MAP2 caps tau fibrils and inhibits aggregation

Fibrils of the microtubule-associated protein tau are intimately linked to the pathology of Alzheimer's disease (AD) and related neurodegenerative disorders. A current paradigm for pathology spreading in the human brain is that short tau fibrils transfer between neurons and then recruit naive tau monomers onto their tips, perpetuating the fibrillar conformation with high fidelity and speed. Although it is known that the propagation could be modulated in a cell-specific manner and thereby contribute to phenotypic diversity, there is still limited understanding of how select molecules are involved in this process. MAP2 is a neuronal protein that shares significant sequence homology with the repeat-bearing amyloid core region of tau. There is discrepancy about MAP2's involvement in pathology and its relationship with tau fibrillization. Here, we employed the entire repeat regions of 3R and 4R MAP2, to investigate their modulatory role in tau fibrillization. We find that both proteins block the spontaneous and seeded aggregation of 4R tau, with 4R MAP2 being slightly more potent. The inhibition of tau seeding is observed in vitro, in HEK293 cells, and in AD brain extracts, underscoring its broader scope. MAP2 monomers specifically bind to the end of tau fibrils, preventing recruitment of further tau and MAP2 monomers onto the fibril tip. The findings uncover a new function for MAP2 as a tau fibril cap that could play a significant role in modulating tau propagation in disease and may hold promise as a potential intrinsic protein inhibitor.

A thiol-based intramolecular redox switch in four-repeat tau controls fibril assembly and disassembly

Oxidative stress has been implicated in the pathogenesis and progression of several tauopathies, including Alzheimer's disease. The deposition of fibrillar inclusions made of tau protein is one of the pathological hallmarks of these disorders. Although it is becoming increasingly evident that the specific fibril structure may vary from one tauopathy to another and it is recognized that different types of isoforms (three-repeat and four-repeat tau) can be selectively deposited, little is known about the role oxidation may play in aggregation. Four-repeat tau contains two cysteines that can form an intramolecular disulfide bond, resulting in a structurally restrained compact monomer. There is discrepancy as to whether this monomer can aggregate or not. Using isolated four-repeat tau monomers (htau40) with intramolecular disulfide bonds, we demonstrate that these proteins form fibrils. The fibrils are less stable than fibrils formed under reducing conditions but are highly effective in seeding oxidized tau monomers. Conversely, a strong seeding barrier prevents incorporation of reduced tau monomers, tau mimics in which the cysteines have been replaced by alanines or serines, and three-repeat tau (htau23), a single-cysteine isoform. The barrier also holds true when seed and monomer types are reversed, indicating that oxidized and reduced tau are incompatible with each other. Surprisingly, fibrils composed of compact tau disaggregate upon reduction, highlighting the importance of the intramolecular disulfide bond for fibril stability. The findings uncover a novel binary redox switch that controls the aggregation and disaggregation of these fibrils and extend the conformational spectrum of tau aggregates.

Conformational fingerprinting of tau variants and strains by Raman spectroscopy

Tauopathies are a group of disorders in which the deposition of abnormally folded tau protein accompanies neurodegeneration. The development of methods for detection and classification of pathological changes in protein conformation are desirable for understanding the factors that influence the structural polymorphism of aggregates in tauopathies. We have previously demonstrated the utility of Raman spectroscopy for the characterization and discrimination of different protein aggregates, including tau, based on their unique conformational signatures. Building on this, in the present study, we assess the utility of Raman spectroscopy for characterizing and distinguishing different conformers of the same protein which in the case of tau are unique tau strains generated in vitro. We now investigate the impact of aggregation environment, cofactors, post-translational modification and primary sequence on the Raman fingerprint of tau fibrils. Using quantitative conformational fingerprinting and multivariate statistical analysis, we found that the aggregation of tau in different buffer conditions resulted in the formation of distinct fibril strains. Unique spectral markers were identified for tau fibrils generated using heparin or RNA cofactors, as well as for phosphorylated tau. We also determined that the primary sequence of the tau monomer influenced the conformational signature of the resulting tau fibril, including 2N4R, 0N3R, K18 and P301S tau variants. These results highlight the conformational polymorphism of tau fibrils, which is reflected in the wide range of associated neurological disorders. Furthermore, the analyses presented in this study provide a benchmark for the Raman spectroscopic characterization of tau strains, which may shed light on how the aggregation environment, cofactors and post-translational modifications influence tau conformation in vivo in future studies.

