Expression of separate isoforms of human tau protein: correlation with the tau pattern in brain and effects on tubulin polymerization.

We have expressed six previously cloned isoforms of human microtubule-associated tau protein in Escherichia coli and purified them to homogeneity in a biologically active form. They range from 352 to 441 amino acids in length and differ from each other by the presence of three or four tandem repeats in the carboxy-terminal half and by the presence or absence of 29 or 58 amino acid inserts in the amino-terminus. When mixed together they gave a set of six bands on SDS-PAGE gels with apparent molecular weights of 48-67 kd and with a characteristic pattern of spacings. Four of these bands aligned with the major tau bands found in adult human cerebral cortex following perchloric acid extraction and alkaline phosphatase treatment. They consisted of isoforms with three repeats and no insertions, four repeats and no amino-terminal insertions and three- and four-repeat containing isoforms with the 29 amino acid insertion. In fetal human brain extracts treated with alkaline phosphatase one of the two major tau bands aligned with the three-repeat containing isoform with no insertions, whereas the molecular nature of the second major tau band remains to be established. The recombinant tau isoforms were biologically active at micromolar concentrations, as assessed by their ability to promote microtubule assembly. The rates of assembly were 2.5-3.0 times faster for isoforms containing four repeats when compared with three-repeat containing isoforms, with no significant contribution by the amino-terminal insertions.     

Cloning and sequencing of the cDNA encoding an isoform of microtubule-associated protein tau containing four tandem repeats: differential expression of tau protein mRNAs in human brain.

We have isolated cDNA clones encoding a 383-amino acid isoform of the human microtubule-associated protein tau. It differs from previously determined tau sequences by the presence of an additional repeat of 31 amino acids, giving four, rather than three, tandem repeats in its carboxy-terminal half. The extra repeat is encoded by a separate exon. Probes derived from cDNA clones encoding the three (type I) and four repeat (type II) tau protein isoforms detected mRNAs for both forms in all adult human brain areas examined. However, in foetal brain only type I mRNA was found. Type I and type II mRNAs were present in pyramidal cells in cerebral cortex. In the hippocampal formation, type I mRNA was found in pyramidal and granule cells; type II mRNA was detected in most, though not all, pyramidal cells but not in granule cells. These observations indicate that tau protein mRNAs are expressed in a stage- and cell-specific manner. Tau protein is found in the protease-resistant core of the paired helical filament, the major constituent of the neurofibrillary tangle in Alzheimer's disease. Taken in conjunction with previous findings, the present results indicate that both the three and four repeat-containing tau protein isoforms are present in the core of the paired helical filament.     

Molecular cloning and functional characterization of chicken brain tau: isoforms with up to five tandem repeats

Tau is a major microtubule-associated protein in mammalian brain, where it exists as multiple isoforms that are produced from a single gene by alternative mRNA splicing. Here we present the first report on the structure and function of tau protein from a nonmammalian vertebrate. In the adult chicken brain, five main tau isoforms are expressed. One isoform has three tandem repeats, two isoforms have four repeats each, and two isoforms have five repeats each. Similar to mammalian tau, some chicken tau isoforms contain an amino-terminal insert of 53 amino acids. Unlike mammalian tau, a 34 amino acid insert in the proline-rich region upstream of the repeats is alternatively spliced in chicken tau. It is preceded by a constitutively expressed sequence of 17 amino acids that is absent in tau from human and rodent brains. The expression of chicken tau isoforms and their phosphorylation are developmentally regulated, similar to what has been described in mammalian brain. Functionally, chicken tau isoforms with five repeats have the greatest ability to promote microtubule assembly, followed by isoforms with four and three repeats, respectively. The 34 amino acid insert positively influences both the rate and the extent of microtubule assembly, whereas the 53 amino acid insert only influences the extent of assembly.     

Assembly of paired helical filaments from mouse tau: implications for the neurofibrillary pathology in transgenic mouse models for Alzheimer's disease.

