Structural Flexibility of Tau in Its Interaction with Microtubules as Viewed by Site-Directed Spin Labeling EPR Spectroscopy

Tau is a microtubule-associated protein that belongs to the Intrinsically Disordered Proteins (IDPs) family. IDPs or Intrinsically Disordered Regions (IDRs) play key roles in protein interaction networks and their dysfunctions are often related to severe diseases. Defined by their lack of stable secondary and tertiary structures in physiological conditions while being functional, these proteins use their inherent structural flexibility to adapt to and interact with various binding partners. Knowledges on the structural dynamics of IDPs and their different conformers are crucial to finely decipher fundamental biological processes controlled by mechanisms such as conformational adaptations or switches, induced fit, or conformational selection events. Different mechanisms of binding have been proposed: among them, the so-called folding-upon-binding in which the IDP adopts a certain conformation upon interacting with a partner protein, or the formation of a "fuzzy" complex in which the IDP partly keeps its dynamical character at the surface of its partner. The dynamical nature and physicochemical properties of unbound as well as bound IDPs make this class of proteins particularly difficult to characterize by classical bio-structural techniques and require specific approaches for the fine description of their inherent dynamics.Among other techniques, Site-Directed Spin Labeling combined with Electron Paramagnetic Resonance (SDSL-EPR) spectroscopy has gained much interest in this last decade for the study of IDPs. SDSL-EPR consists in grafting a paramagnetic label (mainly a nitroxide radical) at selected site(s) of the macromolecule under interest followed by its observation using and/or combining different EPR strategies. These nitroxide spin labels detected by continuous wave (cw) EPR spectroscopy are used as perfect reporters or "spy spins" of their local environment, being able to reveal structural transitions, folding/unfolding events, etc. Another approach is based on the measurement of inter-label distance distributions in the 1.5-8.0 nm range using pulsed dipolar EPR experiments, such as Double Electron-Electron Resonance (DEER) spectroscopy. The technique is then particularly well suited to study the behavior of Tau in its interaction with its physiological partner: microtubules (MTs). In this chapter we provide a detailed experimental protocol for the labeling of Tau protein and its EPR study while interacting with preformed (Paclitaxel-stabilized) MTs, or using Tau as MT inducer. We show how the choice of nitroxide label can be crucial to obtain functional information on Tau/tubulin complexes.

2-Aminothiazole-Flavonoid Hybrid Derivatives Binding to Tau Protein and Responsible for Antitumor Activity in Glioblastoma

Tau protein has been described for several decades as a promoter of tubulin assembly into microtubules. Dysregulation or alterations in Tau expression have been related to various brain cancers, including the highly aggressive and lethal brain tumor glioblastoma multiform (GBM). In this respect, Tau holds significant promise as a target for the development of novel therapies. Here, we examined the structure-activity relationship of a new series of seventeen 2-aminothiazole-fused to flavonoid hybrid compounds (TZF) on Tau binding, Tau fibrillation, and cellular effects on Tau-expressing cancer cells. By spectrofluorometric approach, we found that two compounds, 2 and 9, demonstrated high affinity for Tau and exhibited a strong propensity to inhibit Tau fibrillation. Then, the biological activity of these compounds was evaluated on several Tau-expressing cells derived from glioblastoma. The two lead compounds displayed a high anti-metabolic activity on cells related to an increased fission of the mitochondria network. Moreover, we showed that both compounds induced microtubule bundling within newly formed neurite-like protrusions, as well as with defection of cell migration. Taken together, our results provide a strong experimental basis to develop new potent molecules targeting Tau-expressing cancer cells, such as GBM.

Myotoxin-3 from the Pacific Rattlesnake Crotalus oreganus oreganus Venom Is a New Microtubule-Targeting Agent

Microtubule targeting agents (MTA) are anti-cancer molecules that bind tubulin and interfere with the microtubule functions, eventually leading to cell death. In the present study, we used an in vitro microtubule polymerization assay to screen several venom families for the presence of anti-microtubule activity. We isolated myotoxin-3, a peptide of the crotamine family, and three isoforms from the venom of the Northern Pacific rattlesnake Crotalus oreganus oreganus, which was able to increase tubulin polymerization. Myotoxin-3 turned out to be a cell-penetrating peptide that slightly diminished the viability of U87 glioblastoma and MCF7 breast carcinoma cells. Myotoxin 3 also induced remodeling of the U87 microtubule network and decreased MCF-7 microtubule dynamic instability. These effects are likely due to direct interaction with tubulin. Indeed, we showed that myotoxin-3 binds to tubulin heterodimer with a Kd of 5.3 µM and stoichiometry of two molecules of peptide per tubulin dimer. Our results demonstrate that exogenous peptides are good candidates for developing new MTA and highlight the richness of venoms as a source of pharmacologically active molecules.