Assessment of Long-Term Effects of Sports-Related Concussions: Biological Mechanisms and Exosomal Biomarkers

Concussion or mild traumatic brain injury (mTBI) in athletes can cause persistent symptoms, known as post-concussion syndrome (PCS), and repeated injuries may increase the long-term risk for an athlete to develop neurodegenerative diseases such as chronic traumatic encephalopathy (CTE), and Alzheimer's disease (AD). The Center for Disease Control estimates that up to 3.8 million sport-related mTBI are reported each year in the United States. Despite the magnitude of the phenomenon, there is a current lack of comprehensive prognostic indicators and research has shown that available monitoring tools are moderately sensitive to short-term concussion effects but less sensitive to long-term consequences. The overall aim of this review is to discuss novel, quantitative, and objective measurements that can predict long-term outcomes following repeated sports-related mTBIs. The specific objectives were (1) to provide an overview of the current clinical and biomechanical tools available to health practitioners to ensure recovery after mTBIs, (2) to synthesize potential biological mechanisms in animal models underlying the long-term adverse consequences of mTBIs, (3) to discuss the possible link between repeated mTBI and neurodegenerative diseases, and (4) to discuss the current knowledge about fluid biomarkers for mTBIs with a focus on novel exosomal biomarkers. The conclusions from this review are that current post-concussion clinical tests are not sufficiently sensitive to injury and do not accurately quantify post-concussion alterations associated with repeated mTBIs. In the current review, it is proposed that current practices should be amended to include a repeated symptom inventory, a cognitive assessment of executive function and impulse control, an instrumented assessment of balance, vestibulo-ocular assessments, and an improved panel of blood or exosome biomarkers.

Time-resolved multirotational dynamics of single solution-phase tau proteins reveals details of conformational variation

Intrinsically disordered proteins (IDPs) are crucial to many cellular processes and have been linked to neurodegenerative diseases. Single molecules of tau, an IDP associated with Alzheimer's disease, are trapped in solution using a microfluidic device, and a time-resolved fluorescence anisotropy decay is recorded for each molecule. Multiple rotational components are resolved and a novel k-means algorithm is used to sort the molecules into two families of conformations. Differences in rotational dynamics suggest a change in the rigidity and steric hindrance surrounding a sequence (306VQIVYK311) which is central to paired helical filament formation. This single-molecule approach can be applied to other IDPs to resolve heterogeneous populations and underlying differences in conformational dynamics.

Driving tau into phase-separated liquid droplets

Liquid-liquid phase separation of tau protein has been implicated in normal biological function as well as neurodegenerative diseases, including Alzheimer's. However, knowledge about these links is still scant, and the mechanisms driving tau into liquid droplets are poorly understood. A simplified in vitro system that uses unmodified human tau protein now suggests electrostatic interactions provide the basic instructions underlying liquid droplet formation.

Structural disorder in four-repeat Tau fibrils reveals a new mechanism for barriers to cross-seeding of Tau isoforms

The intracellular deposition of fibrils composed of the microtubule-associated protein Tau is a characteristic feature of Alzheimer's disease (AD) and other fatal neurodegenerative disorders collectively known as tauopathies. Short Tau fibrils spread intracerebrally through transfer between interconnected neurons. Once taken up by a recipient cell, Tau fibrils recruit Tau monomers onto their ends. Based on the number of microtubule-binding repeats, there are two distinct groups of Tau isoforms: three-repeat (3R) Tau and four-repeat (4R) Tau. In AD, all Tau isoforms are deposited, whereas in other tauopathies, only 3R or 4R Tau isoforms are deposited. The molecular basis for these isoform-specific depositions is poorly understood, although conformation-based cross-seeding barriers are key. Here, we used sedimentation assays, EPR spectroscopy, and other structural readouts to better understand the cross-seeding barriers of 4R Tau fibrils. We observed that fibrils formed from truncated Tau (K18), but not full-length Tau (htau40), exhibit a barrier that inhibits 3R Tau recruitment. Investigating an array of differently sized fragments, we found that the Tau C terminus modulates the cross-seeding barrier and that the N terminus plays a synergistic role. Two disease-associated Tau variants, P301S and P301L, also established strong cross-seeding barriers. EPR analysis indicated that fibrils seeded with truncated and mutated Tau, but not htau40, are structurally disordered in the second half of repeat four and onward. These findings suggest that the disorder in this region diminishes the ability of 4R Tau fibrils to recruit 3R Tau monomers, revealing a new mechanism for Tau cross-seeding barriers.

The distinct structural preferences of tau protein repeat domains

The tau fibrillar structures from the brain of an Alzheimer's patient have a core with a C-shaped motif of the third and fourth repeat domains (R3-R4). Our simulations indicated that the C-shaped motif is only stable for R3-R4, while R1-R2 tends to be linear in shape. These two structural motifs appear in the most stable K18 protofilament. Heparin can further stabilize the C-shaped R3-R4 motif, but not other repeats.

Revealing Conformational Variants of Solution-Phase Intrinsically Disordered Tau Protein at the Single-Molecule Level

Intrinsically disordered proteins, such as tau protein, adopt a variety of conformations in solution, complicating solution-phase structural studies. We employed an anti-Brownian electrokinetic (ABEL) trap to prolong measurements of single tau proteins in solution. Once trapped, we recorded the fluorescence anisotropy to investigate the diversity of conformations sampled by the single molecules. A distribution of anisotropy values obtained from trapped tau protein is conspicuously bimodal while those obtained by trapping a globular protein or individual fluorophores are not. Time-resolved fluorescence anisotropy measurements were used to provide an explanation of the bimodal distribution as originating from a shift in the compaction of the two different families of conformations.

How Does Hyperphopsphorylation Promote Tau Aggregation and Modulate Filament Structure and Stability?

