Tau and neurodegeneration

First identified in 1975, tau was implicated in Alzheimer's disease 10 years later. Filamentous tangle inclusions were known to be made of hyperphosphorylated tau by 1991, with similar inclusions gaining recognition for being associated with other neurodegenerative diseases. In 1998, mutations in MAPT, the gene that encodes tau, were identified as the cause of a dominantly inherited form of frontotemporal dementia with abundant filamentous tau inclusions. While this result indicated that assembly of tau into aberrant filaments is sufficient to drive neurodegeneration and dementia, most cases of tauopathy are sporadic. More recent work in experimental systems showed that filamentous assemblies of tau may first form in one brain area, and then spread to others in a prion-like fashion. Beginning in 2017, work on human brains using high-resolution techniques has led to a structure-based classification of tauopathies, which has opened the door to a better understanding of the significance of tau filament formation.

Cryo-EM structures of tau filaments from the brains of mice transgenic for human mutant P301S Tau

Mice transgenic for human mutant P301S tau are widely used as models for human tauopathies. They develop neurodegeneration and abundant filamentous inclusions made of human mutant four-repeat tau. Here we used electron cryo-microscopy (cryo-EM) to determine the structures of tau filaments from the brains of Tg2541 and PS19 mice. Both lines express human P301S tau (0N4R for Tg2541 and 1N4R for PS19) on mixed genetic backgrounds and downstream of different promoters (murine Thy1 for Tg2541 and murine Prnp for PS19). The structures of tau filaments from Tg2541 and PS19 mice differ from each other and those of wild-type tau filaments from human brains. The structures of tau filaments from the brains of humans with mutations P301L, P301S or P301T in MAPT are not known. Filaments from the brains of Tg2541 and PS19 mice share a substructure at the junction of repeats 2 and 3, which comprises residues I297-V312 of tau and includes the P301S mutation. The filament core from the brainstem of Tg2541 mice consists of residues K274-H329 of tau and two disconnected protein densities. Two non-proteinaceous densities are also in evidence. The filament core from the cerebral cortex of line PS19 extends from residues G271-P364 of tau. One strong non-proteinaceous density is also present. Unlike the tau filaments from human brains, the sequences following repeat 4 are missing from the cores of tau filaments from the brains of Tg2541 and PS19 mice.

Heparin-induced tau filaments are polymorphic and differ from those in Alzheimer's and Pick's diseases

Assembly of microtubule-associated protein tau into filamentous inclusions underlies a range of neurodegenerative diseases. Tau filaments adopt different conformations in Alzheimer’s and Pick’s diseases. Here, we used cryo- and immuno- electron microscopy to characterise filaments that were assembled from recombinant full-length human tau with four (2N4R) or three (2N3R) microtubule-binding repeats in the presence of heparin. 2N4R tau assembles into multiple types of filaments, and the structures of three types reveal similar ‘kinked hairpin’ folds, in which the second and third repeats pack against each other. 2N3R tau filaments are structurally homogeneous, and adopt a dimeric core, where the third repeats of two tau molecules pack in a parallel manner. The heparin-induced tau filaments differ from those of Alzheimer’s or Pick’s disease, which have larger cores with different repeat compositions. Our results illustrate the structural versatility of amyloid filaments, and raise questions about the relevance of in vitro assembly.

Structures of filaments from Pick's disease reveal a novel tau protein fold

The ordered assembly of tau protein into abnormal filamentous inclusions underlies many human neurodegenerative diseases1. Tau assemblies seem to spread through specific neural networks in each disease2, with short filaments having the greatest seeding activity3. The abundance of tau inclusions strongly correlates with disease symptoms4. Six tau isoforms are expressed in the normal adult human brain-three isoforms with four microtubule-binding repeats each (4R tau) and three isoforms that lack the second repeat (3R tau)1. In various diseases, tau filaments can be composed of either 3R or 4R tau, or of both. Tau filaments have distinct cellular and neuroanatomical distributions5, with morphological and biochemical differences suggesting that they may be able to adopt disease-specific molecular conformations6,7. Such conformers may give rise to different neuropathological phenotypes8,9, reminiscent of prion strains10. However, the underlying structures are not known. Using electron cryo-microscopy, we recently reported the structures of tau filaments from patients with Alzheimer's disease, which contain both 3R and 4R tau11. Here we determine the structures of tau filaments from patients with Pick's disease, a neurodegenerative disorder characterized by frontotemporal dementia. The filaments consist of residues Lys254-Phe378 of 3R tau, which are folded differently from the tau filaments in Alzheimer's disease, establishing the existence of conformers of assembled tau. The observed tau fold in the filaments of patients with Pick's disease explains the selective incorporation of 3R tau in Pick bodies, and the differences in phosphorylation relative to the tau filaments of Alzheimer's disease. Our findings show how tau can adopt distinct folds in the human brain in different diseases, an essential step for understanding the formation and propagation of molecular conformers.

Propagation of Tau Aggregates and Neurodegeneration

A pathway from the natively unfolded microtubule-associated protein Tau to a highly structured amyloid fibril underlies human Tauopathies. This ordered assembly causes disease and represents the gain of toxic function. In recent years, evidence has accumulated to suggest that Tau inclusions form first in a small number of brain cells, from where they propagate to other regions, resulting in neurodegeneration and disease. Propagation of pathology is often called prion-like, which refers to the capacity of an assembled protein to induce the same abnormal conformation in a protein of the same kind, initiating a self-amplifying cascade. In addition, prion-like encompasses the release of protein aggregates from brain cells and their uptake by neighboring cells. In mice, the intracerebral injection of Tau inclusions induces the ordered assembly of monomeric Tau, followed by its spreading to distant brain regions. Conformational differences between Tau aggregates from transgenic mouse brain and in vitro assembled recombinant protein account for the greater seeding potency of brain aggregates. Short fibrils constitute the major species of seed-competent Tau in the brains of transgenic mice. The existence of multiple human Tauopathies with distinct fibril morphologies has led to the suggestion that different molecular conformers (or strains) of aggregated Tau exist.

Cryo-EM structures of tau filaments from Alzheimer's disease

Alzheimer's disease is the most common neurodegenerative disease, and there are no mechanism-based therapies. The disease is defined by the presence of abundant neurofibrillary lesions and neuritic plaques in the cerebral cortex. Neurofibrillary lesions comprise paired helical and straight tau filaments, whereas tau filaments with different morphologies characterize other neurodegenerative diseases. No high-resolution structures of tau filaments are available. Here we present cryo-electron microscopy (cryo-EM) maps at 3.4-3.5 Å resolution and corresponding atomic models of paired helical and straight filaments from the brain of an individual with Alzheimer's disease. Filament cores are made of two identical protofilaments comprising residues 306-378 of tau protein, which adopt a combined cross-β/β-helix structure and define the seed for tau aggregation. Paired helical and straight filaments differ in their inter-protofilament packing, showing that they are ultrastructural polymorphs. These findings demonstrate that cryo-EM allows atomic characterization of amyloid filaments from patient-derived material, and pave the way for investigation of a range of neurodegenerative diseases.