Identification of the three zinc-binding sites on tau protein

Tau protein has been extensively studied due to its key roles in microtubular cytoskeleton regulation and in the formation of aggregates found in some neurodegenerative diseases. Recently it has been shown that zinc is able to induce tau aggregation by interacting with several binding sites. However, the precise location of these sites and the molecular mechanism of zinc-induced aggregation remain unknown. Here we used Nuclear Magnetic Resonance (NMR) to identify zinc binding sites on tau. These experiments revealed three distinct zinc binding sites on tau, located in the N-terminal part, the repeat region and the C-terminal part. Further analysis enabled us to show that the N-terminal and the C-terminal sites are independent of each other. Using molecular simulations, we proposed a model of each site in a complex with zinc. Given the clinical importance of zinc in tau aggregation, our findings pave the way for designing potential therapies for tauopathies.

Tau Regulates Glioblastoma Progression, 3D Cell Organization, Growth and Migration via the PI3K-AKT Axis

Simple Summary

The Microtubule-associated protein Tau is expressed in different cancers; however, its role and prognostic value are still debated. In the present work, we evaluated the role of Tau in glioblastoma by down-regulating its expression in glioblastoma cells. We showed that Tau: (1) is required for tumor progression in nude mice; (2) is necessary for glioblastoma 3D cell organization, growth, and migration; and (3) regulates the PI3K/AKT signaling pathway.

Abstract

The Microtubule-Associated Protein Tau is expressed in several cancers, including low-grade gliomas and glioblastomas. We have previously shown that Tau is crucial for the 2D motility of several glioblastoma cell lines, including U87-MG cells. Using an RNA interference (shRNA), we tested if Tau contributed to glioblastoma in vivo tumorigenicity and analyzed its function in a 3D model of multicellular spheroids (MCS). Tau depletion significantly increased median mouse survival in an orthotopic glioblastoma xenograft model. This was accompanied by the inhibition of MCS growth and cell evasion, as well as decreased MCS compactness, implying N-cadherin mislocalization. Intracellular Signaling Array analysis revealed a defective activation of the PI3K/AKT pathway in Tau-depleted cells. Such a defect in PI3K/AKT signaling was responsible for reduced MCS growth and cell evasion, as demonstrated by the inhibition of the pathway in control MCS using LY294002 or Perifosine, which did not significantly affect Tau-depleted MCS. Finally, analysis of the glioblastoma TCGA dataset showed a positive correlation between the amount of phosphorylated Akt-Ser473 and the expression of MAPT RNA encoding Tau, underlining the relevance of our findings in glioblastoma disease. We suggest a role for Tau in glioblastoma by controlling 3D cell organization and functions via the PI3K/AKT signaling axis.

Structural Contour Map of the Iota Carbonic Anhydrase from the Diatom Thalassiosira pseudonana Using a Multiprong Approach

Carbonic anhydrases (CAs) are a family of ubiquitous enzymes that catalyze the interconversion of CO₂ and HCO₃⁻. The “iota” class (ι-CA) was first found in the marine diatom Thalassiosira pseudonana (tpι-CA) and is widespread among photosynthetic microalgae and prokaryotes. The ι-CA has a domain COG4875 (or COG4337) that can be repeated from one to several times and resembles a calcium–calmodulin protein kinase II association domain (CaMKII-AD). The crystal structure of this domain in the ι-CA from a cyanobacterium and a chlorarachniophyte has been recently determined. However, the three-dimensional organization of the four domain-containing tpι-CA is unknown. Using biophysical techniques and 3-D modeling, we show that the homotetrameric tpι-CA in solution has a flat “drone-like” shape with a core formed by the association of the first two domains of each monomer, and four protruding arms formed by domains 3 and 4. We also observe that the short linker between domains 3 and 4 in each monomer confers high flexibility, allowing for different conformations to be adopted. We propose the possible 3-D structure of a truncated tpι-CA containing fewer domain repeats using experimental data and discuss the implications of this atypical shape on the activity and metal coordination of the ι-CA.

Insights into animal septins using recombinant human septin octamers with distinct SEPT9 isoforms

Septin GTP-binding proteins contribute essential biological functions that range from the establishment of cell polarity to animal tissue morphogenesis. Human septins in cells form hetero-octameric septin complexes containing the ubiquitously expressed SEPT9 subunit (also known as SEPTIN9). Despite the established role of SEPT9 in mammalian development and human pathophysiology, biochemical and biophysical studies have relied on monomeric SEPT9, thus not recapitulating its native assembly into hetero-octameric complexes. We established a protocol that enabled, for the first time, the isolation of recombinant human septin octamers containing distinct SEPT9 isoforms. A combination of biochemical and biophysical assays confirmed the octameric nature of the isolated complexes in solution. Reconstitution studies showed that octamers with either a long or a short SEPT9 isoform form filament assemblies, and can directly bind and cross-link actin filaments, raising the possibility that septin-decorated actin structures in cells reflect direct actin-septin interactions. Recombinant SEPT9-containing octamers will make it possible to design cell-free assays to dissect the complex interactions of septins with cell membranes and the actin and microtubule cytoskeleton.

Patterns of left ventricular longitudinal myocardial dysfunction in mitral valve prolapse and effects of valve repair

Background

Regional longitudinal left ventricular (LV) dysfunction in patients with mitral regurgitation (MR) due to valve prolapse (MVP) with normal ejection fraction has been recently described, with data pointing at dysfunction of the LV base related to dilatation of the mitral annulus.