Tau proteins are hyperphosphorylated at common sites in their N- and C-terminal domains in at least three neurodegenerative diseases, Parkinson, dementia with Lewy bodies, and Alzheimer's, suggesting specific pathology but general mechanism. Full-length human tau filament comprises a rigid core and a two-layered fuzzy coat. Tau is categorized into two groups of isoforms, with either four repeats (R1-R4) or three repeats (R1, R3, and R4); their truncated constructs are respectively called K18 and K19. Using multiscale molecular dynamics simulations, we explored the conformational consequences of hyperhposphorylation on tau's repeats. Our lower conformational energy filament models suggest a rigid filament core with a radius of ∼30 to 40 Å and an outer layer with a thickness of ∼140 Å consisting of a double-layered polyelectrolyte. The presence of the phosphorylated terminal domains alters the relative stabilities in the K18 ensemble, thus shifting the populations of the full-length filaments. However, the structure with the straight repeats in the core region is still the most stable, similar to the truncated K18 peptide species without the N- and C-terminus. Our simulations across different scales of resolution consistently reveal that hyperphosphorylation of the two terminal domains decreases the attractive interactions among the N- and C-terminus and repeat domain. To date, the relationship on the conformational level between phosphorylation and aggregation has not been understood. Our results suggest that the exposure of the repeat domain upon hyperphosphorylation could enhance tau filament aggregation. Thus, we discovered that even though these neurodegenerative diseases vary and their associated tau filaments are phosphorylated to different extents, remarkably, the three pathologies appear to share a common tau aggregation mechanism.

Fracture and Growth Are Competing Forces Determining the Fate of Conformers in Tau Fibril Populations

Tau fibrils are pathological aggregates that can transfer between neurons and then recruit soluble Tau monomers by template-assisted conversion. The propagation of different fibril polymorphs is thought to be a contributing factor to phenotypic diversity in Alzheimer disease and other Tauopathies. We found that a homogeneous population of Tau fibrils composed of the truncated version K18 (residues 244-372) gradually converted to a new set of fibril conformers when subjected to multiple cycles of seeding and growth. Using double electron-electron resonance (DEER) spectroscopy, we observed that the distances between spin labels at positions 311 and 328 in the fibril core progressively decreased. The findings were corroborated by changes in turbidity, morphology, and protease sensitivity. Fibrils that were initially formed under stirring conditions exhibited an increased fragility compared with fibrils formed quiescently after multiple cycles of seeding. The quiescently formed fibrils were marked by accelerated growth. The difference in fragility and growth between the different conformers explains how the change in incubation condition could lead to the amplification of a minor subpopulation of fibrils. Under quiescent conditions where fibril breakage is minimal, faster growing fibrils have a selective advantage. The findings are of general importance as they suggest that changes in selective pressures during fibril propagation in the human brain could result in the emergence of new fibril conformers with varied clinicopathological consequences.

Spin Labeling and Characterization of Tau Fibrils Using Electron Paramagnetic Resonance (EPR)

Template-assisted propagation of Tau fibrils is essential for the spreading of Tau pathology in Alzheimer's disease. In this process, small seeds of fibrils recruit Tau monomers onto their ends. The physical properties of the fibrils play an important role in their propagation. Here, we describe two different electron paramagnetic resonance (EPR) techniques that have provided crucial insights into the structure of Tau fibrils. Both techniques rely on the site-directed introduction of one or two spin labels into the protein monomer. Continuous-wave (CW) EPR provides information on which amino acid residues are contained in the fibril core and how they are stacked along the long fibril axis. Double electron-electron resonance (DEER) determines distances between two spin labels within a single protein and hence provides insights into their spatial arrangement in the fibril cross section. Because of the long distance range accessible to DEER (~2-5 nm) populations of distinct fibril conformers can be differentiated.

RNA Binds to Tau Fibrils and Sustains Template-Assisted Growth

Tau fibrils are the main proteinacious components of neurofibrillary lesions in Alzheimer disease. Although RNA molecules are sequestered into these lesions, their relationship to Tau fibrils is only poorly understood. Such understanding, however, is important, as short fibrils can transfer between neurons and nonproteinacious factors including RNA could play a defining role in modulating the latter process. Here, we used sedimentation assays combined with electron paramagnetic resonance (EPR), fluorescence, and absorbance spectroscopy to determine the effects of RNA on Tau fibril structure and growth. We observe that, in the presence of RNA, three-repeat (3R) and four-repeat (4R) Tau form fibrils with parallel, in-register arrangement of β-strands and exhibit an asymmetric seeding barrier in which 4R Tau grows onto 3R Tau seeds but not vice versa. These structural features are similar to those previously observed for heparin-induced fibrils, indicating that basic conformational properties are conserved, despite their being molecular differences of the nucleating agents. Furthermore, RNA sustains template-assisted growth and binds to the fibril surface and can be exchanged by heparin. These findings suggest that, in addition to mediating fibrillization, cofactors decorating the surface of Tau fibrils may modulate biological interactions and thereby influence the spreading of Tau pathology in the human brain.