Structures of hepatitis B virus cores presenting a model epitope and their complexes with antibodies

The core shell of hepatitis B virus is a potent immune stimulator, giving a strong neutralizing immune response to foreign epitopes inserted at the immunodominant region, located at the tips of spikes on the exterior of the shell. Here, we analyze structures of core shells with a model epitope inserted at two alternative positions in the immunodominant region. Recombinantly expressed core protein assembles into T = 3 and T = 4 icosahedral shells, and atomic coordinates are available for the T = 4 shell. Since the modified protein assembles predominantly into T = 3 shells, a quasi-atomic model of the native T = 3 shell was made. The spikes in this T = 3 structure resemble those in T = 4 shells crystallized from expressed protein. However, the spikes in the modified shells exhibit an altered conformation, similar to the DNA containing shells in virions. Both constructs allow full access of antibodies to the foreign epitope, DPAFR from the preS1 region of hepatitis B virus surface antigen. However, one induces a 10-fold weaker immune response when injected into mice. In this construct, the epitope is less constrained by the flanking linker regions and is positioned so that the symmetry of the shell causes pairs of epitopes to come close enough to interfere with one another. In the other construct, the epitope mimics the native epitope conformation and position. The interaction of native core shells with an antibody specific to the immunodominant epitope is compared to the constructs with an antibody against the foreign epitope. Our findings have implications for the design of vaccines based on virus-like particles.

Phenotypic variation of autosomal-dominant corticobasal degeneration

Neurodegenerative tauopathies may be inherited as autosomal-dominant disorders with variable clinicopathological phenotypes, and causative mutations in the microtubule-associated protein tau (MAPT) gene are not regularly seen. Herein, we describe a patient with clinically typical and autopsy-proven corticobasal degeneration (CBD). Her mother was diagnosed to have Parkinson's disease, but autopsy showed CBD pathology as in the index patient. The sister of the index patient had the clinical symptoms of primary progressive aphasia (PPA), but no pathology was available to date. Molecular analysis did not reveal any mutation in the MAPT or progranulin (GRN) genes. Our findings illustrate that CBD, progressive supranuclear palsy and PPA may be overlapping diseases with a common pathological basis rather than distinct entities. Clinical presentation and course might be determined by additional, yet unknown, genetic modifying factors.

Tilt-pair analysis of images from a range of different specimens in single-particle electron cryomicroscopy

The comparison of a pair of electron microscope images recorded at different specimen tilt angles provides a powerful approach for evaluating the quality of images, image-processing procedures, or three-dimensional structures. Here, we analyze tilt-pair images recorded from a range of specimens with different symmetries and molecular masses and show how the analysis can produce valuable information not easily obtained otherwise. We show that the accuracy of orientation determination of individual single particles depends on molecular mass, as expected theoretically since the information in each particle image increases with molecular mass. The angular uncertainty is less than 1° for particles of high molecular mass (~50 MDa), several degrees for particles in the range 1-5 MDa, and tens of degrees for particles below 1 MDa. Orientational uncertainty may be the major contributor to the effective temperature factor (B-factor) describing contrast loss and therefore the maximum resolution of a structure determination. We also made two unexpected observations. Single particles that are known to be flexible showed a wider spread in orientation accuracy, and the orientations of the largest particles examined changed by several degrees during typical low-dose exposures. Smaller particles presumably also reorient during the exposure; hence, specimen movement is a second major factor that limits resolution. Tilt pairs thus enable assessment of orientation accuracy, map quality, specimen motion, and conformational heterogeneity. A convincing tilt-pair parameter plot, where 60% of the particles show a single cluster around the expected tilt axis and tilt angle, provides confidence in a structure determined using electron cryomicroscopy.

Human beta-synuclein rendered fibrillogenic by designed mutations

Filamentous inclusions made of α-synuclein are found in nerve cells and glial cells in a number of human neurodegenerative diseases, including Parkinson disease, dementia with Lewy bodies, and multiple system atrophy. The assembly and spreading of these inclusions are likely to play an important role in the etiology of common dementias and movement disorders. Both α-synuclein and the homologous β-synuclein are abundantly expressed in the central nervous system; however, β-synuclein is not present in the pathological inclusions. Previously, we observed a poor correlation between filament formation and the presence of residues 73-83 of α-synuclein, which are absent in β-synuclein. Instead, filament formation correlated with the mean β-sheet propensity, charge, and hydrophilicity of the protein (global physicochemical properties) and β-strand contiguity calculated by a simple algorithm of sliding averages (local physicochemical property). In the present study, we rendered β-synuclein fibrillogenic via one set of point mutations engineered to enhance global properties and a second set engineered to enhance predominantly β-strand contiguity. Our findings show that the intrinsic physicochemical properties of synucleins influence their fibrillogenic propensity via two distinct but overlapping modalities. The implications for filament formation and the pathogenesis of neurodegenerative diseases are discussed.

Impact of hepatitis B virus surface protein mutations on the diagnosis of occult hepatitis B virus infection

Genotype D occult hepatitis B virus (HBV) infections (OBIs) have a high frequency of amino acid substitutions in the major hydrophilic region of the small surface protein (S protein). This possibly reflects an escape mutation mechanism to evade detection by the host immune system. Mutations may also impact the detection of hepatitis B surface antigen (HBsAg) by commercial assays. To test these hypotheses, 20 recombinant HBV genotype D surface proteins from OBI carriers with or without antibody to hepatitis B surface antigen (anti-HBs) were expressed in yeast. Recombinant surface protein (rS protein) variants were nonreactive with autologous anti-HBs but reacted weakly with vaccine-induced anti-HBs supporting an immune escape mechanism. rS protein variants tested with a wide range of HBs antibodies, and HBsAg commercial assays showed significantly lower antigenic reactivity in anti-HBs carriers than in donors with antibody to hepatitis B core antigen (anti-HBc) only. Eight out of 10 recombinant variants from anti-HBs carriers reacted weakly or were nonreactive with antibodies to HBs as well as with qualitative and quantitative commercial HBsAg assays, whereas eight out of 10 anti-HBc-only plasmas were fully reactive. rS proteins with substitutions of wild-type cysteine at positions 121, 124, and 137 were nonreactive or showed poor reactivity. However, mutation of cysteine 147 did not alter reactivity compared with controls. Restoration of cysteines 124 and 137 by site-directed mutagenesis improved antigenic reactivity.

CONCLUSION

Escape mutation is a mechanism associated with OBI, which also leads to decreased reactivity in HBsAg detection assays. Performance of commercial assays would be improved by the incorporation of OBI mutants in reagent development.