Purpose

To investigate degree and extent of regional LV dysfunction and its mechanisms in patients with MVP and severe acute (MRa, n=27) or chronic (MRc, n=41) MR and no coronary disease, undergoing surgical valve repair with 3 months follow-up (FU); 20 normal subjects were used as controls (N).

Methods

Speckle-tracking echocardiography was performed pre- (Bas), 1 week (1w) and 4 months (4mo) post-operatively to measure longitudinal global (GLPSS, %), regional (RPSS, %) and segmental (SPSS) peak systolic strain. Maximum and minimum mitral annulus (MA) diameters were measured with 3D echo at Bas. We also evaluated: LV end-diastolic volume index (EDVi, ml/m2); ejection fraction (EF, %); left atrial end-systolic volume index (LAVi, ml/m2); non-invasive pulmonary systolic pressure (PSP, mmHg).

Results

Risk factors (hypertension, diabetes, atrial fibrillation, smoke and previous stroke) were similar in MRc and MRa. At Bas EDVi was larger by definition in MRc (MRc: 102±21, MRa: 67±10 ml/m2, p<0.001) as LAVi (101±46 vs 76±31 ml/m2, p=0.035). Both EF (65±8 vs 64±8 ml/m2) and GLPSS (−20±4 vs −21±5%) were normal, but RPSS was reduced, only at the base (−13±6 vs −13±6%, p= ns; N: −18±2, p<0.03 vs MRc and MRa) in MRc and MRa, with reduced SPSS localized at anterior, lateral and posterior – but not septal – segments. At 1w, EF decreased in both MRc (47±14%, p<0.001 vs Bas) and MRa (56±10%, p=0.014 vs Bas), together with GLPSS (MRc: −11±4%, p<0.001 vs Bas; MRa; −13±4, p<0.001 vs Bas) driven by a prevalent marked decrease in RPSS (MRc: −7±4%, p<0.001 vs Bas; MRa; −8±5, p<0.001 vs Bas) of the LV base. All patients were alive at 3 months with no MACEs, similar reduction of mean MR grade (MRc: 4±0 to 1.9±0.7, p<0.001; MRa: 3.9±0.3 to 0.9±0.9, p<0.001) and PSP (MRc: 50±23 to 29±5 mmHg, p<0.001; MRa: 42±22 to 32±6 mmHg, p=0.039), normal EDVi (MRc: 70±27, MRa: 49±10 ml/m2), dilated LAVi (MRc: 101±46, MRa: 54±13 ml/m2), and reduced GLPSS (MRc: −12±5%, p<0.001 vs Bas; MRa; −15±3, p=0.001 vs Bas) and base RPSS (MRc: −7±6%, p=0.004 vs Bas; MRa; −10±4, p= ns vs Bas). At multivariate analysis, regional dysfunction was not related to the prolapsing scallop, presence of flail, commissure involvement, dimension and geometry of the MA, EF or pulmonary pressures.

Conclusions

In patients with MVP and severe MR, there is a specific regional longitudinal dysfunction pattern prevalent at the LV base which may be related to the duration of MR but not to annular dilatation or morphology of the prolapsing leaflets. The dysfunction worsens greatly following acute reduction of preload after surgical repair and is still significant at 4mo FU.

Funding Acknowledgement

Type of funding source: None

Allocolchicinoids bearing a Michael acceptor fragment for possible irreversible binding of tubulin

We describe an attempt to apply the concept of covalent binding towards the highly active allocolchicinoids selected on the basis of SAR analysis of previously synthesized molecules. To achieve the irreversible binding of the agent to the cysteine residues of the colchicine site of tubulin protein, we synthesized a number of new allocolchicinoids bearing the acceptor moiety. Some of the new derivatives possess cytotoxic activity against COLO-357, BxPC-3, HaCaT, and HEK293 cell lines in a low nanomolar range of concentrations. A substoichiometric mode of microtubule assembly inhibition was demonstrated. The most active compounds possess close to colchicine general toxicity on mice.

149 Acute effects on left ventricular systolic and diastolic function of a post-implant echocardiographic biventricular pacemaker optimization protocol following cardiac resynchronization therapy

Background

Cardiac resynchronization therapy (CRT) has demonstrated efficacy in at least 60% of patients with left ventricular (LV) failure and guideline-based indication to CRT. Whereas lack of response to CRT in up to a third of patients is multifactorial, a relevant factor is thought to be inadequate biventricular pacemaker (BIV) optimization (OPT) of either the intraventricular (VVd) or atrioventricular (AVd) delay

Purpose

In this echocardiographic observational study, we compared the acute effects on LV contractility, output, and diastolic function of BIV intra-implant QRS duration-based (OPTq) and post-implant Doppler echocardiography-based (OPTe) OPT of VVd and AVd.

