Structural mapping techniques distinguish the surfaces of fibrillar 1N3R and 1N4R human tau.

The rigid core of intracellular tau filaments from Alzheimer's disease (AD), Pick's disease (PiD), and Corticobasal disease (CBD) brains has been shown to differ in their cryo-EM atomic structure. Despite providing critical information on the intimate arrangement of a fraction of htau molecule within the fibrillar scaffold, the cryo-EM studies neither yield a complete picture of tau fibrillar assemblies structure nor contribute insights into the surfaces that define their interactions with numerous cellular components. Here, using proteomic approaches such as proteolysis and molecular covalent painting, we mapped the exposed amino acid stretches at the surface and those constituting the fibrillar core of in vitro-assembled fibrils of human htau containing one N-terminal domain and three (1N3R) or four (1N4R) C-terminal microtubule-binding repeat domains as a result of alternative splicing. Using limited proteolysis, we identified the proteolytic fragments composing the molecular "bar-code" for each type of fibril. Our results are in agreement with structural data reported for filamentous tau from AD, PiD, and CBD cases predigested with the protease pronase. Finally, we report two amino acid stretches, exposed to the solvent in 1N4R not in 1N3R htau, which distinguish the surfaces of these two kinds of fibrils. Our findings open new perspectives for the design of highly specific ligands with diagnostic and therapeutic potential.

Atomic Force Microscopy Imaging and Nanomechanical Properties of Six Tau Isoform Assemblies.

The amyloid fibrillar form of the protein Tau is involved in a number of neurodegenerative diseases, also known as tauopathies. In this work, six different fibrillar Tau isoforms were assembled in vitro. The morphological and nanomechanical properties of these isoforms were studied using atomic force microscopy at high resolution in air and buffer. Our results demonstrate that all Tau isoform fibrils exhibit paired-helical-filament-like structures consisting of two protofibrils separated by a shallow groove. Interestingly, whereas the N-terminal inserts do not contribute to any morphological or mechanical difference between the isoforms with the same carboxyl-terminal microtubule-binding domain repeats, isoforms with four microtubule repeats (4R) exhibited a persistence length ranging from 2.0 to 2.8 µm, almost twofold higher than those with three repeats (3R). In addition, the axial Young's modulus values derived from the persistence lengths, as well as their radial ones determined via nanoindentation experiments, were very low compared to amyloid fibrils made of other proteins. This sheds light on the weak intermolecular interaction acting between the paired ß-sheets within Tau fibrils. This may play an important role in their association into high molecular weight assemblies, their dynamics, their persistence, their clearance in cells, and their propagation.

Disassembly of Tau fibrils by the human Hsp70 disaggregation machinery generates small seeding-competent species.

The accumulation of amyloid Tau aggregates is implicated in Alzheimer's disease (AD) and other tauopathies. Molecular chaperones are known to maintain protein homeostasis. Here, we show that an ATP-dependent human chaperone system disassembles Tau fibrils in vitro We found that this function is mediated by the core chaperone HSC70, assisted by specific cochaperones, in particular class B J-domain proteins and a heat shock protein 110 (Hsp110)-type nucleotide exchange factor (NEF). The Hsp70 disaggregation machinery processed recombinant fibrils assembled from all six Tau isoforms as well as Sarkosyl-resistant Tau aggregates extracted from cell cultures and human AD brain tissues, demonstrating the ability of the Hsp70 machinery to recognize a broad range of Tau aggregates. However, the chaperone activity released monomeric and small oligomeric Tau species, which induced the aggregation of self-propagating Tau conformers in a Tau cell culture model. We conclude that the activity of the Hsp70 disaggregation machinery is a double-edged sword, as it eliminates Tau amyloids at the cost of generating new seeds.

Increased Immune Activation by Pathologic α-Synuclein in Parkinson's Disease.