From envelopes to atoms: The remarkable progress of biological electron microscopy

The electron microscope has, in principle, provided a powerful method for investigating biological structures for quite sometime, but only recently is its full potential being realized. Technical advances in the microscopes themselves, in methods of specimen preparation, and in computer processing of the recorded micrographs have all been necessary to underpin progress. It is now possible with suitable unstained specimens of two-dimensional crystals, helical or tubular structures, and icosahedral viruses to achieve resolutions of 4Å or better. For nonsymmetrical particles, sub-nanometer resolution is often possible. Tomography is enabling detailed pictures of subcellular organization to be produced. Thus, electron microscopy is now starting to rival X-ray crystallography in the resolution achievable but with the advantage of being applicable to a far wider range of biological specimens. With further improvements already under way, electron microscopy is set to be a centrally important technique for understanding biological structure and function at all levels-from atomic to cellular.

Transmission and spreading of tauopathy in transgenic mouse brain

Hyperphosphorylated tau makes up the filamentous intracellular inclusions of several neurodegenerative diseases, including Alzheimer's disease. In the disease process, neuronal tau inclusions first appear in the transentorhinal cortex from where they seem to spread to the hippocampal formation and neocortex. Cognitive impairment becomes manifest when inclusions reach the hippocampus, with abundant neocortical tau inclusions and extracellular beta-amyloid deposits being the defining pathological hallmarks of Alzheimer's disease. An abundance of tau inclusions, in the absence of beta-amyloid deposits, defines Pick's disease, progressive supranuclear palsy, corticobasal degeneration and other diseases. Tau mutations cause familial forms of frontotemporal dementia, establishing that tau protein dysfunction is sufficient to cause neurodegeneration and dementia. Thus, transgenic mice expressing mutant (for example, P301S) human tau in nerve cells show the essential features of tauopathies, including neurodegeneration and abundant filaments made of hyperphosphorylated tau protein. By contrast, mouse lines expressing single isoforms of wild-type human tau do not produce tau filaments or show neurodegeneration. Here we have used tau-expressing lines to investigate whether experimental tauopathy can be transmitted. We show that injection of brain extract from mutant P301S tau-expressing mice into the brain of transgenic wild-type tau-expressing animals induces assembly of wild-type human tau into filaments and spreading of pathology from the site of injection to neighbouring brain regions.

Structure of hepatitis B surface antigen from subviral tubes determined by electron cryomicroscopy

Hepatitis B virus consists of an icosahedral core containing the double-stranded DNA genome, enveloped by a membrane with embedded surface proteins. The crystal structure of the core protein has been solved but little information about the structure of the surface proteins has so far been available. There are three sizes of surface protein, small (S), medium (M) and large (L), which form disulfide-bonded homo- and heterodimers. The three proteins, expressed from different start sites in the coding sequence, share the common C-terminal S region; the M protein contains an additional preS2 sequence N-terminal to S, and the L protein a further preS1 sequence N-terminal to M. In infected individuals, the surface proteins are produced in huge excess over the amount needed for viral envelopment and are secreted as a heterogeneous mixture of isometric and tubular subviral particles. We have used electron cryomicroscopy to study tubular particles extracted from human serum. Helical Fourier-Bessel analysis was used to calculate a low-resolution map, although it showed that the tubes were quite disordered. From the symmetry derived from this analysis, we used single-particle methods to improve the resolution. We found that the tubes had a diameter of approximately 250 A, with spike-like features projecting from the membrane. In the plane of the membrane the proteins appear to be close packed. We propose a model for the packing arrangement of surface protein dimers in the tubes.

Tau inclusions in retinal ganglion cells of human P301S tau transgenic mice: effects on axonal viability

Tau inclusions play a key role in the pathogenesis of tauopathies. Altered tau levels have been detected in retina and optic nerve of patients with glaucoma, suggesting the possibility of shared pathogenic mechanisms with tauopathies. Here we report that hyperphosphorylated transgenic tau accumulates in the nerve fibre layer and, from 2 months of age, aggregates into filamentous inclusions in retinal ganglion cells of human P301S tau transgenic mice. Axonopathy and accumulation of hyperphosphorylated tau in the nerve fibre layer preceded inclusion formation. Hyperphosphorylated tau and tau inclusions were also detected in cultured retinal explants from 5-month-old transgenic mice. Axonal outgrowth was similar in transgenic and wild-type retinal explants under basal conditions. However, when exposed to growth-promoting stimuli, axon elongation was enhanced in explants from wild-type but not transgenic mice, indicating that the presence of abnormal tau can impair stimulated axonal outgrowth. These findings suggest that the retina is a good model system for investigating tau-driven neurodegeneration and for assessing potential pharmacological modifiers for tauopathies.

Structure and disassembly of filaments formed by the ESCRT-III subunit Vps24

The ESCRT machinery mediates sorting of ubiquitinated transmembrane proteins to lysosomes via multivesicular bodies (MVBs) and also has roles in cytokinesis and viral budding. The ESCRT-III subunits are metastable monomers that transiently assemble on membranes. However, the nature of these assemblies is unknown. Among the core yeast ESCRT-III subunits, Snf7 and Vps24 spontaneously form ordered polymers in vitro. Single-particle EM reconstruction of helical Vps24 filaments shows both parallel and head-to-head subunit arrangements. Mutations of regions involved in intermolecular assembly in vitro result in cargo-sorting defects in vivo, suggesting that these homopolymers mimic interactions formed by ESCRT-III heteropolymers during MVB biogenesis. The C terminus of Vps24 is at the surface of the filaments and is not required for filament assembly. When this region is replaced by the MIT-interacting motif from the Vps2 subunit of ESCRT-III, the AAA-ATPase Vps4 can both bundle and disassemble the chimeric filaments in a nucleotide-dependent fashion.

The Leeuwenhoek lecture 2006. Microscopy goes cold: frozen viruses reveal their structural secrets

The electron microscope provides a powerful tool for investigating the structure of biological complexes such as viruses. A modern instrument is fully capable of atomic resolution on suitable non-biological specimens, but biological materials are difficult to preserve, owing to their fragility, and to image, owing to their radiation, sensitivity. The act of imaging the specimen severely damages it. Originally, samples were prepared by staining with a heavy metal salt, which provides a stable specimen but limits the amount of details that can be retrieved. Now particulate specimens, such as viruses, are prepared by rapid freezing of unstained material and observed in a frozen state with low doses of electrons. The resulting images require extensive computer processing to extract fully detailed three-dimensional information about the specimen. The whole process is referred to as single-particle electron cryomicroscopy. Using this approach, the structure of the human hepatitis B virus core was solved at the level of the protein fold. By comparing maps of RNA- and DNA-containing cores, it was possible to propose a model for the maturation and control of the envelopment of the virus during assembly. These examples show that cryomicroscopy offers great potential for understanding the structure and function of complex biological assemblies.