Methods

In 160 patients with ischemic (n = 86) or idiopathic (n = 74) dilated cardiomyopathy, guideline-based different de novo CRT systems were implanted followed by immediate OPTq. Post-implant (10 days) OPTe was performed measuring: transmitral velocity-time integral (MVFi), % diastolic filling time (MVFt%), and E/A ratio, LV outflow integral (LVOTi), ejection time (LVOTt), and stroke volume (SV), isovolumic contraction (IVCT) and relaxation (IVRT) times, and LV myocardial performance index (MPI). The protocol included, sequentially: 1) Doppler measurements with OPTq settings; 2) measurements (separated by 3’ intervals) during a range (80/200 ms) of AVd with synchronous VVd; 3) algorithm-based AVd selection (at least 2 of following: increase in MVFi or SV, decrease in MPI); 4) measurements, with set AVd, during range of VVd: LV-first (-20, -40ms); RV-first (20, 40ms); synchronous; 5) VVd selection based on same algorithm used for AVd selection. Results. At OPTq, 58.6% of patients were set synchronous, 38.6% LV-first and 3% RV-first, with a 126 ± 29 mean AVd. This increased to 137 ± 36 after OPTe, when 49.1% were set synchronous, 38% LV-first and 12.4% RV-first, resulting in modifications of AVd and VVd in 59% and 36% of patients. Further, gain in SV with OPTe, compared to OPTq, was 8.3% (p<.001), paralleled by an increase in MVFi (21.2 ± 8 cm vs 20.5 ± 8, p<.001) and decrease in E/A (1.25 vs 1.45, p<.001). The greatest increase in SV with OPTe was found in patients in whom both AVd and VVd were modified (n = 48; 81 ± 26 ml vs 71 ± 23, p<.001) vs. patients without modifications (n = 42), or with change of either AVd or VVd (n = 70; 77 ± 20 vs 72 ± 20, p<.01). Only in the first patient group both MVFi (22 ± 9 vs 20 ± 9, p<.001) and MVFt% (52 ± 7 vs 49 ± 8, p=.004) increased, along with a decrease in MPI (.82±.31 vs .92 ± 36, p=.007) and IVRT (144 ± 51 vs 156 ± 62, p=.02.

Conclusions

These preliminary results point to a significant incremental role of post-implant OPTe to enhance LV output, contractility, and diastolic function in patients with CRT. The prognostic role of OPTe-determined AVd and VVD changes remains to be determined.

P1754 Regional left ventricular longitudinal myocardial dysfunction in mitral valve prolapse could be primary

Background

Regional left ventricular dysfunction in patients with mitral valve prolapse (MVP) and normal ejection fraction has been described by different Authors, and recent data point to a dysfunction (prevalently longitudinal strain) of the myocardium of the LV base secondary to dilatation of the mitral annulus.

Purpose

To investigate degree and extent of regional LV dysfunction and its mechanisms in patients with MVP, severe regurgitation and normal global systolic function, compared to patients with equivalent degree of regurgitation but functional etiology (FMR).

Methods

Speckle-tracking echocardiography was performed in 30 controls (N), and in severe primary (MVP, n= 50) or functional (FMR, n= 20) mitral regurgitation, to measure global, regional and segmental longitudinal peak systolic strain (LPSS, %), and time delay of peak maximum strain (TTPd, ms, calculated as time to peak maximum strain - time of aortic valve closure). Maximum and minimum mitral annulus diameters and area were measured with 3D echo. We also evaluated as recommended: LV end-diastolic volume index (EDVi, ml/m2), ejection fraction (EF, %), and left atrial end-systolic volume index (LAESVi, ml/m2) with 2D echo; LV stroke volume index, and non-invasive pulmonary systolic (PSP, mmHg) and diastolic pressures (PDP), mmHg) with Doppler echo.

Results

Age, heart rate, BSA and systolic blood pressure were similar between groups. Atrial fibrillation was present in 34% of MVP and 71% of FMR patients. LV EF was normal in MVP and reduced in FMR (43 ± 14 % vs N, p<.001). LV EDVi (MVP: 77 ± 20 ml/m2; FMR: 107 ± 35, both p<.001 vs N) and LAESVi (MVP: 91 ± 26 ml/m2; FMR: 80 ± 30, both p<.001 vs N) were similarly increased (volume overload) in MVP and FMR, as were PSP (MVP: 42 ± 23 ml/m2; FMR: 52 ± 25, both p<.001 vs N) and PDP (MVP: 16 ± 6 ml/m2; MVP: 15 ± 5, both p<.001 vs N). In FMR, LPSS was reduced globally (-12.8 ± 3.3, p<.001 vs N and MVP) and similarly at LV base, papillary and apical levels. In contrast, in MVP global (-19.4 ± 3.7%) and apical (-23.4 ± 4.5%) LPSS were normal, whereas LV base (-12.3 ± 5.8%, p=.003 vs N) and papillary (-17.1 ± 4%, p=.024 vs N) LPSS were reduced; further, LPSS reduction was localized to the anterior (-16 ± 4, p=.028 vs N), lateral (-17 ± 5, p=.006 vs N) and posterior (-16 ± 6, p=.007 vs N) segments, and was associated with an increased TTPd in the same segments in MVP but not in FMR patients. At multivariate analysis, degree and localisation of regional myocardial dysfunction in patients with MVP was not related to the prolapsing scallop, dimension of the mitral annulus, degree of volume overload or pulmonary pressures, or stroke volume index.

Conclusions

In patients with MVP, severe regurgitation and normal EF, there is a specific dysfunction pattern of regional LV longitudinal function which appears to be primary and not dependent on the degree of preload increase, mitral annulus dilatation, or localization of the prolapsing scallop.