OBJECTIVE: Excessive inflammation in the central nervous system (CNS) and the periphery can result in neurodegeneration and parkinsonism. Recent evidence suggests that immune responses in Parkinson disease patients are dysregulated, leading to an increased inflammatory reaction to unspecific triggers. Although α-synuclein pathology is the hallmark of Parkinson disease, it has not been investigated whether pathologic α-synuclein is a specific trigger for excessive inflammatory responses in Parkinson disease. METHODS: We investigated the immune response of primary human monocytes and a microglial cell line to pathologic forms of α-synuclein by assessing cytokine release upon exposure. RESULTS: We show that pathologic α-synuclein (mutations, aggregation) results in a robust inflammatory activation of human monocytes and microglial BV2 cells. The activation is conformation- dependent, with increasing fibrillation and early onset mutations having the strongest effect on immune activation. We also found that activation of immune cells by extracellular α-synuclein is potentiated by extracellular vesicles, possibly by facilitating the uptake of α-synuclein. Blood extracellular vesicles from Parkinson disease patients induce a stronger activation of monocytes than blood extracellular vesicles from healthy controls. Most importantly, monocytes from Parkinson disease patients are dysregulated and hyperactive in response to stimulation with pathologic α-synuclein. Furthermore, we demonstrate that α-synuclein pathology in the CNS is sufficient to induce the monocyte dysregulation in the periphery of a mouse model. INTERPRETATION: Taken together, our data suggest that α-synuclein pathology and dysregulation of monocytes in Parkinson disease can act together to induce excessive inflammatory responses to α-synuclein. ANN NEUROL 2019;86:593-606.

Clustering of Tau fibrils impairs the synaptic composition of α3-Na+/K+-ATPase and AMPA receptors.

Tau assemblies have prion-like properties: they propagate from one neuron to another and amplify by seeding the aggregation of endogenous Tau. Although key in prion-like propagation, the binding of exogenous Tau assemblies to the plasma membrane of naïve neurons is not understood. We report that fibrillar Tau forms clusters at the plasma membrane following lateral diffusion. We found that the fibrils interact with the Na+/K+-ATPase (NKA) and AMPA receptors. The consequence of the clustering is a reduction in the amount of α3-NKA and an increase in the amount of GluA2-AMPA receptor at synapses. Furthermore, fibrillar Tau destabilizes functional NKA complexes. Tau and α-synuclein aggregates often co-exist in patients' brains. We now show evidences for cross-talk between these pathogenic aggregates with α-synuclein fibrils dramatically enhancing fibrillar Tau clustering and synaptic localization. Our results suggest that fibrillar α-synuclein and Tau cross-talk at the plasma membrane imbalance neuronal homeostasis.

Structural and functional properties of prefibrillar α-synuclein oligomers.

The deposition of fibrillar alpha-synuclein (α-syn) within inclusions (Lewy bodies and Lewy neurites) in neurons and glial cells is a hallmark of synucleinopathies. α-syn populates a variety of assemblies ranging from prefibrillar oligomeric species to fibrils whose specific contribution to neurodegeneration is still unclear. Here, we compare the specific structural and biological properties of distinct soluble prefibrillar α-syn oligomers formed either spontaneously or in the presence of dopamine and glutaraldehyde. We show that both on-fibrillar assembly pathway and distinct dopamine-mediated and glutaraldehyde-cross-linked α-syn oligomers are only slightly effective in perturbing cell membrane integrity and inducing cytotoxicity, while mature fibrils exhibit the highest toxicity. In contrast to low-molecular weight and unstable oligomers, large stable α-syn oligomers seed the aggregation of soluble α-syn within reporter cells although to a lesser extent than mature α-syn fibrils. These oligomers appear elongated in shape. Our findings suggest that α-syn oligomers represent a continuum of species ranging from unstable low molecular weight particles to mature fibrils via stable elongated oligomers composed of more than 15 α-syn monomers that possess seeding capacity.

The 26S Proteasome Degrades the Soluble but Not the Fibrillar Form of the Yeast Prion Ure2p In Vitro.