Abnormally phosphorylated tau is associated with neuronal and axonal loss in experimental autoimmune encephalomyelitis and multiple sclerosis

The pathological correlate of clinical disability and progression in multiple sclerosis is neuronal and axonal loss; however, the underlying mechanisms are unknown. Abnormal phosphorylation of tau is a common feature of some neurodegenerative disorders, such as Alzheimer's disease. We investigated the presence of tau hyperphosphorylation and its relationship with neuronal and axonal loss in chronic experimental autoimmune encephalomyelitis (CEAE) and in brain samples from patients with secondary progressive multiple sclerosis. We report the novel finding of abnormal tau phosphorylation in CEAE. We further show that accumulation of insoluble tau is associated with both neuronal and axonal loss that correlates with progression from relapsing-remitting to chronic stages of EAE. Significantly, analysis of secondary progressive multiple sclerosis brain tissue also revealed abnormally phosphorylated tau and the formation of insoluble tau. Together, these observations provide the first evidence implicating abnormal tau in the neurodegenerative phase of tissue injury in experimental and human demyelinating disease.

The tauopathy associated with mutation +3 in intron 10 of Tau: characterization of the MSTD family

Multiple system tauopathy with presenile dementia (MSTD) is an inherited disease caused by a (g) to (a) transition at position +3 in intron 10 of Tau. It belongs to the spectrum of frontotemporal dementia and parkinsonism linked to chromosome 17 with mutations in Tau (FTDP-17T). Here we present the longitudinal clinical, neuropsychological, neuroimaging, neuropathological, biochemical and genetic characterization of the MSTD family. Presenting signs were consistent with the behavioural variant of frontotemporal dementia in 17 of 21 patients. Two individuals presented with an atypical form of progressive supranuclear palsy and two others with either severe postural imbalance or an isolated short-term memory deficit. Memory impairment was present at the onset in 15 patients, with word finding difficulties and stereotyped speech also being common. Parkinsonism was first noted 3 years after the onset of symptoms. Neuroimaging showed the most extensive grey matter loss in the hippocampus, parahippocampal gyrus and frontal operculum/insular cortex of the right hemisphere and, to a lesser extent, in the anterior cingulate gyrus, head of the caudate nucleus and the posterolateral orbitofrontal cortex and insular cortex bilaterally. Neuropathologically, progressive nerve cell loss, gliosis and coexistent neuronal and/or glial deposits consisting mostly of 4-repeat tau were present in frontal, cingulate, temporal and insular cortices, white matter, hippocampus, parahippocampus, basal ganglia, selected brainstem nuclei and spinal cord. Tau haplotyping indicated that specific haplotypes of the wild-type allele may act as modifiers of disease presentation. Quantitative neuroimaging has been used to analyse the progression of atrophy in affected individuals and for predicting disease onset in an asymptomatic mutation carrier. This multidisciplinary study provides a comprehensive description of the natural history of disease in one of the largest known families with FTDP-17T.

Dimerisation of HIV-2 genomic RNA is linked to efficient RNA packaging, normal particle maturation and viral infectivity

Background

Retroviruses selectively encapsidate two copies of their genomic RNA, the Gag protein binding a specific RNA motif in the 5' UTR of the genome. In human immunodeficiency virus type 2 (HIV-2), the principal packaging signal (Psi) is upstream of the major splice donor and hence is present on all the viral RNA species. Cotranslational capture of the full length genome ensures specificity. HIV-2 RNA dimerisation is thought to occur at the dimer initiation site (DIS) located in stem-loop 1 (SL-1), downstream of the main packaging determinant. However, the HIV-2 packaging signal also contains a palindromic sequence (pal) involved in dimerisation. In this study, we analysed the role of the HIV-2 packaging signal in genomic RNA dimerisation in vivo and its implication in viral replication.

Results

Using a series of deletion and substitution mutants in SL-1 and the Psi region, we show that in fully infectious HIV-2, genomic RNA dimerisation is mediated by the palindrome pal. Mutation of the DIS had no effect on dimerisation or viral infectivity, while mutations in the packaging signal severely reduce both processes as well as RNA encapsidation. Electron micrographs of the Psi-deleted virions revealed a significant reduction in the proportion of mature particles and an increase in that of particles containing multiple cores.

Conclusion

In addition to its role in RNA encapsidation, the HIV-2 packaging signal contains a palindromic sequence that is critical for genomic RNA dimerisation. Encapsidation of a dimeric genome seems required for the production of infectious mature particles, and provides a promising therapeutic target.

Analysis of tau phosphorylation and truncation in a mouse model of human tauopathy

Recent evidence has suggested that truncation of tau protein at the caspase cleavage site D421 precedes hyperphosphorylation and may be necessary for the assembly of tau into filaments in Alzheimer's disease and other tauopathies. Here we have investigated the time course of the appearance of phosphorylated and truncated tau in the brain and spinal cord of mice transgenic for mutant human P301S tau protein. This mouse line recapitulates the essential molecular and cellular features of the human tauopathies, including tau hyperphosphorylation, tau filament formation, and neurodegeneration. Soluble tau was strongly phosphorylated at 1 to 6 months of age. Low levels of phosphorylated, sarkosyl-insoluble tau were detected at 2 months, with a steady increase up to 6 months of age. Tau truncated at D421 was detected at low levels in Tris-soluble and detergent-soluble tau at 3 to 6 months of age. By immunoblotting, it was not detected in sarkosyl-insoluble tau. However, by immunoelectron microscopy, a small percentage of tau in filaments from brain and spinal cord of transgenic mice was truncated at D421. Similar findings were obtained using dispersed filaments from Alzheimer's disease and FTDP-17 brains. The late appearance and low abundance of tau ending at D421 indicate that it is unlikely that truncation at this site is necessary for the assembly of tau into filaments.

Sequence Determinants for Amyloid Fibrillogenesis of Human alpha-Synuclein

Parkinson's disease (PD) and dementia with Lewy bodies (DLB) are characterized by the presence of filamentous inclusions in nerve cells. These filaments are amyloid fibrils that are made of the protein alpha-synuclein, which is genetically linked to rare cases of PD and DLB. beta-Synuclein, which shares 60% identity with alpha-synuclein, is not found in the inclusions. Furthermore, while recombinant alpha-synuclein readily assembles into amyloid fibrils, beta-synuclein fails to do so. It has been suggested that this may be due to the lack in beta-synuclein of a hydrophobic region that spans residues 73-83 of alpha-synuclein. Here, fibril assembly of recombinant human alpha-synuclein, alpha-synuclein deletion mutants, beta-synuclein and beta/alpha-synuclein chimeras was assayed quantitatively by thioflavin T fluorescence and semi-quantitatively by transmission electron microscopy. Deletion of residues 73-83 from alpha-synuclein did not abolish filament formation. Furthermore, a chimera of beta-synuclein with alpha-synuclein(73-83) inserted was significantly less fibrillogenic than wild-type alpha-synuclein. These findings, together with results obtained using a number of recombinant synucleins, showed a correlation between fibrillogenesis and mean beta-strand propensity, hydrophilicity and charge of the amino acid sequences. The combination of these simple physicochemical properties with a previously described calculation of beta-strand contiguity allowed us to design mutations that changed the fibrillogenic propensity of alpha-synuclein in predictable ways.