Amplification of Tau fibrils from minute quantities of seeds

The propagation of Tau pathology in Alzheimer’s disease (AD) is thought to proceed through templated conversion of Tau protein into fibrils and cell-to-cell transfer of elongation-competent seeds. To investigate the efficiency of Tau conversion, we adapted the protein misfolding cyclic amplification assay used for the conversion of prions. Utilizing heparin as a cofactor and employing repetitive cycles of shearing and growth, synthetic Tau fibrils and Tau fibrils in AD brain extract are progressively amplified. Concurrently, self-nucleation is suppressed. The results highlight breakage-induced replication of Tau fibrils as a potential facilitator of disease spread.

Single mutations in tau modulate the populations of fibril conformers through seed selection

Seeded conversion of tau monomers into fibrils is a central step in the progression of tau pathology in Alzheimer's disease and other neurodegenerative disorders. Self-assembly is mediated by the microtubule binding repeats in tau. There are either three or four repeats present depending on the protein isoform. Here, double electron-electron resonance spectroscopy was used to investigate the conformational ensemble of four-repeat tau fibrils. Single point mutations at key positions in the protein (ΔK280, P301S, P312I, D314I) markedly change the distribution of fibril conformers after template-assisted growth, whereas other mutations in the protein (I308M, S320F, G323I, G326I, Q336R) do not. These findings provide unprecedented insights into the seed selection of tau disease mutants and establish conformational compatibility as an important driving force in tau fibril propagation.

Molecular insights into the reversible formation of tau protein fibrils

We computationally and experimentally showed that tau protein fibrils can be formed at high temperature. When cooled, the fibrils dissociate back to monomers. Heparin promotes tau fibril formation and prevents its reversion. Our results revealed the physicochemical mechanism of reversible formation of tau fibrils.

Small misfolded Tau species are internalized via bulk endocytosis and anterogradely and retrogradely transported in neurons

The accumulation of Tau into aggregates is associated with key pathological events in frontotemporal lobe degeneration (FTD-Tau) and Alzheimer disease (AD). Recent data have shown that misfolded Tau can be internalized by cells in vitro (Frost, B., Jacks, R. L., and Diamond, M. I. (2009) J. Biol. Chem. 284, 12845-12852) and propagate pathology in vivo (Clavaguera, F., Bolmont, T., Crowther, R. A., Abramowski, D., Frank, S., Probst, A., Fraser, G., Stalder, A. K., Beibel, M., Staufenbiel, M., Jucker, M., Goedert, M., and Tolnay, M. (2009) Nat. Cell Biol. 11, 909-913; Lasagna-Reeves, C. A., Castillo-Carranza, D. L., Sengupta, U., Guerrero-Munoz, M. J., Kiritoshi, T., Neugebauer, V., Jackson, G. R., and Kayed, R. (2012) Sci. Rep. 2, 700). Here we show that recombinant Tau misfolds into low molecular weight (LMW) aggregates prior to assembly into fibrils, and both extracellular LMW Tau aggregates and short fibrils, but not monomers, long fibrils, nor long filaments purified from brain extract are taken up by neurons. Remarkably, misfolded Tau can be internalized at the somatodendritic compartment, or the axon terminals and it can be transported anterogradely, retrogradely, and can enhance tauopathy in vivo. The internalized Tau aggregates co-localize with dextran, a bulk-endocytosis marker, and with the endolysosomal compartments. Our findings demonstrate that exogenous Tau can be taken up by cells, uptake depends on both the conformation and size of the Tau aggregates and once inside cells, Tau can be transported. These data provide support for observations that tauopathy can spread trans-synaptically in vivo, via cell-to-cell transfer.

Conformational basis for asymmetric seeding barrier in filaments of three- and four-repeat tau

Tau pathology in Alzheimer's disease is intimately linked to the deposition of proteinacious filaments, which akin to infectious prions, have been proposed to spread via seeded conversion. Here we use double electron-electron resonance (DEER) spectroscopy in combination with extensive computational analysis to show that filaments of three- (3R) and four-repeat (4R) tau are conformationally distinct. Distance measurements between spin labels in the third repeat, reveal tau amyloid filaments as ensembles of known β-strand-turn-β-strand U-turn motifs. Whereas filaments seeded with 3R tau are structurally homogeneous, filaments seeded with 4R tau are heterogeneous, composed of at least three distinct conformers. These findings establish a molecular basis for the seeding barrier between different tau isoforms and offer a new powerful approach for investigating the composition and dynamics of amyloid fibril ensembles.