Yeast prions are self-perpetuating protein aggregates that cause heritable and transmissible phenotypic traits. Among these, [PSI+] and [URE3] stand out as the most studied yeast prions, and result from the self-assembly of the translation terminator Sup35p and the nitrogen catabolism regulator Ure2p, respectively, into insoluble fibrillar aggregates. Protein quality control systems are well known to govern the formation, propagation and transmission of these prions. However, little is known about the implication of the cellular proteolytic machineries in their turnover. We previously showed that the 26S proteasome degrades both the soluble and fibrillar forms of Sup35p and affects [PSI+] propagation. Here, we show that soluble native Ure2p is degraded by the proteasome in an ubiquitin-independent manner. Proteasomal degradation of Ure2p yields amyloidogenic N-terminal peptides and a C-terminal resistant fragment. In contrast to Sup35p, fibrillar Ure2p resists proteasomal degradation. Thus, structural variability within prions may dictate their ability to be degraded by the cellular proteolytic systems.

Structural and functional characterization of two alpha-synuclein strains.

α-Synuclein aggregation is implicated in a variety of diseases including Parkinson's disease, dementia with Lewy bodies, pure autonomic failure and multiple system atrophy. The association of protein aggregates made of a single protein with a variety of clinical phenotypes has been explained for prion diseases by the existence of different strains that propagate through the infection pathway. Here we structurally and functionally characterize two polymorphs of α-synuclein. We present evidence that the two forms indeed fulfil the molecular criteria to be identified as two strains of α-synuclein. Specifically, we show that the two strains have different structures, levels of toxicity, and in vitro and in vivo seeding and propagation properties. Such strain differences may account for differences in disease progression in different individuals/cell types and/or types of synucleinopathies.

G51D α-synuclein mutation causes a novel parkinsonian-pyramidal syndrome.

OBJECTIVE: To date, 3 rare missense mutations in the SNCA (α-synuclein) gene and the more frequent duplications or triplications of the wild-type gene are known to cause a broad array of clinical and pathological symptoms in familial Parkinson disease (PD). Here, we describe a French family with a parkinsonian-pyramidal syndrome harboring a novel heterozygous SNCA mutation. METHODS: Whole exome sequencing of DNA from 3 patients in a 3-generation pedigree was used to identify a new PD-associated mutation in SNCA. Clinical and pathological features of the patients were analyzed. The cytotoxic effects of the mutant and wild-type proteins were assessed by analytical ultracentrifugation, thioflavin T binding, transmission electron microscopy, cell viability assay, and caspase-3 activation. RESULTS: We identified a novel SNCA G51D (c.152 G>A) mutation that cosegregated with the disease and was absent from controls. G51D was associated with an unusual PD phenotype characterized by early disease onset, moderate response to levodopa, rapid progression leading to loss of autonomy and death within a few years, marked pyramidal signs including bilateral extensor plantar reflexes, occasionally spasticity, and frequently psychiatric symptoms. Pathological lesions predominated in the basal ganglia and the pyramidal tracts and included fine, diffuse cytoplasmic inclusions containing phospho-α-synuclein in superficial layers of the cerebral cortex, including the entorhinal cortex. Functional studies showed that G51D α-synuclein oligomerizes more slowly and its fibrils are more toxic than those of the wild-type protein. INTERPRETATION: We have identified a novel SNCA G51D mutation that causes a form of PD with unusual clinical, neuropathological, and biochemical features.

Structural basis for morpheein-type allosteric regulation of Escherichia coli glucosamine-6-phosphate synthase: equilibrium between inactive hexamer and active dimer.

The amino-terminal cysteine of glucosamine-6-phosphate synthase (GlmS) acts as a nucleophile to release and transfer ammonia from glutamine to fructose 6-phosphate through a channel. The crystal structure of the C1A mutant of Escherichia coli GlmS, solved at 2.5 Å resolution, is organized as a hexamer, where the glutaminase domains adopt an inactive conformation. Although the wild-type enzyme is active as a dimer, size exclusion chromatography, dynamic and quasi-elastic light scattering, native polyacrylamide gel electrophoresis, and ultracentrifugation data show that the dimer is in equilibrium with a hexameric state, in vitro and in cellulo. The previously determined structures of the wild-type enzyme, alone or in complex with glucosamine 6-phosphate, are also consistent with a hexameric assembly that is catalytically inactive because the ammonia channel is not formed. The shift of the equilibrium toward the hexameric form in the presence of cyclic glucosamine 6-phosphate, together with the decrease of the specific activity with increasing enzyme concentration, strongly supports product inhibition through hexamer stabilization. Altogether, our data allow us to propose a morpheein model, in which the active dimer can rearrange into a transiently stable form, which has the propensity to form an inactive hexamer. This would account for a physiologically relevant allosteric regulation of E. coli GlmS. Finally, in addition to cyclic glucose 6-phosphate bound at the active site, the hexameric organization of E. coli GlmS enables the binding of another linear sugar molecule. Targeting this sugar-binding site to stabilize the inactive hexameric state is therefore suggested for the development of specific antibacterial inhibitors.