The novel Tau mutation G335S: clinical, neuropathological and molecular characterization

Mutations in Tau cause the inherited neurodegenerative disease, frontotemporal dementia and Parkinsonism linked to chromosome 17 (FTDP-17). Known coding region mutations cluster in the microtubule-binding region, where they alter the ability of tau to promote microtubule assembly. Depending on the tau isoforms, this region consists of three or four imperfect repeats of 31 or 32 amino acids, each of which contains a characteristic and invariant PGGG motif. Here, we report the novel G335S mutation, which changes the PGGG motif of the third tau repeat to PGGS, in an individual who developed social withdrawal, emotional bluntness and stereotypic behavior at age 22, followed by disinhibition, hyperorality and ideomotor apraxia. Abundant tau-positive inclusions were present in neurons and glia in the frontotemporal cortex, hippocampus and brainstem. Sarkosyl-insoluble tau showed paired helical and straight filaments, as well as more irregular rope-like filaments. The pattern of pathological tau bands was like that of Alzheimer disease. Experimentally, the G335S mutation resulted in a greatly reduced ability of tau to promote microtubule assembly, while having no significant effect on heparin-induced assembly of recombinant tau into filaments.

Tau protein, the paired helical filament and Alzheimer's disease

In 1906, Alzheimer described the clinical and neuropathological characteristics of the disease that was subsequently named after him. Although the paired helical filament was identified as the major component of the neurofibrillary pathology of Alzheimer's disease in 1963, its molecular composition was only uncovered in the 1980s. In 1988, work at the MRC Laboratory of Molecular Biology in Cambridge (UK) provided direct proof that tau protein is an integral component of the paired helical filament. The paper highlighted here [Goedert M., Wischik C.M., Crowther R.A., Walker J.E. and Klug A. (1988) Cloning and sequencing of a core protein of the paired helical filament of Alzheimer disease: Identification as the microtubule-associated protein tau. Proc. Natl. Acad. Sci. USA 85, 4051-4055] also reported the first sequerce of a human tau isoform and paved the way for the identification of the six brain tau isoforms that are expressed by alternative mRNA splicing from a single gene. By the early 1990s, it was clear that tau protein is the major component of the paired helical filament and that the latter is made of all six tau isoforms, each full-length and hyperphosphorylated.

Synuclein proteins of the pufferfish Fugu rubripes: sequences and functional characterization

In humans, three genes encode the related alpha-, beta-, and gamma-synucleins, which function as lipid-binding proteins in vitro. They are being widely studied, mainly because of the central involvement of alpha-synuclein in a number of neurodegenerative diseases, including Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy. In these diseases, the normally soluble alpha-synuclein assembles into abnormal filaments. Here, we have identified and characterized the synuclein gene family from the pufferfish Fugu rubripes. It consists of four genes, which encode alpha-, beta-, gamma1-, and gamma2-synucleins. They range from 113 to 127 amino acids in length and share many of the characteristics of human synucleins, including the presence of imperfect amino-terminal repeats of 11 amino acids, a hydrophobic middle region, and a negatively charged carboxy-terminus. All four synucleins are expressed in the Fugu brain. Recombinant Fugu synucleins exhibited differential liposome binding, which was strongest for alpha-synuclein, followed by beta-, gamma2-, and gamma1-synucleins. In assembly experiments, Fugu alpha-, gamma1-, and gamma2-synucleins formed filaments more readily than human alpha-synuclein. Fugu beta-synuclein, by contrast, failed to assemble in bulk. Filament assembly of synucleins was directly proportional to their degree of hydrophobicity and their tendency to form beta-sheet structure, and correlated inversely with their net charge.

CTF determination and correction in electron cryotomography

Electron cryotomography (cryoET) has the potential to elucidate the structure of complex biological specimens at molecular resolution but technical and computational improvements are still needed. This work addresses the determination and correction of the contrast transfer function (CTF) of the electron microscope in cryoET. Our approach to CTF detection and defocus determination depends on strip-based periodogram averaging, extended throughout the tilt series to overcome the low contrast conditions found in cryoET. A method for CTF correction that deals with the defocus gradient in images of tilted specimens is also proposed. These approaches to CTF determination and correction have been applied here to several examples of cryoET of pleomorphic specimens and of single particles. CTF correction is essential for improving the resolution, particularly in those studies that combine cryoET with single particle averaging techniques.

A structural model for maturation of the hepatitis B virus core

Hepatitis B virus, a widespread and serious human pathogen, replicates by reverse transcription of an RNA intermediate. The virus consists of an inner nucleocapsid or core, surrounded by a lipid envelope containing virally encoded surface proteins. Using electron cryomicroscopy, we compare the structures of the bacterially expressed RNA-containing core particle and the mature DNA-containing core particle extracted from virions. We show that the mature core contains 240 subunits in a T = 4 arrangement similar to that in expressed core (T is the triangulation number and the icosahedral shell contains 60 T subunits). During the infective cycle, the core assembles in an immature state around a complex of viral pregenomic RNA and polymerase. After reverse transcription with concomitant degradation of the RNA, the now mature core buds through a cellular membrane containing the surface proteins to become enveloped. Envelopment must not happen before reverse transcription is completed, so it has been hypothesized that a change in capsid structure may signal maturation. Our results show significant differences in structure between the RNA- and DNA-containing cores. One such difference is in a hydrophobic pocket, formed largely from residues that, on mutation, lead to abnormal secretion. We suggest that the changes we see are related to maturation and control of envelopment, and we propose a mechanism based on DNA synthesis for their triggering.

Abundant neuritic inclusions and microvacuolar changes in a case of diffuse Lewy body disease with the A53T mutation in the alpha-synuclein gene

We report here a case of diffuse Lewy body disease with the A53T mutation in the alpha-synuclein gene. The proband presented at the age of 41 years with parkinsonism that was poorly responsive to levodopa. She subsequently developed cognitive impairment and moderate dementia, and died at the age of 50. Her father, paternal grandfather and uncle were all reported to have suffered from Parkinson's disease. Staining of tissue sections from the proband's brain with hematoxylin-eosin and alpha-synuclein antibodies showed small numbers of Lewy bodies in a few brain regions. This contrasted with large numbers of Lewy neurites and neuroaxonal spheroids in many brain regions. By electron microscopy, Lewy neurites consisted of abnormal filaments and dense granular material. Isolated filaments resembled those previously described in idiopathic Parkinson's disease and dementia with Lewy bodies. They were decorated by antibodies specific for the N and C termini of alpha-synuclein, indicating the presence of the full-length protein. Nucleus accumbens and the lower layers in limbic areas of the cerebral cortex showed prominent vacuolation, with frequent clustering of microvacuoles around Lewy neurites. Nerve cell loss was most extensive in dorsal motor nucleus of the vagus nerve, substantia nigra and nucleus basalis of Meynert. Neurofibrillary tangles and senile plaques were not observed. However, in several brain regions, a few widely scattered tau-positive nerve cell bodies and neurites were present. By electron microscopy, Alzheimer-type paired helical and straight filaments were seen.