Cross-seeding and conformational selection between three- and four-repeat human Tau proteins

In Alzheimer's disease and frontotemporal dementias, the microtubule-associated protein Tau forms intracellular paired helical filaments. The filaments can form not only by the full-length human Tau protein, but also by the three repeated (K19) or four repeated (K18) Tau segments. However, of interest, experimentally, K19 can seed K18, but not vice versa. To obtain insight into the cross-seeding between K18 and K19 aggregates, here, K18 and K19 octamers with repeat 3 (R3) in U-shaped, L-shaped, and long straight line-shaped (SL-shape) conformations are assembled into different structures. The simulation results show that K18-8/K19-8 (K18 and K19 assemblies number 8) with R3 in an L shape and K18-9/K19-9 with R3 in an SL shape are highly populated and present the highest structural similarity among all simulated K18 and K19 octamers, suggesting that similar folding of K18/K19 may serve as structural core for the K18-K19 co-assembled heterogeneous filament. We demonstrate that formation of stable R2 and R3 conformations is the critical step for K18 aggregation, and R3 is critical for K19 fibrillization. The different core units in K18 and K19 may create a cross-seeding barrier for the K18 seed to trigger K19 fibril growth because R2 is not available for K19. Our study provides insights into cross-seeding involving heterogeneous structures. The polymorphic nature of protein aggregation could be magnified in the cross-seeding process. If the seeding conformations lead to too much divergence in the energy landscape, it could impede fibril formation. Such an effect could also contribute to the asymmetric barrier between K18 and K19.

Variations in filament conformation dictate seeding barrier between three- and four-repeat tau

Tau filaments are the pathological hallmark of >20 neurodegenerative diseases including Alzheimer's disease. Six tau isoforms exist that can be grouped into 4-repeat (4R) tau and 3-repeat (3R) tau based on the presence or absence of the second of four microtubule binding repeats. Recent evidence suggests that tau filaments can transfer between cells and spread through the brain. Here we demonstrate in vitro that seeded filament growth, a prerequisite for tau spreading, is crucially dependent on the isoform composition of individual seeds. Seeds of 3R tau and 3R/4R tau recruit both types of isoforms. Seeds of 4R tau recruit 4R tau, but not 3R tau, establishing an asymmetric barrier. Conformational templating of 4R tau onto 3R tau seeds eliminates this barrier, giving rise to a new type of tau filament. These findings provide fundamental mechanistic insights into the seeding, propagation, and diversification of tau filaments.

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.

Investigation of alpha-synuclein fibril structure by site-directed spin labeling

The misfolding and fibril formation of alpha-synuclein plays an important role in neurodegenerative diseases such as Parkinson disease. Here we used electron paramagnetic resonance spectroscopy, together with site-directed spin labeling, to investigate the structural features of alpha-synuclein fibrils. We generated fibrils from a total of 83 different spin-labeled derivatives and observed single-line, exchange-narrowed EPR spectra for the majority of all sites located within the core region of alpha-synuclein fibrils. Such exchange narrowing requires the orbital overlap between multiple spin labels in close contact. The core region of alpha-synuclein fibrils must therefore be arranged in a parallel, in-register structure wherein same residues from different molecules are stacked on top of each other. This parallel, in-register core region extends from residue 36 to residue 98 and is tightly packed. Only a few sites within the core region, such as residues 62-67 located at the beginning of the NAC region, as well as the N- and C-terminal regions outside the core region, are significantly less ordered. Together with the accessibility measurements that suggest the location of potential beta-sheet regions within the fibril, the data provide significant structural constraints for generating three-dimensional models. Furthermore, the data support the emerging view that parallel, in-register structure is a common feature shared by a number of naturally occurring amyloid fibrils.

Side chain-dependent stacking modulates tau filament structure

The misfolding of proteins into highly ordered fibrils with similar physical properties is a hallmark of many degenerative diseases. Here, we use the microtubule associated protein tau as a model system to investigate the role of amino acid side chains in the formation of such fibrils. We identify a region (positions 272-289) in the tau protein that, in the fibrillar state, either forms part of a core of parallel, in-register, beta-strands, or remains unfolded. Single point mutations are sufficient to control this conformational switch with disease mutants G272V and DeltaK280 (found in familial forms of dementia) inducing a folded state. Through systematic mutagenesis we derive a propensity scale for individual amino acids to form fibrils with parallel, in-register, beta-strands. This scale should not only apply to tau fibrils but generally to all fibrils with same strand arrangement.

Spin labeling analysis of amyloids and other protein aggregates

Because of the enormous size of amyloid fibrils and their low tendency to form crystal lattices, it has been difficult to obtain high-resolution structural information on these aggregates. Magnetic resonance methods, such as solid-state nuclear magnetic resonance spectroscopy and electron paramagnetic resonance (EPR) spectroscopy, are promising new technologies by which to obtain molecular models. This chapter will focus on the application of EPR spectroscopy to amyloids and other protein aggregates. Site-directed spin labeling (SDSL), in combination with EPR spectroscopy, has been successfully used to study protein structure and the dynamics of soluble as well as membrane proteins. Recent studies indicate that this strategy is also well suited for studying amyloid fibrils. For example, an important outcome of the SDSL studies performed in our laboratory is that fibrils of amyloid beta, islet amyloid polypeptide, alpha-synuclein, and tau have their beta-strands aligned in an in-register, parallel fashion. Future studies promise to yield molecular information about fibril topography and protofilament arrangement and can be extended to include oligomeric structures.