P1748 Left atrial diastolic and systolic functions modulate the response to the standardised Valsalva maneuver in normal subjects

Background

Although the Valsalva maneuver (VM) is being advocated by current guidelines to identify with echocardiography patients with increased left ventricular (LV) filling pressures using a decrease in mitral E/A velocity > 0.5 as cutoff, there are limited published data for both patients and the normal response to the maneuver in healthy subjects.

Purpose

To assess LV and left atrial (LA) physiology during a standardized VM (VMs) in normal subjects.

Methods

The VMs was performed in 50 healthy subjects (M:F 38:12; age 40 ± 12 y.; HR 70 ± 11 bpm; BSA 1.81 ± 11 m2), instructed to forcefully exhale for 20 seconds without an initial deep breath into a tube connected to a sphygmomanometer, maintaining a 25-35 mmHg pressure. The VM was repeated 2 times at 5 minute intervals to record sequentially in the apical 4-chamber view: 1. LV and LA volumes; 2. Transmitral flow velocities. LA diastolic reservoir function (LAres) was calculated as: (maximum – minimum volume) / minimum volume x 100. Results. During the VMs, in all subjects LV indexed end-diastolic (-14 ± 7 ml/m2, -31 ± 15 %) and end-systolic (-6 ± 4 ml/m2, -31 ± 18 %) volumes, and stroke volume index (-9 ± 5 ml/m2, -30 ± 15 %) decreased similarly with unchanged LV ejection fraction %, and LA maximum and minimum volume indices both decreased (respectively -8 ± 6 ml/m2, -3 ± 3 ml/m2;-32 ± 25 %) with high variability. Mitral peak E velocity also decreased (-22 ± 13 cm/s, -27 ± 14 %) in all subjects, whereas peak A velocity change varied, such that a "pseudo-abnormal" decrease of E/A > 0.5 was seen in 18 subjects (37 %). At baseline, this subgroup had lower heart rate (66 ± 11 vs 73 ± 10 bpm, p= .026), higher LAres (193 ± 67 vs 145 ± 47 %, p= .006), lower peak A velocity (50 ± 12 vs 58 ± 12 cm/s, p= .04) and higher E/A (1.8±.6 vs 1.4±.3, p= .004). During VMs, LV and LA volumes decreased similarly in all subjects, but increase in heart rate was higher (12 ± 8 vs 6 ± 5 bpm, p= .023), and peak A wave increased instead of decreasing (20 ± 20 % vs -8 ± 18 %, p< .001) in the subjects with "pseudo-abnormal" decrease of E/A. During VMs, decrease in E/A was mainly determined (regression analysis, r: .76, p= .029) by baseline LAres (B= -.71) and change in LAres during VMs (B= -.47), whereas an increase in peak A velocity (r: .46, p= .031) was mainly determined by degree of HR increase (B= .41) and baseline LV EF (B= .3).

Conclusions

During VMs, a "pseudo-abnormal" decrease of the E/A velocity ratio is present in almost 40 % of normal subjects, and is determined by the interplay of the baseline diastolic compliance and the increase in systolic function of the LA during VM. These results may influence the accuracy of the VMs in the detection of increased LV filling pressures in patients.

Role of Tau as a Microtubule-Associated Protein: Structural and Functional Aspects

Microtubules (MTs) play a fundamental role in many vital processes such as cell division and neuronal activity. They are key structural and functional elements in axons, supporting neurite differentiation and growth, as well as transporting motor proteins along the axons, which use MTs as support tracks. Tau is a stabilizing MT associated protein, whose functions are mainly regulated by phosphorylation. A disruption of the MT network, which might be caused by Tau loss of function, is observed in a group of related diseases called tauopathies, which includes Alzheimer’s disease (AD). Tau is found hyperphosphorylated in AD, which might account for its loss of MT stabilizing capacity. Since destabilization of MTs after dissociation of Tau could contribute to toxicity in neurodegenerative diseases, a molecular understanding of this interaction and its regulation is essential.

Tau regulates the microtubule-dependent migration of glioblastoma cells via the Rho-ROCK signaling pathway

The pathological significance of Tau (encoded by MAPT) in mechanisms driving cell migration in glioblastoma is unclear. By using an shRNA approach to deplete microtubule-stabilizing Tau in U87 cells, we determined its impact on cytoskeletal coordination during migration. We demonstrated here that the motility of these Tau-knockdown cells (shTau cells) was significantly (36%) lower than that of control cells. The shTau cells displayed a slightly changed motility in the presence of nocodazole, which inhibits microtubule formation. Such reduced motility of shTau cells was characterized by a 28% lower number of microtubule bundles at the non-adhesive edges of the tails. In accordance with Tau-stabilized microtubules being required for cell movement, measurements of the front, body and rear section displacements of cells showed inefficient tail retraction in shTau cells. The tail retraction was restored by treatment with Y27632, an inhibitor of Rho-ROCK signaling. Moreover, we clearly identified that shTau cells displayed relocation of the active phosphorylated form of p190-RhoGAP (also known as ARHGAP35), which inhibits Rho-ROCK signaling, and focal adhesion kinase (FAK, also known as PTK2) in cell bodies. In conclusion, our findings indicate that Tau governs the remodeling of microtubule and actin networks for the retraction of the tail of cells, which is necessary for effective migration.