Tau and α-Synuclein Inclusions in a Case of Familial Frontotemporal Dementia and Progressive Aphasia

Recent studies have shown that neurofibrillary tangles are frequently accompanied by alpha-synuclein inclusions in sporadic and familial Alzheimer disease, in Down syndrome, in progressive supranuclear palsy, and Parkinsonism dementia complex of Guam. Here we report the cases of 2 brothers with familial progressive aphasia who developed features of frontotemporal dementia with predominant tau pathology but also alpha-synuclein pathology. The 2 patients' brains revealed abundant tau pathology in the hippocampus and basal ganglia, whereas tau and alpha-synuclein aggregates coexisted only in the nucleus basalis of Meynert, the only region where alpha-synuclein was present. In this brain region, abundant Lewy bodies, Lewy neurites, and tau inclusions were found; the pathology was more abundant in the older than in the younger brother. Sarkosyl-insoluble tau extracted from brains of the 2 patients showed the presence of tau filaments that contained 3 major tau bands of 60, 64, and 68 kDa on Western blot analysis. These bands contained mainly tau with 3 and 4 repeats and no amino-terminal inserts and tau with 4 repeats and one amino-terminal insert. No mutations were identified in the tau, alpha-synuclein, beta-synuclein, or parkin genes. We think that this is the first report showing a specific colocalization of neurofibrillary tangles and Lewy bodies in a family with progressive aphasia.

Viruses and the development of quantitative biological electron microscopy

The electron microscope has become an important tool for determining the structure of biological materials of all kinds. Many technical advances in specimen preparation and in sophisticated methods of image analysis, initially based on optical systems but latterly on computer processing, have contributed to the development of the subject. Viruses of various kinds have often provided a convenient and appropriate test specimen. This paper describes the major technical advances and shows how viruses have had an important role in most of the developments.

Mutation E46K increases phospholipid binding and assembly into filaments of human α-synuclein

Missense mutations (A30P and A53T) in alpha-synuclein and the overproduction of the wild-type protein cause familial forms of Parkinson's disease and dementia with Lewy bodies. Alpha-synuclein is the major component of the filamentous Lewy bodies and Lewy neurites that define these diseases at a neuropathological level. Recently, a third missense mutation (E46K) in alpha-synuclein was described in an inherited form of dementia with Lewy bodies. Here, we have investigated the functional effects of this novel mutation on phospholipid binding and filament assembly of alpha-synuclein. When compared to the wild-type protein, the E46K mutation caused a significantly increased ability of alpha-synuclein to bind to negatively charged liposomes, unlike the previously described mutations. The E46K mutation increased the rate of filament assembly to the same extent as the A53T mutation. Filaments formed from E46K alpha-synuclein often had a twisted morphology with a cross-over spacing of 43 nm. The observed effects on lipid binding and filament assembly may explain the pathogenic nature of the E46K mutation in alpha-synuclein.

An amino-terminal segment of hantavirus nucleocapsid protein presented on hepatitis B virus core particles induces a strong and highly cross-reactive antibody response in mice

Previously, we have demonstrated that hepatitis B virus (HBV) core particles tolerate the insertion of the amino-terminal 120 amino acids (aa) of the Puumala hantavirus nucleocapsid (N) protein. Here, we demonstrate that the insertion of 120 amino-terminal aa of N proteins from highly virulent Dobrava and Hantaan hantaviruses allows the formation of chimeric core particles. These particles expose the inserted foreign protein segments, at least in part, on their surface. Analysis by electron cryomicroscopy of chimeric particles harbouring the Puumala virus (PUUV) N segment revealed 90% T = 3 and 10% T = 4 shells. A map computed from T = 3 shells shows additional density splaying out from the tips of the spikes producing the effect of an extra shell of density at an outer radius compared with wild-type shells. The inserted Puumala virus N protein segment is flexibly linked to the core spikes and only partially icosahedrally ordered. Immunisation of mice of two different haplotypes (BALB/c and C57BL/6) with chimeric core particles induces a high-titered and highly cross-reactive N-specific antibody response in both mice strains.

Early-onset dementia with Lewy bodies

The clinical and neuropathological characteristics of an atypical form of dementia with Lewy bodies (DLB) are described. The proband experienced difficulties in her school performance at 13 years of age. Neurological examination revealed cognitive dysfunction, dysarthria, parkinsonism and myoclonus. By age 14 years, the symptoms had worsened markedly and the proband died at age 15 years. On neuropathological examination, the brain was severely atrophic. Numerous intracytoplasmic and intraneuritic Lewy bodies, as well as Lewy neurites, were present throughout the cerebral cortex and subcortical nuclel; vacuolar changes were seen in the upper layers of the neocortex and severe neuronal loss and gliosis were evident in the cerebral cortex and substantia nigra. Lewy bodies and Lewy neurites were strongly immunoreactive for alpha-synuclein and ubiquitin. Lewy bodies were composed of filamentous and granular material and isolated filaments were decorated by alpha-synuclein antibodies. Immunohistochemistry for tau or beta-amyloid yielded negative results. The etiology of this atypical form of DLB is unknown, since there was no family history and since sequencing of the exonic regions of alpha-Synuclein, beta-Synuclein, Synphilin-1, Parkin, Ubiquitin C-terminal hydrolase L1 and Neurofilament-M failed to reveal a pathogenic mutation. This study provides further evidence of the clinical and pathological heterogeneity of DLB.

Comparison of the structures of three circoviruses: chicken anemia virus, porcine circovirus type 2, and beak and feather disease virus

Circoviruses are small, nonenveloped icosahedral animal viruses characterized by circular single-stranded DNA genomes. Their genomes are the smallest possessed by animal viruses. Infections with circoviruses, which can lead to economically important diseases, frequently result in virus-induced damage to lymphoid tissue and immunosuppression. Within the family Circoviridae, different genera are distinguished by differences in genomic organization. Thus, Chicken anemia virus is in the genus Gyrovirus, while porcine circoviruses and Beak and feather disease virus belong to the genus CIRCOVIRUS: Little is known about the structures of circoviruses. Accordingly, we investigated the structures of these three viruses with a view to determining whether they are related. Three-dimensional maps computed from electron micrographs showed that all three viruses have a T=1 organization with capsids formed from 60 subunits. Porcine circovirus type 2 and beak and feather disease virus show similar capsid structures with flat pentameric morphological units, whereas chicken anemia virus has stikingly different protruding pentagonal trumpet-shaped units. It thus appears that the structures of viruses in the same genus are related but that those of viruses in different genera are unrelated.

