structural evidence that a PHF, whether found in disease or assembled in vitro, is composed of two distinct intertwined filaments.

Induction of Alzheimer-specific Tau epitope AT100 in apoptotic human fetal astrocytes

In Alzheimer's and other neurodegenerative diseases, hyperphosphorylated tau accumulates in affected neuronal and glial cells in the form of paired helical filaments (PHFs). This tau binds antibody AT100, which recognizes the double phosphorylation site (Thr212/Ser214) that is not present in normal biopsy tau. In primary cultures, highly enriched (>98%) in astrocytes of human fetal brain, three polypeptides of 52, 64, and 70 kD showed immunoreactivity with tau antibodies against non-phosphorylated epitopes, accounting for 88, 12, and <1%, respectively, of the total reactivity. All three polypeptides were phosphorylated at the PHF-1 epitope but not at the epitopes Tau-1, 12E8, AT8, and AT100. Treatment of cultures with okadaic acid resulted in apoptosis characterized by the blebbing of the plasma membrane, condensation of nuclear chromatin, and fragmentation of the nucleus. This treatment also resulted in a 3- to 5-fold increase in the content of both tau protein and phosphorylation. The increases were observed in all phosphorylation sites examined, and included the AT100 site. The AT100 site has been proposed to be generated by protein kinase B/Akt and Cdc2. Since okadaic acid can induce an AD-like hyperphosphorylated state of normal tau in primary cultures of human brain cells, a simple cellular model is available permitting study of self-aggregation of tau and phosphorylation events characteristic of neurodegeneration.

The pathogenic activation of calpain: a marker and mediator of cellular toxicity and disease states

Over-activation of calpain, a ubiquitous calcium-sensitive protease, has been linked to a variety of degenerative conditions in the brain and several other tissues. Dozens of substrates for calpain have been identified and several of these have been used to measure activation of the protease in the context of experimentally induced and naturally occurring pathologies. Calpain-mediated cleavage of the cytoskeletal protein spectrin, in particular, results in a set of large breakdown products (BDPs) that are unique in that they are unusually stable. Over the last 15 years, measurements of BDPs in experimental models of stroke-type excitotoxicity, hypoxia/ischemia, vasospasm, epilepsy, toxin exposure, brain injury, kidney malfunction, and genetic defects, have established that calpain activation is an early and causal event in the degeneration that ensues from acute, definable insults. The BDPs also have been found to increase with normal ageing and in patients with Alzheimer's disease, and the calpain activity may be involved in related apoptotic processes in conjunction with the caspase family of proteases. Thus, it has become increasingly clear that regardless of the mode of disturbance in calcium homeostasis or the cell type involved, calpain is critical to the development of pathology and therefore a distinct and powerful therapeutic target. The recent development of antibodies that recognize the site at which spectrin is cleaved has greatly facilitated the temporal and spatial resolution of calpain activation in situ. Accordingly, sensitive spectrin breakdown assays now are utilized to identify potential toxic side-effects of compounds and to develop calpain inhibitors for a wide range of indications including stroke, cerebral vasospasm, and kidney failure.

Tau protein isoforms, phosphorylation and role in neurodegenerative disorders

Tau proteins belong to the family of microtubule-associated proteins. They are mainly expressed in neurons where they play an important role in the assembly of tubulin monomers into microtubules to constitute the neuronal microtubules network. Microtubules are involved in maintaining the cell shape and serve as tracks for axonal transport. Tau proteins also establish some links between microtubules and other cytoskeletal elements or proteins. Tau proteins are translated from a single gene located on chromosome 17. Their expression is developmentally regulated by an alternative splicing mechanism and six different isoforms exist in the human adult brain. Tau proteins are the major constituents of intraneuronal and glial fibrillar lesions described in Alzheimer's disease and numerous neurodegenerative disorders referred to as 'tauopathies'. Molecular analysis has revealed that an abnormal phosphorylation might be one of the important events in the process leading to their aggregation. Moreover, a specific set of pathological tau proteins exhibiting a typical biochemical pattern, and a different regional and laminar distribution could characterize each of these disorders. Finally, a direct correlation has been established between the progressive involvement of the neocortical areas and the increasing severity of dementia, suggesting that pathological tau proteins are reliable marker of the neurodegenerative process. The recent discovery of tau gene mutations in frontotemporal dementia with parkinsonism linked to chromosome 17 has reinforced the predominant role attributed to tau proteins in the pathogenesis of neurodegenerative disorders, and underlined the fact that distinct sets of tau isoforms expressed in different neuronal populations could lead to different pathologies.

Conformation of paired helical filaments blocks dephosphorylation of epitopes shared with fetal tau except Ser199/202 and Ser202/Thr205

To determine if the high phosphate content of paired helical filaments (PHFs) in Alzheimer's disease (AD) is a result of limited access to filament phosphorylation sites, we studied in vitro dephosphorylation of intact PHFs, PHFs with filamentous structure abolished by formic acid treatment (PHF(FA)) and fetal human tau protein. Samples were treated with alkaline phosphatase for up to 24 h at 37 degrees C and then immunoblotted with eight well characterized tau antibodies, that recognize two phosphorylation-insensitive sites and six phosphorylation-sensitive epitopes at Thr181, Ser199/202, Ser202/Thr205, Thr231, Ser262/356 and Ser396/404. Intact PHFs were effectively dephosphorylated only at the two N-terminal epitopes Ser199/202 and Ser202/Thr205, with little change in electrophoretic mobility. In contrast, PHF(FA) were dephosphorylated at all epitopes, with particular effectiveness at those in the C-terminus and with significant increase in electrophoretic mobility. The fetal tau epitopes were effectively dephosphorylated except at Thr181 and Thr231 with marked increase in mobility. The extent of dephosphorylation of PHF(FA) was equal or more effective than in fetal tau, except for Thr181 that was minimally dephosphorylated in both proteins. The results indicate that intact PHFs, but not PHF(FA) or fetal tau display differential dephosphorylation of the N- and C-terminal epitopes. The results confirm that the filamentous conformation may significantly contribute to hyperphosphorylation of PHFs in the C-terminus. The filamentous conformation, however, does not limit access to two N-terminal epitopes Ser199/202 and Ser202/Thr205. The access to these sites in AD may be limited by other factors, e.g., inhibition of phosphatase binding.