In Alzheimer's disease and related dementias, human tau protein aggregates into paired helical filaments and neurofibrillary tangles. However, such tau aggregates have not yet been demonstrated in transgenic mouse models of the disease. One of the possible explanations would be that mouse tau has different properties which prevents it from aggregating. We have cloned several murine tau isoforms, containing three or four repeats and different combinations of inserts, expressed them in Escherichia coli and show here that they can all be assembled into paired helical filaments similar to those in Alzheimer's disease, using the same protocols as with human tau. Therefore, the absence of pathologically aggregated tau in transgenic mice cannot be explained by intrinsic differences in mouse tau protein and instead must be explained by other as yet unknown factors.     

Molecular cloning of XTP, a tau-like microtubule-associated protein from Xenopus laevis tadpoles

The microtubules of the mammalian nervous system are stabilised by several microtubule-associated proteins (MAPs), including the tau and MAP-2 protein families. The most prominent feature of mammalian tau and MAP-2 proteins is a common and highly homologous microtubule-binding region consisting of three or four imperfect tandem repeats. In this paper we report the cloning and characterisation of a Xenopus laevis tau-like protein (XTP) from tadpole tails. This protein encompasses two isoforms of 673 or 644 amino acids with four tandem repeats that are highly homologous to mammalian tau repeats. Both isoforms share a common amino terminal half, whereas the carboxyl terminus downstream of the repeat region is unique for each isoform. Northern blot analysis revealed that both isoforms are preferentially expressed in the tail of X. laevis tadpoles, whereas a shorter version of XTP is expressed in the head. Recombinant proteins of both XTP isoforms were able to bind microtubules. The longest isoform, however, was more effective at promoting tubulin polymerisation, indicating that sequences downstream of the repeat region affect the microtubule assembling capacity. These results demonstrate that tau-like proteins are found in non-mammalian vertebrate species, where they may support the stability of microtubules.     

Alzheimer-like paired helical filaments and antiparallel dimers formed from microtubule-associated protein tau in vitro

Recent evidence from several laboratories shows that the paired helical filaments of Alzheimer's disease brains consist mainly of the protein tau in an abnormally phosphorylated form, but the mode of assembly is not understood. Here we use EM to study several constructs derived from human brain tau and expressed in Escherichia coli. All constructs or tau isoforms are rodlike molecules with a high tendency to dimerize in an antiparallel fashion, as shown by antibody labeling and chemical crosslinking. The length of the rods is largely determined by the region of internal repeats that is also responsible for microtubule binding. One unit length of the repeat domain (three or four repeats) is around 22-25 nm, comparable to the cross-section of Alzheimer PHF cores. Constructs corresponding roughly to the repeat region of tau can form synthetic paired helical filaments resembling those from Alzheimer brain tissue. A similar self-assembly occurs with the chemically cross-linked dimers. In both cases there is no need for phosphorylation of the protein.     

Three- and four-repeat Tau coassemble into heterogeneous filaments: an implication for Alzheimer disease

Tau filaments are the pathological hallmark of numerous neurodegenerative diseases including Alzheimer disease, Pick disease, and progressive supranuclear palsy. In the adult human brain, six isoforms are expressed that differ by the presence or absence of the second of four semiconserved repeats. As a consequence, half of the tau isoforms have three repeats (3R tau), whereas the other half of the isoforms have four repeats (4R tau). Tauopathies can be characterized based on the isoform composition of their filaments. Alzheimer disease filamentous inclusions contain all isoforms. Pick disease filaments contain 3R tau. Progressive supranuclear palsy filaments contain 4R tau. Here, we used site-directed spin labeling of recombinant tau in conjunction with electron paramagnetic resonance spectroscopy to obtain structural insights into these filaments. We find that filaments of 4R tau and 3R tau share a highly ordered core structure in the third repeat with parallel, in-register arrangement of β-strands. This structure is conserved regardless of whether full-length isoforms (htau40 and htau23) or truncated constructs (K18 and K19) are used. When mixed, 3R tau and 4R tau coassemble into heterogeneous filaments. These filaments share the highly ordered core in the third repeat; however, they differ in their overall composition. Our findings indicate that at least three distinct types of filaments exist: homogeneous 3R tau, homogeneous 4R tau, and heterogeneous 3R/4R tau. These results suggest that individual filaments found in Alzheimer disease are structurally distinct from those in the 3R and 4R tauopathies.     