Two Tau binding sites on tubulin revealed by thiol-disulfide exchanges

Tau is a Microtubule-associated protein that induces and stabilizes the formation of the Microtubule cytoskeleton and plays an important role in neurodegenerative diseases. The Microtubules binding region of Tau has been determined for a long time but where and how Tau binds to its partner still remain a topic of debate. We used Site Directed Spin Labeling combined with EPR spectroscopy to monitor Tau upon binding to either Taxol-stabilized MTs or to αβ-tubulin when Tau is directly used as an inducer of MTs formation. Using maleimide-functionalized labels grafted on the two natural cysteine residues of Tau, we found in both cases that Tau remains highly flexible in these regions confirming the fuzziness of Tau:MTs complexes. More interestingly, using labels linked by a disulfide bridge, we evidenced for the first time thiol disulfide exchanges between αβ-tubulin or MTs and Tau. Additionally, Tau fragments having the two natural cysteines or variants containing only one of them were used to determine the role of each cysteine individually. The difference observed in the label release kinetics between preformed MTs or Tau-induced MTs, associated to a comparison of structural data, led us to propose two putative binding sites of Tau on αβ-tubulin.

Tau Interaction with Tubulin and Microtubules: From Purified Proteins to Cells

Microtubules (MTs) play an important role in many cellular processes and are dynamic structures regulated by an important network of microtubules-associated proteins, MAPs, such as Tau. Tau has been discovered as an essential factor for MTs formation in vitro, and its region implicated in binding to MTs has been identified. By contrast, the affinity, the stoichiometry, and the topology of Tau-MTs interaction remain controversial. Indeed, depending on the experiment conditions a wide range of values have been obtained. In this chapter, we focus on three biophysical methods, turbidimetry, cosedimentation assay, and Förster Resonance Energy Transfer to study Tau-tubulin interaction both in vitro and in cell. We highlight precautions that must be taken in order to avoid pitfalls and we detail the nature of the conclusions that can be drawn from these methods about Tau-tubulin interaction.

Interactions of long-chain homologues of colchicine with tubulin

Several colchicine analogues in which the N-acetyl residue has been replaced by aliphatic, straight-chain acyl moieties, have been synthesized. These compounds show high cytotoxic activity at the nanomolar level against the tumoral cell lines HT-29, MCF-7 and A549. Some of them exhibit activities in the picomolar range against the HT-29 line and are thus two to three orders of magnitude more cytotoxic than colchicine. In this specific cell line, the activities were found to be closely related to the length of the acyl carbon chain, an increase in the latter giving rise to an increase in the cytotoxicity with a maximum in the range of 10-12 carbon atoms, followed by a decrease in activity with still longer chains. Some of the compounds inhibit microtubule assembly and induce the formation of abnormal polymers and present in most cases better apparent affinity constants than colchicine. In addition, at IC₅₀ concentrations the analogues block the cell cycle of A549 cells in the G2/M phase. Molecular docking studies suggest that, while interactions of the colchicine analogues with the colchicine binding site at β-tubulin are still present, the increase in the acyl chain length leads to the progressive development of new interactions, not present in colchicine itself, with the neighboring α-tubulin subunit. Indeed, sufficiently long acyl chains span the intradimer interface and contact with a hydrophobic groove in α-tubulin. It is worth noting that some of the compounds show cytotoxicity at concentrations three orders of magnitude lower than colchicine. Their pharmacological use in cancer therapy could possibly be performed with lower dosages and be thus endowed with less acute toxicity problems than in the case of colchicine.

Tau antagonizes end-binding protein tracking at microtubule ends through a phosphorylation-dependent mechanism

Tau antagonizes tracking of end-binding proteins (EBs) at microtubule ends, a process requiring the C-terminal part of EBs and the microtubule-binding sites of tau. The inhibiting activity of tau on EB properties is regulated by tau phosphorylation. The interplay between EBs and tau proteins results in modulation of microtubule dynamics.

Proper regulation of microtubule dynamics is essential for cell functions and involves various microtubule-associated proteins (MAPs). Among them, end-binding proteins (EBs) accumulate at microtubule plus ends, whereas structural MAPs bind along the microtubule lattice. Recent data indicate that the structural MAP tau modulates EB subcellular localization in neurons. However, the molecular determinants of EB/tau interaction remain unknown, as is the effect of this interplay on microtubule dynamics. Here we investigate the mechanisms governing EB/tau interaction in cell-free systems and cellular models. We find that tau inhibits EB tracking at microtubule ends. Tau and EBs form a complex via the C-terminal region of EBs and the microtubule-binding sites of tau. These two domains are required for the inhibitory activity of tau on EB localization to microtubule ends. Moreover, the phosphomimetic mutation S262E within tau microtubule-binding sites impairs EB/tau interaction and prevents the inhibitory effect of tau on EB comets. We further show that microtubule dynamic parameters vary, depending on the combined activities of EBs and tau proteins. Overall our results demonstrate that tau directly antagonizes EB function through a phosphorylation-dependent mechanism. This study highlights a novel role for tau in EB regulation, which might be impaired in neurodegenerative disorders.