Fibrillar α-synuclein and huntingtin exon 1 assemblies are toxic to the cells.

The aggregation of alpha-synuclein (α-syn) and huntingtin (htt) into fibrillar assemblies in nerve and glial cells is a molecular hallmark of Parkinson's and Huntington's diseases. Within the aggregation process, prefibrillar and fibrillar oligomeric species form. Prefibrillar assemblies rather than fibrils are nowadays considered cytotoxic. However, recent reports describing spreading of fibrillar assemblies from one cell to another, in cell cultures, animal models, and brains of grafted patients suggest a critical role for fibrillar assemblies in pathogenesis. Here we compare the cytotoxic effect of defined and comparable particle concentrations of on-assembly pathway oligomeric and fibrillar α-syn and Htt fragment corresponding to the first exon of the protein (HttEx1). We show that homogeneous populations of α-syn and HttEx1 fibrils, rather than their precursor on-assembly pathway oligomers, are highly toxic to cultured cells and induce apoptotic cell death. We document the reasons that make fibrils toxic. We show that α-syn and HttEx1 fibrils bind and permeabilize lipid vesicles. We also show that fibrils binding to the plasma membrane in cultured cells alter Ca(2+) homeostasis. Overall, our data indicate that fibrillar α-syn and HttEx1, rather than their precursor oligomers, are highly cytotoxic, the toxicity being associated to their ability to bind and permeabilize the cell membranes.

Characterization of monomeric intermediates during VSV glycoprotein structural transition.

Entry of enveloped viruses requires fusion of viral and cellular membranes, driven by conformational changes of viral glycoproteins. Crystal structures provide static pictures of pre- and post-fusion conformations of these proteins but the transition pathway remains elusive. Here, using several biophysical techniques, including analytical ultracentrifugation, circular dichroïsm, electron microscopy and small angle X-ray scattering, we have characterized the low-pH-induced fusogenic structural transition of a soluble form of vesicular stomatitis virus (VSV) glycoprotein G ectodomain (G(th), aa residues 1-422, the fragment that was previously crystallized). While the post-fusion trimer is the major species detected at low pH, the pre-fusion trimer is not detected in solution. Rather, at high pH, G(th) is a flexible monomer that explores a large conformational space. The monomeric population exhibits a marked pH-dependence and adopts more elongated conformations when pH decreases. Furthermore, large relative movements of domains are detected in absence of significant secondary structure modification. Solution studies are complemented by electron micrographs of negatively stained viral particles in which monomeric ectodomains of G are observed at the viral surface at both pH 7.5 and pH 6.7. We propose that the monomers are intermediates during the conformational change and thus that VSV G trimers dissociate at the viral surface during the structural transition.

Hsc70 protein interaction with soluble and fibrillar alpha-synuclein.