Tau mutations in frontotemporal dementia FTDP-17 and their relevance for Alzheimer's disease

Alzheimer's disease is characterised by the degeneration of selected populations of nerve cells that develop filamentous inclusions prior to degeneration. The neuronal inclusions of Alzheimer's disease are made of the microtubule-associated protein tau, in a hyperphosphorylated state. Abundant filamentous tau inclusions are not limited to Alzheimer's disease. They are the defining neuropathological characteristic of frontotemporal dementias, such as Pick's disease, and of progressive supranuclear palsy and corticobasal degeneration. The discovery of mutations in the tau gene in familial frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) has provided a direct link between tau dysfunction and dementing disease. Known mutations produce either a reduced ability of tau to interact with microtubules, or an overproduction of tau isoforms with four microtubule-binding repeats. This leads in turn to the assembly of tau into filaments similar or identical to those found in Alzheimer's disease brain. Several missense mutations also have a stimulatory effect on heparin-induced tau filament formation. Assembly of tau into filaments may be the gain of toxic function that is believed to underlie the demise of affected brain cells.     

Characterization of tau in cerebrospinal fluid using mass spectrometry

The neurodegenerative disorder Alzheimer's disease (AD) is the most common cause of dementia in the elderly. The presence of neurofibrillary tangles, consisting of hyperphosphorylated tau protein, is one of the major neuropathologic characteristics of the disease, making this protein an attractive biomarker for AD and a possible target for therapy. Here, we describe an optimized immunoprecipitation mass spectrometry method that enables, for the first time, detailed characterization of tau in human cerebrospinal fluid. The identities of putative tau fragments were confirmed using nanoflow liquid chromatography and tandem mass spectrometry. Nineteen tryptic fragments of tau were detected, of which 16 are found in all tau isoforms while 3 represented unique tau isoforms. These results pave the way for clinical CSF studies on the tauopathies.     

Proline-directed phosphorylation of human Tau protein.

The primary sequence of the microtubule-associated protein tau contains multiple repeats of the sequence -X-Ser/Thr-Pro-X-, the consensus sequence for the proline-directed protein kinase (p34cdc2/p58cyclin A). When phosphorylated by proline-directed protein kinase in vitro, tau was found to incorporate up to 4.4 mol of phosphate/mol of protein. Isoelectric focusing of the tryptic phosphopeptides demonstrated the presence of five distinct peptides with pI values of approximately 6.9, 6.5, 5.6-5.9, 4.7, and 3.6. Mapping of the tryptic phosphopeptides by high performance liquid chromatography techniques demonstrated three distinct peaks. Data from gas phase sequencing, amino acid analysis, and phosphoamino acid analysis suggest that proline-directed protein kinase phosphorylates tau at four sites. Each site demonstrates the presence of a proline residue on the carboxyl-terminal side of the phosphorylated residue. Two phosphorylation sites are located adjacent to the three-repeat microtubule-binding domain that has been found to be required for the in vivo co-localization of tau protein to microtubules. Two other putative phosphorylation sites are located within the identified epitope of the monoclonal antibody Tau-1. Phosphorylation of these sites altered the immunoreactivity of tau to Tau-1 antibody. Since the neuronal microtubule-associated protein tau is multiply phosphorylated in Alzheimer's disease, and Tau-1 immunoreactivity is similarly reduced in neurofibrillary tangles and enhanced after dephosphorylation, phosphorylation at one or more of these sites may correlate with abnormally phosphorylated sites in tau protein in Alzheimer's disease.     

Interaction of tau isoforms with Alzheimer's disease abnormally hyperphosphorylated tau and in vitro phosphorylation into the disease-like protein