Template-assisted filament growth by parallel stacking of tau

Tau filaments are found in >20 neurodegenerative diseases. Yet, because of their enormous molecular weights and poor tendency to form highly ordered 3D crystal lattices, they have evaded high-resolution structure determination. Here, we studied 25 derivatized tau mutants by using electron paramagnetic resonance and fluorescence spectroscopy to report structural details of tau filaments. Based on strong spin exchange and pyrene excimer formation of core residues, we find that individual tau proteins form single molecule layers along the fiber axis that perfectly stack on top of each other by in-register, parallel alignment of beta-strands. We suggest a model of filament growth wherein the existing filament serves as a template for the incoming, unfolded tau molecule, resulting in a new structured layer with maximized hydrogen-bonded contact surface and side-chain stacking.

Determinants of liposome fusion mediated by synaptic SNARE proteins

Synaptic exocytosis requires the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins syntaxin 1, SNAP-25, and synaptobrevin (VAMP). Assembly of the SNAREs into a stable core complex is supposed to catalyze membrane fusion, and proteoliposomes reconstituted with synaptic SNARE proteins spontaneously fuse with each other. We now show that liposome fusion mediated by synaptic SNAREs is inhibited by botulinum neurotoxin E (BoNT/E) but can be rescued by supplementing the C-terminal portion of SNAP-25. Furthermore, fusion is prevented by a SNAP-25-specific antibody known to block exocytosis in chromaffin cells, and it is competed for by soluble fragments of the R-SNAREs synaptobrevin 2, endobrevin/VAMP-8, and tomosyn. No accumulation of clustered vesicles is observed during the reaction. Rapid artificial clustering of SNARE-containing proteoliposomes enhances the fusion rate at low but not at saturating liposome concentrations. We conclude that the rate of liposome fusion is dominated by the intrinsic properties of the SNAREs rather than by the preceding docking step.

A Transient N-terminal Interaction of SNAP-25 and Syntaxin Nucleates SNARE Assembly

The SNARE proteins syntaxin, SNAP-25, and synaptobrevin play a central role during Ca(2+)-dependent exocytosis at the nerve terminal. Whereas syntaxin and SNAP-25 are located in the plasma membrane, synaptobrevin resides in the membrane of synaptic vesicles. It is thought that gradual assembly of these proteins into a membrane-bridging ternary SNARE complex ultimately leads to membrane fusion. According to this model, syntaxin and SNAP-25 constitute an acceptor complex for synaptobrevin. In vitro, however, syntaxin and SNAP-25 form a stable complex that contains two syntaxin molecules, one of which is occupying and possibly obstructing the binding site of synaptobrevin. To elucidate the assembly pathway of the synaptic SNAREs, we have now applied a combination of fluorescence and CD spectroscopy. We found that SNARE assembly begins with the slow and rate-limiting interaction of syntaxin and SNAP-25. Their interaction was prevented by N-terminal but not by C-terminal truncations, suggesting that for productive assembly all three participating helices must come together simultaneously. This suggests a complicated nucleation process that might be the reason for the observed slow assembly rate. N-terminal truncations of SNAP-25 and syntaxin also prevented the formation of the ternary complex, whereas neither N- nor C-terminal shortened synaptobrevin helices lost their ability to interact. This suggests that binding of synaptobrevin occurs after the establishment of the syntaxin-SNAP-25 interaction. Moreover, binding of synaptobrevin was inhibited by an excess of syntaxin, suggesting that a 1:1 interaction of syntaxin and SNAP-25 serves as the on-pathway SNARE assembly intermediate.

Single-molecule fluorescence resonance energy transfer reveals a dynamic equilibrium between closed and open conformations of syntaxin 1

Protein conformational transitions form the molecular basis of many cellular processes, such as signal transduction and membrane traffic. However, in many cases, little is known about their structural dynamics. Here we have used dynamic single-molecule fluorescence to study at high time resolution, conformational transitions of syntaxin 1, a soluble N-ethylmaleimide-sensitive factor attachment protein receptors protein essential for exocytotic membrane fusion. Sets of syntaxin double mutants were randomly labeled with a mix of donor and acceptor dye and their fluorescence resonance energy transfer was measured. For each set, all fluorescence information was recorded simultaneously with high time resolution, providing detailed information on distances and dynamics that were used to create structural models. We found that free syntaxin switches between an inactive closed and an active open configuration with a relaxation time of 0.8 ms, explaining why regulatory proteins are needed to arrest the protein in one conformational state.

The Habc domain and the SNARE core complex are connected by a highly flexible linker

Syntaxin 1a is a member of the SNARE superfamily of small, mostly membrane-bound proteins that mediate membrane fusion in all eukaryotic cells. Upon membrane fusion, syntaxin 1 forms a stable complex with its partner SNAREs. Syntaxin contains a C-terminal transmembrane domain, an adjacent SNARE motif that interacts with its partner SNAREs, and an N-terminal Habc domain. The Habc domain reversibly folds back upon the SNARE motif, resulting in a "closed" conformation that is stabilized by binding to the protein munc18. The SNARE motif and the Habc domain are separated by a linker region of about 40 amino acids. When syntaxin is complexed with munc18, the linker is structured and consists of a mix of turns and small alpha-helices. When syntaxin is complexed with its partner SNAREs, the Habc domain is dissociated, but the structure of the linker region is not known. Here we used site-directed spin labeling and EPR spectroscopy to determine the structure of the linker region of syntaxin in the SNARE complex. We found that the entire linker region of syntaxin is unstructured except for three residues at the N-terminal and six residues at the C-terminal boundary whereas the structures of the flanking regions in the Habc domain and the SNARE motif correspond to the high-resolution structures of the isolated fragments. We conclude that the linker region exhibits a high degree of conformational flexibility.