The aggregation of α-synuclein (α-Syn), the primary component of Lewy bodies, into high molecular weight assemblies is strongly associated with Parkinson disease. This event is believed to result from a conformational change within native α-Syn. Molecular chaperones exert critical housekeeping functions in vivo including refolding, maintaining in a soluble state, and/or pacifying protein aggregates. The influence of the stress-induced heat shock protein 70 (Hsp70) on α-Syn aggregation has been notably investigated. The constitutively expressed chaperone Hsc70 acts as an antiaggregation barrier before cells are overwhelmed with α-Syn aggregates and Hsp70 expression induced. Here, we investigate the interaction between Hsc70 and α-Syn, the consequences of this interaction, and the role of nucleotides and co-chaperones Hdj1 and Hdj2 as modulators. We show that Hsc70 sequesters soluble α-Syn in an assembly incompetent complex in the absence of ATP. The affinity of Hsc70 for soluble α-Syn diminishes upon addition of ATP alone or together with its co-chaperones Hdj1 or Hdj2 allowing faster binding and release of client proteins thus abolishing α-Syn assembly inhibition by Hsc70. We show that Hsc70 binds α-Syn fibrils with a 5-fold tighter affinity compared with soluble α-Syn. This suggests that Hsc70 preferentially interacts with high molecular weight α-Syn assemblies in vivo. Hsc70 binding certainly has an impact on the physicochemical properties of α-Syn assemblies. We show a reduced cellular toxicity of α-Syn fibrils coated with Hsc70 compared with "naked" fibrils. Hsc70 may therefore significantly affect the cellular propagation of α-Syn aggregates and their spread throughout the central nervous system in Parkinson disease.

Cyan fluorescent protein carries a constitutive mutation that prevents its dimerization.

The tendency of GFP-like fluorescent proteins to dimerize in vitro is a permanent concern as it may lead to artifacts in FRET imaging applications. However, we have found recently that CFP and YFP (the couple of GFP variants mostly used in FRET studies) show no trace of association in the cytosol of living cells up to millimolar concentrations. In this study, we investigated the oligomerization properties of purified CFP, by fluorescence anisotropy and sedimentation velocity. Surprisingly, we found that CFP has a much weaker homoaffinity than other fluorescent proteins (K(d) ≥ 3 × 10(-3) M), and that this is due to the constitutive N146I mutation, originally introduced into CFP to improve its brightness.

The Pih1-Tah1 cochaperone complex inhibits Hsp90 molecular chaperone ATPase activity.

Hsp90 (heat shock protein 90) is an ATP-dependent molecular chaperone regulated by collaborating proteins called cochaperones. This machinery is involved in the conformational activation of client proteins like signaling kinases, transcription factors, or ribonucleoproteins (RNP) such as telomerase. TPR (TetratricoPeptide Repeat)-containing protein associated with Hsp90 (Tah1) and protein interacting with Hsp90 (Pih1) have been identified in Saccharomyces cerevisiae as two Hsp90 cochaperones involved in chromatin remodeling complexes and small nucleolar RNP maturation. Tah1 possesses a minimal TPR domain and binds specifically to the Hsp90 C terminus, whereas Pih1 displays no homology to other protein motifs and has been involved in core RNP protein interaction. While Pih1 alone was unstable and was degraded from its N terminus, we showed that Pih1 and Tah1 form a stable heterodimeric complex that regulates Hsp90 ATPase activity. We used different biophysical approaches such as analytical ultracentrifugation, microcalorimetry, and noncovalent mass spectrometry to characterize the Pih1-Tah1 complex and its interaction with Hsp90. We showed that the Pih1-Tah1 heterodimer binds to Hsp90 with a similar affinity and the same stoichiometry as Tah1 alone. However, the Pih1-Tah1 complex antagonizes Tah1 activity on Hsp90 and inhibits the chaperone ATPase activity. We further identified the region within Pih1 responsible for interaction with Tah1 and inhibition of Hsp90, allowing us to suggest an interaction model for the Pih1-Tah1/Hsp90 complex. These results, together with previous reports, suggest a role for the Pih1-Tah1 cochaperone complex in the recruitment of client proteins such as core RNP proteins to Hsp90.

Two different signaling pathways for thaxtomin A-induced cell death in Arabidopsis and tobacco BY2.

Thaxtomin A (TXT) is a phytotoxin produced by all plant-pathogenic Streptomyces scabies involved in the potato scab disease. Their pathogenicity was previously correlated with the production of TXT. Calcium is known to be an essential second messenger associated with pathogen-induced plant responses and cell death. We have effectively shown that in Arabidopsis thaliana cell suspensions, TXT induces an early short lived Ca(2+) influx which is involved in the cell death process and other TXT-induced responses. We extended our study to Nicotiana tabacum BY2 by monitoring cell death and changes in cytosolic calcium concentration on cells expressing the apoaequorine Ca(2+) reporter protein to compare the responses to TXT of the two model plants, tobacco and A. thaliana. Our investigations show that cell death in BY2 appeared to be dose dependent with a lag of sensitivity comparing to A. thaliana. Moreover, pathway leading to cell death in BY2 does not involve calcium signaling. Our results suggest that different pathways are engaged in A. thaliana and N. tabacum BY2 to achieve the same response to TXT.