Viruses and the development of quantitative biological electron microscopy

The electron microscope has become an important tool for determining the structure of biological materials of all kinds. Many technical advances in specimen preparation and in sophisticated methods of image analysis, initially based on optical systems but latterly on computer processing, have contributed to the development of the subject. Viruses of various kinds have often provided a convenient and appropriate test specimen. This paper describes the major technical advances and shows how viruses have had an important role in most of the developments.

Variable phenotypic expression and extensive tau pathology in two families with the novel tau mutation L315R

Mutations in the tau gene cause familial frontotemporal dementia and parkinsonism linked to chromosome 17. Here, we describe two Dutch families with familial frontotemporal dementia associated with the novel missense mutation L315R in exon 11 of tau. The age at onset of disease showed a large variation within each family, ranging from 25 to 64 years. Incomplete penetrance was established in an 82-year-old mutation carrier with no signs of dementia and appeared probable in two additional subjects. The brains of two affected subjects were studied and showed extensive tau pathology in neurons (Pick-like inclusions) and astroglial cells, particularly in the frontotemporal cortex and the hippocampal formation. Sarkosyl-insoluble tau extracted from the cerebral cortex showed the presence of straight and twisted tau filaments and a pattern of pathological tau bands similar to that of Pick's disease. Upon dephosphorylation, only five of the six brain tau isoforms were observed, with the shortest isoform being undetectable. All six tau isoforms were present in soluble brain tau. Recombinant tau proteins with the L315R mutation showed a reduced ability to promote microtubule assembly.

Tau Protein in Frontotemporal Dementia Linked to Chromosome 3 (FTD-3)

Recent work on frontotemporal dementia (FTD) has revealed the existence of at least 3 genetically distinct groups of inherited FTD: FTDP-17, FTD and motor neuron disease linked to chromosome 9, and FTD linked to chromosome 3 (FTD-3). Tau, on chromosome 17, is the only gene where mutations have been identified and its involvement in FTD has been firmly established. The genes on chromosome 9 and chromosome 3 associated with familial forms of FTD remain to be identified. Abnormal aggregates of tau protein characterize the brain lesions of FTDP-17 patients and ubiquitin inclusions have been found in FTD with motor neuron disease linked to chromosome 9. In this study the frontal cortices of 3 FTD-3 patients from a unique Danish family were examined for characteristic neuropathological features. In these brains tau inclusions were present in neurons and some glial cells in the absence of beta-amyloid deposits. The presence of filamentous tau protein in the frontal cortex of these patients suggests a possible link between tau and the genetic defect present on chromosome 3 and associated with FTD-3, although the limited amount of tau deposits observed makes it difficult to define this as a tauopathy.

Tau filaments from human brain and from in vitro assembly of recombinant protein show cross-beta structure

Abnormal filaments consisting of hyperphosphorylated microtubule-associated protein tau form in the brains of patients with Alzheimer's disease, Down's syndrome, and various dementing tauopathies. In Alzheimer's disease and Down's syndrome, the filaments have two characteristic morphologies referred to as paired helical and straight filaments, whereas in tauopathies, there is a wider range of morphologies. There has been controversy in the literature concerning the internal molecular fine structure of these filaments, with arguments for and against the cross-beta structure demonstrated in many other amyloid fibers. The difficulty is to produce from brain pure preparations of filaments for analysis. One approach to avoid the need for a pure preparation is to use selected area electron diffraction from small groups of filaments of defined morphology. Alternatively, it is possible to assemble filaments in vitro from expressed tau protein to produce a homogeneous specimen suitable for analysis by electron diffraction, x-ray diffraction, and Fourier transform infrared spectroscopy. Using both these approaches, we show here that native filaments from brain and filaments assembled in vitro from expressed tau protein have a clear cross-beta structure.

Abundant tau filaments and nonapoptotic neurodegeneration in transgenic mice expressing human P301S tau protein

The identification of mutations in the Tau gene in frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) has made it possible to express human tau protein with pathogenic mutations in transgenic animals. Here we report on the production and characterization of a line of mice transgenic for the 383 aa isoform of human tau with the P301S mutation. At 5-6 months of age, homozygous animals from this line developed a neurological phenotype dominated by a severe paraparesis. According to light microscopy, many nerve cells in brain and spinal cord were strongly immunoreactive for hyperphosphorylated tau. According to electron microscopy, abundant filaments made of hyperphosphorylated tau protein were present. The majority of filaments resembled the half-twisted ribbons described previously in cases of FTDP-17, with a minority of filaments resembling the paired helical filaments of Alzheimer's disease. Sarkosyl-insoluble tau from brains and spinal cords of transgenic mice ran as a hyperphosphorylated 64 kDa band, the same apparent molecular mass as that of the 383 aa tau isoform in the human tauopathies. Perchloric acid-soluble tau was also phosphorylated at many sites, with the notable exception of serine 214. In the spinal cord, neurodegeneration was present, as indicated by a 49% reduction in the number of motor neurons. No evidence for apoptosis was obtained, despite the extensive colocalization of hyperphosphorylated tau protein with activated MAP kinase family members. The latter may be involved in the hyperphosphorylation of tau.

Functional effects of tau gene mutations deltaN296 and N296H

Mutations in the tau gene cause frontotemporal dementia and parkinsonism linked to chromosome-17 (FTDP-17). Functionally, about half of the known mutations increase the alternative mRNA splicing of exon 10 of the tau gene, resulting in the overproduction of tau isoforms with four microtubule-binding repeats. The other mutations reduce the ability of tau to interact with microtubules, with some mutations also increasing the propensity of tau to assemble into filaments. Here we have examined the functional effects of the recently described tau gene mutations deltaN296 and N296H. Both mutations reduced the ability of tau to promote microtubule assembly, without having a significant effect on tau filament formation. By exon trapping, they increased the splicing of exon 10. DeltaN296 and N296H thus define a class of tau mutations with effects at both the RNA and the protein level.