Hsp90 oligomerization process: How can p23 drive the chaperone machineries?

The 90-kDa heat shock protein (Hsp90) is a highly flexible dimer that is able to self-associate in the presence of divalent cations or under heat shock. In a previous work, we focused on the Mg2+-induced oligomerization process of Hsp90, and characterized the oligomers. Combining analytical ultracentrifugation, size-exclusion chromatography coupled to multi-angle laser light scattering and high-mass matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, we studied the interaction of p23 with both Hsp90 dimer and oligomers. Even if p23 predominantly binds the Hsp90 dimer, we demonstrated, for the first time, that p23 is also able to interact with Hsp90 oligomers, shifting the Hsp90 dimer-oligomers equilibrium toward dimer. Our results showed that the Hsp90:p23 binding stoichiometry decreases with the Hsp90 oligomerization degree. Therefore, we propose a model in which p23 would act as a "protein wedge" regarding the Hsp90 dimer closure and the Hsp90 oligomerization process.

Hsp90 Oligomers Interacting with the Aha1 Cochaperone: An Outlook for the Hsp90 Chaperone Machineries

The 90-kDa heat shock protein (Hsp90) is a highly flexible dimer able to self-associate in the presence of divalent cations or under heat shock. This study investigated the relationship between Hsp90 oligomers and the Hsp90 cochaperone Aha1 (activator of Hsp90 ATPase). The interactions of Aha1 with Hsp90 dimers and oligomers were evaluated by ultracentrifugation, size-exclusion chromatography coupled to multiangle laser light scattering and high-mass matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Hsp90 dimer was able to bind up to four Aha1 molecules, and Hsp90 oligomers are also able to interact with Aha1. The binding of Aha1 did not interfere with the Hsp90 oligomerization process. Except for Hsp90 dimer, the stoichiometry of the interaction remained constant, at 2 Aha1 molecules per Hsp90 dimer, regardless of the degree of Hsp90 oligomerization. Moreover, Aha1 predominantly bound to Hsp90 oligomers. Thus, the ability of Hsp90 oligomers to bind the Aha1 ATPase activator reinforces their role within the Hsp90 chaperone machineries.

Insights into the coiled-coil organization of the Hendra virus phosphoprotein from combined biochemical and SAXS studies

Nipah and Hendra viruses are recently emerged paramyxoviruses belonging to the Henipavirus genus. The Henipavirus phosphoprotein (P) consists of a large intrinsically disordered domain and a C-terminal domain (PCT) containing alternating disordered and ordered regions. Among these latter is the P multimerization domain (PMD). Using biochemical, analytical ultracentrifugation and small-angle X-ray scattering (SAXS) studies, we show that Hendra virus (HeV) PMD forms an elongated coiled-coil homotrimer in solution, in agreement with our previous findings on Nipah virus (NiV) PMD. However, the orientation of the N-terminal region differs from that observed in solution for NiV PMD, consistent with the ability of this region to adopt different conformations. SAXS studies provided evidence for a trimeric organization also in the case of PCT, thus extending and strengthening our findings on PMD. The present results are discussed in light of conflicting reports in the literature pointing to a tetrameric organization of paramyxoviral P proteins.

Synthesis of indole-derived allocolchicine congeners exhibiting pronounced anti-proliferative and apoptosis-inducing properties

The synthesis and biological assessment of indole-based allocolchicine congeners with potent anti-proliferative and apoptosis-inducing activity are reported.

Mechanism of Tau-promoted microtubule assembly as probed by NMR spectroscopy

Determining the molecular mechanism of the neuronal Tau protein in the tubulin heterodimer assembly has been a challenge owing to the dynamic character of the complex and the large size of microtubules. We use here defined constructs comprising one or two tubulin heterodimers to characterize their association with a functional fragment of Tau, named TauF4. TauF4 binds with high affinities to the tubulin heterodimer complexes, but NMR spectroscopy shows that it remains highly dynamic, partly because of the interaction with the acidic C-terminal tails of the tubulin monomers. When bound to a single tubulin heterodimer, TauF4 is characterized by an overhanging peptide corresponding to the first of the four microtubule binding repeats of Tau. This peptide becomes immobilized in the complex with two longitudinally associated tubulin heterodimers. The longitudinal associations are favored by the fragment and contribute to Tau's functional role in microtubule assembly.

Quantitative analysis of tau-microtubule interaction using FRET

The interaction between the microtubule associated protein, tau and the microtubules is investigated. A fluorescence resonance energy transfer (FRET) assay was used to determine the distance separating tau to the microtubule wall, as well as the binding parameters of the interaction. By using microtubules stabilized with Flutax-2 as donor and tau labeled with rhodamine as acceptor, a donor-to-acceptor distance of 54 ± 1 Å was found. A molecular model is proposed in which Flutax-2 is directly accessible to tau-rhodamine molecules for energy transfer. By titration, we calculated the stoichiometric dissociation constant to be equal to 1.0 ± 0.5 µM. The influence of the C-terminal tails of αβ-tubulin on the tau-microtubule interaction is presented once a procedure to form homogeneous solution of cleaved tubulin has been determined. The results indicate that the C-terminal tails of α- and β-tubulin by electrostatic effects and of recruitment seem to be involved in the binding mechanism of tau.