Anion channel activation and proton pumping inhibition involved in the plasma membrane depolarization induced by ABA in Arabidopsis thaliana suspension cells are both ROS dependent.

In Arabidopsis thaliana suspension cells, ABA was previously shown to promote the activation of anion channels and the reduction of proton pumping that both contribute to the plasma membrane depolarization. These two ABA responses were shown to induce two successive [Ca(2+)](cyt) spikes. As reactive oxygen species (ROS) have emerged as components of ABA signaling pathways especially by promoting [Ca(2+)](cyt) variations, we studied whether ROS were involved in the regulation of anion channels and proton pumps activities. Here we demonstrated that ABA induced ROS production which triggered the second of the two [Ca(2+)](cyt) increases observed in response to ABA. Blocking ROS generation using diphenyleneiodonium (DPI) impaired the proton pumping reduction, the anion channel activation and the RD29A gene expression in response to ABA. Furthermore, H(2)O(2) was shown to activate anion channels and to inhibit plasma membrane proton pumping, as did ABA. However, ROS partially mimicked ABA's effects since H(2)O(2) treatment elicited anion channel activation but not the subsequent expression of the RD29A gene as did ABA. This suggests that expression of the RD29A gene in response to ABA results from the activation of multiple concomitant signaling pathways: blocking of one of them would impair gene expression whereas stimulating only one would not. We conclude that ROS are a central messenger of ABA in the signaling pathways leading to the plasma membrane depolarization induced by ABA.

Toxic and signalling effects of oxalic acid: Oxalic acid-Natural born killer or natural born protector?

Oxalic acid is thought to be a key factor of the early pathogenic stage in a wide range of necrotrophic fungi. We have recently published that oxalic acid induces Programmed Cell Death (PCD) in Arabidopsis thaliana cells. This cell death results from an early anionic efflux which is a prerequisite for the synthesis of ethylene and the PCD. Complementary experiments have been carried out by using seedlings of A. thaliana. The effects of millimolar concentrations of oxalic acid were analysed on A. thaliana seedlings. A treatment with a 3 mM oxalic acid solution does not alter the development of the plants but induces the transcription of defence related genes which are anion channel dependant. Moreover, our results suggest that a pre-treatment of the seedlings with oxalic acid is able to confer the resistance of A. thaliana against Sclerotium rolfsii. Regarding our results, we suggest that oxalic acid plays two distinct roles, depending on the concentration: a high concentration of oxalic acid induces a large PCD and then contribute to the progression of the fungi. However, at low concentration it is able to induce the establishment of a resistance of the plant against the fungi.

A putative role for fusaric acid in biocontrol of the parasitic angiosperm Orobanche ramosa.

Fusarium spp. are ubiquitous fungi found in soil worldwide as both pathogenic and nonpathogenic strains. The signals leading to disease or the absence of disease are poorly understood. We recently showed that fusaric acid (FA), a nonspecific toxin produced by most Fusarium spp., could elicit various plant defense responses at 100 nM without toxic effect. In this study, we checked for the effect of FA on root and root hairs, probable first site of contact between the fungi and the host. Large FA concentrations reduce root and root-hair growth and induce a rapid transient membrane hyperpolarization, followed by a large depolarization, due to the inhibition of H(+)-ATPase currents. Nanomolar concentrations of FA induced only an early transient membrane hyperpolarization of root hairs compatible with the induction of a signal transduction pathway. FA at 10(-7) M failed to induce salicylic acid- and jasmonic acid/ethylene-dependent defense-related genes but inhibited the germination of the angiosperm parasite Orobanche ramosa in contact of FA-pretreated Arabidopsis thaliana seedlings. These data suggest that FA at nontoxic concentrations could activate signal transduction components necessary for plant-defense responses that could contribute to biocontrol activity of Fusarium spp.