SNAREs in native plasma membranes are active and readily form core complexes with endogenous and exogenous SNAREs

During neuronal exocytosis, the vesicle-bound soluble NSF attachment protein (SNAP) receptor (SNARE) synaptobrevin 2 forms complexes with the plasma membrane-bound SNAREs syntaxin 1A and SNAP25 to initiate the fusion reaction. However, it is not known whether in the native membrane SNAREs are constitutively active or whether they are unable to enter SNARE complexes unless activated before membrane fusion. Here we used binding of labeled recombinant SNAREs to inside-out carrier supported plasma membrane sheets of PC12 cells to probe for the activity of endogenous SNAREs. Binding was specific, saturable, and depended on the presence of membrane-resident SNARE partners. Our data show that virtually all of the endogenous syntaxin 1 and SNAP-25 are highly reactive and readily form SNARE complexes with exogenously added SNAREs. Furthermore, complexes between endogenous SNAREs were not detectable when the membranes are freshly prepared, but they slowly form upon prolonged incubation in vitro. We conclude that the activity of membrane-resident SNAREs is not downregulated by control proteins but is constitutively active even if not engaged in fusion events.

Rapid and selective binding to the synaptic SNARE complex suggests a modulatory role of complexins in neuroexocytosis

The Ca(2+)-triggered release of neurotransmitters is mediated by fusion of synaptic vesicles with the plasma membrane. The molecular machinery that translates the Ca(2+) signal into exocytosis is only beginning to emerge. The soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins syntaxin, SNAP-25, and synaptobrevin are central components of the fusion apparatus. Assembly of a membrane-bridging ternary SNARE complex is thought to initiate membrane merger, but the roles of other factors are less understood. Complexins are two highly conserved proteins that modulate the Ca(2+) responsiveness of neurotransmitter release. In vitro, they bind in a 1:1 stoichiometry to the assembled synaptic SNARE complex, making complexins attractive candidates for controlling the exocytotic fusion apparatus. We have now performed a detailed structural, kinetic, and thermodynamic analysis of complexin binding to the SNARE complex. We found that no major conformational changes occur upon binding and that the complexin helix is aligned antiparallel to the four-helix bundle of the SNARE complex. Complexins bound rapidly (approximately 5 x 10(7) m(-1) s(-1)) and with high affinity (approximately 10 nm), making it one of the fastest protein-protein interactions characterized so far in membrane trafficking. Interestingly, neither affinity nor binding kinetics was substantially altered by Ca(2+) ions. No interaction of complexins was detectable either with individual SNARE proteins or with the binary syntaxin x SNAP-25 complex. Furthermore, complexin did not promote the formation of SNARE complex oligomers. Together, our data suggest that complexins modulate neuroexocytosis after assembly of membrane-bridging SNARE complexes.

Homo- and heterooligomeric SNARE complexes studied by site-directed spin labeling

SNARE (soluble NSF acceptor protein receptor) proteins are thought to mediate membrane fusion by assembling into heterooligomeric complexes that connect the fusing membranes and initiate the fusion reaction. Here we used site-directed spin labeling to map conformational changes that occur upon homo- and heterooligomeric complex formation of neuronal SNARE proteins. We found that the soluble domains of synaptobrevin, SNAP-25, and syntaxin 1 are unstructured. At higher concentrations, the SNARE motif of syntaxin 1 forms homooligomeric helical bundles with at least some of the alpha-helices aligned in parallel. In the assembled SNARE complex, mapping of thirty side chain positions yielded spectra which are in good agreement with the recently published crystal structure. The loop region of SNAP-25 that connects the two SNARE motifs is largely unstructured. C-terminal truncation of synaptobrevin resulted in complexes that are completely folded N-terminal of the truncation but become unstructured at the C-terminal end. The binary complex of syntaxin and SNAP-25 consists of a parallel four helix-bundle with properties resembling that of the ternary complex.

Inhibition of SNARE complex assembly differentially affects kinetic components of exocytosis

In chromaffin cells, an increase in intracellular Ca2+ leads to an exocytotic burst followed by sustained secretion. The burst can be further resolved into two kinetically distinct components, which suggests the presence of two separate pools of vesicles. To investigate how these components relate to SNARE complex formation, we introduced an antibody that blocks SNARE assembly but not disassembly. In the presence of the antibody, the sustained component was largely blocked, the burst was slightly reduced, and one of its kinetic components was eliminated. We conclude that SNARE complexes form before Ca(2+)-triggered membrane fusion and exist in a dynamic equilibrium between a loose and a tight state, both of which support exocytosis. Interaction of the antibody with preformed SNARE complexes favors the loose state.