Pick's disease associated with the novel Tau gene mutation K369I

Exonic and intronic mutations in Tau cause neurodegenerative syndromes characterized by frontotemporal dementia and filamentous tau protein deposits. We describe a K369I missense mutation in exon 12 of Tau in a patient with a pathology typical of sporadic Pick's disease. The proband presented with severe personality changes, followed by loss of cognitive function. Detailed postmortem examination of the brain showed atrophy, which was most pronounced in the temporal lobes; and numerous tau-immunoreactive Pick bodies and Pick cells in the neocortex and the hippocampal formation, as well as in subcortical brain regions. Their appearance and staining characteristics were indistinguishable from those of sporadic Pick's disease. However, immunoblot analysis of sarkosyl-insoluble tau showed three major bands of 60, 64, and 68 kDa, consistent with the presence of 3- and 4-repeat tau isoforms, as in Alzheimer's disease. Isolated tau filaments were irregularly twisted ribbons, with a small number of Alzheimer-type paired helical filaments. In the presence of heparin, tau proteins with the K369I mutation formed short, slender filaments. Biochemically, recombinant tau proteins with the K369I mutation showed reduced ability to promote microtubule assembly, suggesting that this may be the primary effect of the mutation by providing a pool of aberrant tau for filament assembly. Taken together, results indicate that the K369I mutation in Tau can cause a dementing disease with a neuropathology like that of Pick's disease.

From genetics to pathology: tau and alpha-synuclein assemblies in neurodegenerative diseases

The most common degenerative diseases of the human brain are characterized by the presence of abnormal filamentous inclusions in affected nerve cells and glial cells. These diseases can be grouped into two classes, based on the identity of the major proteinaceous components of the filamentous assemblies. The filaments are made of either the microtubule-associated protein tau or the protein alpha-synuclein. Importantly, the discovery of mutations in the tau gene in familial forms of frontotemporal dementia and of mutations in the alpha-synuclein gene in familial forms of Parkinson's disease has established that dysfunction of tau protein and alpha-synuclein can cause neurodegeneration.

The natural osmolyte trimethylamine N-oxide (TMAO) restores the ability of mutant tau to promote microtubule assembly

Coding region and intronic mutations in the gene for microtubule-associated protein tau cause frontotemporal dementia and Parkinsonism linked to chromosome 17 (FTDP-17). Most coding region mutations effect a reduced ability of tau protein to interact with microtubules and lead to the formation of a filamentous pathology made of hyperphosphorylated tau. Here we show that trimethylamine N-oxide (TMAO) restores the ability of tau with FTDP-17 mutations to promote microtubule assembly. To mimic phosphorylation, serine and threonine residues in tau were singly or multiply mutated to glutamic acid, resulting in a reduced ability of tau to promote microtubule assembly. With the exception of the most heavily substituted protein (27 glutamic acid residues), TMAO increased the ability of mutant tau to promote microtubule assembly. However, it had no significant effect on heparin-induced assembly of tau into filaments.

Tau gene mutation K257T causes a tauopathy similar to Pick's disease

Exonic and intronic mutations in Tau cause neurodegenerative syndromes characterized by frontotemporal dementia and filamentous tau protein deposits. Here we describe a K257T missense mutation in exon 9 of Tau. The proband, a 47-yr-old male, presented with severe personality changes followed by semantic memory loss. A diagnosis of Pick's disease was made. The symptoms progressed until death at age 51. The proband's brain showed a marked frontotemporal atrophy that was most pronounced in the temporal lobes. Numerous tau-immunoreactive Pick bodies were present in the neocortex and the hippocampal formation, as well as in some subcortical brain regions. Their appearance and staining characteristics were indistinguishable from those of sporadic Pick's disease. Diffuse staining for hyperphosphorylated tau was also observed in some nerve cell bodies. Immunoblot analysis of sarkosyl-insoluble tau showed 2 major bands of 60 and 64 kDa and 2 very minor bands of 68 and 72 kDa. Upon dephosphorylation, these bands resolved into 6 bands consisting of 3-repeat and 4-repeat tau isoforms, with an overall preponderance of 3-repeat tau. Isolated tau filaments were narrow, irregularly twisted ribbons. Biochemically, recombinant tau proteins with the K257T mutation showed a reduced ability to promote microtubule assembly, suggesting that this may be the primary effect of the mutation. In addition, the K257T mutation was found to stimulate heparin-induced assembly of 3-repeat tau into filaments. Taken together, the present findings indicate that the K257T mutation in Tau can cause a dementing condition similar to Pick's disease.

Characterisation of isolated alpha-synuclein filaments from substantia nigra of Parkinson's disease brain

The defining neuropathological deposits of Parkinson's disease, dementia with Lewy bodies and multiple system atrophy are strongly immunoreactive for alpha-synuclein. We have shown previously that isolated filaments from dementia with Lewy bodies and multiple system atrophy brains are labelled in a characteristic fashion by a number of alpha-synuclein antibodies. Here we have extracted filaments from substantia nigra of patients with idiopathic Parkinson's disease. Antibodies directed against the carboxy-terminal region of alpha-synuclein labelled isolated filaments along their entire lengths. By contrast, an antibody directed against the amino-terminal region of alpha-synuclein only labelled one filament end. These characteristics were identical to those of filaments extracted from brains of patients with dementia with Lewy bodies and multiple system atrophy.

Abnormal tau-containing filaments in neurodegenerative diseases

It has been known for some time that the neurofibrillary pathology in Alzheimer's disease consists of so-called paired helical and straight filaments made up of the microtubule-associated protein tau. The degree of dementia observed in the disease correlates better with the extent of neurofibrillary pathology than with the Abeta amyloid deposits, the other characteristic defining pathological fibrous deposit in Alzheimer's disease. However, no familial cases of Alzheimer's disease have been genetically linked to the tau protein locus. Recently a group of frontotemporal dementias with parkinsonism linked to chromosome 17 has been shown to be caused by mutations in the tau gene. Some are missense mutations giving altered tau proteins, whereas others affect the splicing of the pre-mRNA and change the balance between different tau isoforms. Histologically these diseases are all characterised by various kinds of filamentous tau protein deposits, mostly in the complete absence of Abeta deposits. The abnormal tau filaments show different morphologies, depending on the nature of the tau mutation. These diseases show that tau mutations can be a prime cause of inherited dementing illness and may throw some light on the pathological process in the much larger number of sporadic cases of Alzheimer's disease.

Axonopathy and amyotrophy in mice transgenic for human four-repeat tau protein

Coding region and intronic mutations in the tau gene cause frontotemporal dementia and parkinsonism linked to chromosome 17. Some of these mutations lead to an overproduction of tau isoforms with four microtubule-binding repeats. Here we have expressed the longest four-repeat human brain tau isoform in transgenic mice under the control of the murine Thy1 promoter. Transgenic mice aged 3 weeks to 25 months overexpressed human tau protein in nerve cells of brain and spinal cord. Numerous abnormal, tau-immunoreactive nerve cell bodies and dendrites were seen. In addition, large numbers of pathologically enlarged axons containing neurofilament- and tau-immunoreactive spheroids were present, especially in spinal cord. Signs of Wallerian degeneration and neurogenic muscle atrophy were observed. When motor function was tested, transgenic mice showed signs of muscle weakness. Taken together, these findings demonstrate that overexpression of human four-repeat tau leads to a central and peripheral axonopathy that results in nerve cell dysfunction and amyotrophy.