The microtubule-associated protein tau is a family of six isoforms that becomes abnormally hyperphosphorylated and accumulates in neurons undergoing neurodegeneration in the brains of patients with Alzheimer disease (AD). We investigated the isoform-specific interaction of normal tau with AD hyperphosphorylated tau (AD P-tau). We found that the binding of AD P-tau to normal human recombinant tau was tau4L > tau4S > tau4 and tau3L > tau3S > tau3, and that its binding to tau4L was greater than to tau3L. AD P-tau also inhibited the assembly of microtubules promoted by each tau isoform and caused disassembly when added to preassembled microtubules. This inhibition and depolymerization of microtubules by the AD P-tau corresponded directly to the degree of its interaction with the different tau isoforms. In vitro hyperphosphorylation of recombinant tau (P-tau) conferred AD P-tau-like characteristics. Like AD P-tau, P-tau interacted with and sequestered normal tau and inhibited microtubule assembly. These studies suggest that the AD P-tau interacts preferentially with the tau isoforms that have the amino-terminal inserts and four microtubule binding domain repeats and that hyperphosphorylation of tau appears to be sufficient to acquire AD P-tau characteristics. Thus, lack of amino-terminal inserts and extra microtubule binding domain repeat in fetal human brain might be protective from Alzheimer's neurofibrillary degeneration.     

The core of tau-paired helical filaments studied by scanning transmission electron microscopy and limited proteolysis

In Alzheimer's disease and frontotemporal dementias the microtubule-associated protein tau forms intracellular paired helical filaments (PHFs). The filaments formed in vivo consist mainly of full-length molecules of the six different isoforms present in adult brain. The substructure of the PHF core is still elusive. Here we applied scanning transmission electron microscopy (STEM) and limited proteolysis to probe the mass distribution of PHFs and their surface exposure. Tau filaments assembled from the three repeat domain have a mass per length (MPL) of approximately 60 kDa/nm and filaments from full-length tau (htau40DeltaK280 mutant) have approximately 160 kDa/nm, compared with approximately 130 kDa/nm for PHFs from Alzheimer's brain. Polyanionic cofactors such as heparin accelerate assembly but are not incorporated into PHFs. Limited proteolysis combined with N-terminal sequencing and mass spectrometry of fragments reveals a protease-sensitive N-terminal half and semiresistant PHF core starting in the first repeat and reaching to the C-terminus of tau. Continued proteolysis leads to a fragment starting at the end of the first repeat and ending in the fourth repeat. PHFs from tau isoforms with four repeats revealed an additional cleavage site within the middle of the second repeat. Probing the PHFs with antibodies detecting epitopes either over longer stretches in the C-terminal half of tau or in the fourth repeat revealed that they grow in a polar manner. These data describe the physical parameters of the PHFs and enabled us to build a model of the molecular arrangement within the filamentous structures.     

Cell cycle and activation of CK2

Microtubule associated protein tau is considered to play roles in some types of human transmissible spongiform encephalopathies (TSE). In this study, the full-length and several truncated human tau proteins were expressed from E. coli and purified. Using GST pull down, co-immunoprecipitation assay and tau-coated ELISA, the molecular interaction between tau protein and PrP was confirmed in the context of the full-length human tau. The N terminus (amino acids 1–91) and tandem repeats region (amino acids 186–283) of tau protein were responsible for the interaction with PrP. The octapeptide repeats within PrP directly affected the binding activity of PrP with tau. GSS-related mutant PrP102L and fCJD- related mutants with two and seven extra octarepeats showed more active binding capacity with tau than wild-type PrP. The molecular interactions between PrP and tau protein highlight a potential role of tau in the biological function of PrP and the pathogenesis of TSE.     

Cyclic AMP-dependent protein kinase regulates the alternative splicing of tau exon 10: a mechanism involved in tau pathology of Alzheimer disease

Hyperphosphorylation and deposition of tau into neurofibrillary tangles is a hallmark of Alzheimer disease (AD). Alternative splicing of tau exon 10 generates tau isoforms containing three or four microtubule binding repeats (3R-tau and 4R-tau), which are equally expressed in adult human brain. Dysregulation of exon 10 causes neurofibrillary degeneration. Here, we report that cyclic AMP-dependent protein kinase, PKA, phosphorylates splicing factor SRSF1, modulates its binding to tau pre-mRNA, and promotes tau exon 10 inclusion in cultured cells and in vivo in rat brain. PKA-Cα, but not PKA-Cβ, interacts with SRSF1 and elevates SRSF1-mediated tau exon 10 inclusion. In AD brain, the decreased level of PKA-Cα correlates with the increased level of 3R-tau. These findings suggest that a down-regulation of PKA dysregulates the alternative splicing of tau exon 10 and contributes to neurofibrillary degeneration in AD by causing an imbalance in 3R-tau and 4R-tau expression.