Mixed and non-cognate SNARE complexes. Characterization of assembly and biophysical properties

Assembly of soluble N-ethylmaleimide-sensitive fusion attachment protein receptor (SNARE) proteins between two opposing membranes is thought to be the key event that initiates membrane fusion. Many new SNARE proteins have recently been localized to distinct intracellular compartments, supporting the view that sets of specific SNAREs are specialized for distinct trafficking steps. We have now investigated whether other SNAREs can form complexes with components of the synaptic SNARE complex including synaptobrevin/VAMP 2, SNAP-25, and syntaxin 1. When the Q-SNAREs syntaxin 2, 3, and 4, and the R-SNARE endobrevin/VAMP 8 were used in various combinations, heat-resistant complexes were formed. Limited proteolysis revealed that these complexes contained a protease-resistant core similar to that of the synaptic complex. All complexes were disassembled by the ATPase N-ethylmaleimide-sensitive fusion protein and its cofactor alpha-SNAP. Circular dichroism spectroscopy showed that major conformational changes occur during assembly, which are associated with induction of structure from unstructured monomers. Furthermore, no preference for synaptobrevin was observed during the assembly of the synaptic complex when endobrevin/VAMP 8 was present in equal concentrations. We conclude that cognate and non-cognate SNARE complexes are very similar with respect to biophysical properties, assembly, and disassembly, suggesting that specificity of membrane fusion in intracellular membrane traffic is not due to intrinsic specificity of SNARE pairing.

The synaptophysin-synaptobrevin complex: a hallmark of synaptic vesicle maturation

Exocytosis of synaptic vesicles requires the formation of a fusion complex consisting of the synaptic vesicle protein synaptobrevin (vesicle-associated membrane protein, or VAMP) and the plasma membrane proteins syntaxin and soluble synaptosomal-associated protein of 25 kDa (or SNAP 25). In search of mechanisms that regulate the assembly of the fusion complex, it was found that synaptobrevin also binds to the vesicle protein synaptophysin and that synaptophysin-bound synaptobrevin cannot enter the fusion complex. Using a combination of immunoprecipitation, cross-linking, and in vitro interaction experiments, we report here that the synaptophysin-synaptobrevin complex is upregulated during neuronal development. In embryonic rat brain, the complex is not detectable, although synaptophysin and synaptobrevin are expressed and are localized to the same nerve terminals and to the same pool of vesicles. In contrast, the ability of synaptobrevin to participate in the fusion complex is detectable as early as embryonic day 14. The binding of synaptoporin, a closely related homolog of synaptophysin, to synaptobrevin changes in a similar manner during development. Recombinant synaptobrevin binds to synaptophysin derived from adult brain extracts but not to that derived from embryonic brain extracts. Furthermore, the soluble cytosol fraction of adult, but not of embryonic, synaptosomes contains a protein that induces synaptophysin-synaptobrevin complex formation in embryonic vesicle fractions. We conclude that complex formation is regulated during development and is mediated by a posttranslational modification of synaptophysin. Furthermore, we propose that the synaptophysin-synaptobrevin complex is not essential for exocytosis but rather provides a reserve pool of synaptobrevin for exocytosis that can be readily recruited during periods of high synaptic activity.

A stable interaction between syntaxin 1a and synaptobrevin 2 mediated by their transmembrane domains

The proteins synaptobrevin (VAMP), SNAP-25 and syntaxin 1 are essential for neuronal exocytosis. They assemble into a stable ternary complex which is thought to initiate membrane fusion. In vitro, the transmembrane domains of syntaxin and synaptobrevin are not required for association. Here we report a novel interaction between synaptobrevin and syntaxin that requires the presence of the transmembrane domains. When co-reconstituted into liposomes, the proteins form a stable binary complex that cannot be disassembled by NSF and that is resistant to denaturation by SDS. Cleavage of synaptobrevin with tetanus toxin does not affect the interaction. Furthermore, the complex is formed when a truncated version of syntaxin is used that contains only 12 additional amino acid residues outside the membrane anchor. We conclude that the interaction is mediated by the transmembrane domains.

Immunoglobulin light chain class multiplicity and alternative organizational forms in early vertebrate phylogeny

The prototypic chondrichthyan immunoglobulin (Ig) light chain type (type I) isolated from Heterodontus francisci (horned shark) has a clustered organization in which variable (V), joining (J), and constant (C) elements are in relatively close linkage (V-J-C). Using a polymerase chain reaction-based approach on a light chain peptide sequence from the holocephalan, Hydrolagus colliei (spotted ratfish), it was possible to isolate members of a second light chain gene family. A probe to this light chain (type II) detects homologs in two orders of elasmobranchs, Heterodontus, a galeomorph and Raja erinacea (little skate), a batoid, suggesting that this light chain type may be present throughout the cartilaginous fishes. In all cases, V, J, and C regions of the type II gene are arranged in closely linked clusters typical of all known Ig genes in cartilaginous fishes. All representatives of this type II gene family are joined in the germline. A third (kappa-like) light chain type from Heterodontus is described. These findings establish that a degree of light chain class complexity comparable to that of the mammals is present in the most phylogenetically distant extant jawed vertebrates and that the phenomenon of germline-joined (pre-rearranged) genes, described originally in the heavy chain genes of cartilaginous fishes, extends to light chain genes.