Biochemical and structural studies of the oligomerization domain of the Nipah virus phosphoprotein: evidence for an elongated coiled-coil homotrimer

Nipah virus (NiV) is a recently emerged severe human pathogen that belongs to the Henipavirus genus within the Paramyxoviridae family. The NiV genome is encapsidated by the nucleoprotein (N) within a helical nucleocapsid that is the substrate used by the polymerase for transcription and replication. The polymerase is recruited onto the nucleocapsid via its cofactor, the phosphoprotein (P). The NiV P protein has a modular organization, with alternating disordered and ordered domains. Among these latter, is the P multimerization domain (PMD) that was predicted to adopt a coiled-coil conformation. Using both biochemical and biophysical approaches, we show that NiV PMD forms a highly stable and elongated coiled-coil trimer, a finding in striking contrast with respect to the PMDs of Paramyxoviridae members investigated so far that were all found to tetramerize. The present results therefore represent the first report of a paramyxoviral P protein forming trimers.

Molecular mechanisms of Tau binding to microtubules and its role in microtubule dynamics in live cells

Despite extensive studies, the molecular mechanisms of Tau binding to microtubules (MTs) and its consequences on MT stability still remain unclear. It is especially true in cells where the spatiotemporal distribution of Tau-MT interactions is unknown. Using Förster resonance energy transfer (FRET), we showed that the Tau-MT interaction was distributed along MTs in periodic hotspots of high and low FRET intensities. Fluorescence recovery after photobleaching (FRAP) revealed a two-phase exchange of Tau with MTs as a rapid diffusion followed by a slower binding phase. A real-time FRET assay showed that high FRET occurred simultaneously with rescue and pause transitions at MT ends. To further explore the functional interaction of Tau with MTs, the binding of paclitaxel (PTX), tubulin acetylation induced by trichostatin A (TSA), and the expression of non-acetylatable tubulin were used. With PTX and TSA, FRAP curves best fitted a single phase with a long time constant, whereas with non-acetylatable α-tubulin, curves best fitted a two phase recovery. Upon incubation with PTX and TSA, the number of high and low FRET hotspots decreased by up to 50% and no hotspot was observed during rescue and pause transitions. In the presence of non-acetylatable α-tubulin, a 34% increase in low FRET hotspots occurred, and our real-time FRET assay revealed that low FRET hotspots appeared with MTs recovering growth. In conclusion, we have identified, by FRET and FRAP, a discrete Tau-MT interaction, in which Tau could induce conformational changes of MTs, favoring recovery of MT self-assembly.

Synthesis and biological evaluation of 4-arylcoumarin analogues of combretastatins. Part 2

A series of A-ring variously methoxylated 4-(3-hydroxy-4-methoxyphenyl)coumarins related to combretastatin A-4 was prepared by cross-coupling reactions. Cytotoxicity studies indicated a potent activity against HBL100 cell line. Substitution patterns on A-ring had only a slight effect on antiproliferative activity. For most cytotoxic compounds, the activity as potential modulators of P-gp and BCRP efflux pumps was evaluated. The results show that compounds 2 and 7 were able to restore mitoxantrone accumulation (BCRP) at concentrations similar to that of cyclosporine A. Compound 7 was the most efficient to reverse P-gp activity. All compounds were found to potently inhibit in vitro microtubule formation via a substoichiometric mode of action for the most part. Compounds 1 and 2 were found to have an apparent affinity binding constant similar to that of combretastatin A-4, i.e., 1 × 10 (6) M(-1). The molecular modeling of coumarin derivatives was performed on the basis of the molecular structure of 7, as determined by single-crystal X-ray crystallography. The calculations suggested that the presence of a methoxy group out of the plane of the chromenone moiety is an important steric hindrance factor embedding the accessibility of those molecules inside the binding pocket on tubulin.

Stathmin and interfacial microtubule inhibitors recognize a naturally curved conformation of tubulin dimers

Tubulin is able to switch between a straight microtubule-like structure and a curved structure in complex with the stathmin-like domain of the RB3 protein (T(2)RB3). GTP hydrolysis following microtubule assembly induces protofilament curvature and disassembly. The conformation of the labile tubulin heterodimers is unknown. One important question is whether free GDP-tubulin dimers are straightened by GTP binding or if GTP-tubulin is also curved and switches into a straight conformation upon assembly. We have obtained insight into the bending flexibility of tubulin by analyzing the interplay of tubulin-stathmin association with the binding of several small molecule inhibitors to the colchicine domain at the tubulin intradimer interface, combining structural and biochemical approaches. The crystal structures of T(2)RB3 complexes with the chiral R and S isomers of ethyl-5-amino-2-methyl-1,2-dihydro-3-phenylpyrido[3,4-b]pyrazin-7-yl-carbamate, show that their binding site overlaps with colchicine ring A and that both complexes have the same curvature as unliganded T(2)RB3. The binding of these ligands is incompatible with a straight tubulin structure in microtubules. Analytical ultracentrifugation and binding measurements show that tubulin-stathmin associations (T(2)RB3, T(2)Stath) and binding of ligands (R, S, TN-16, or the colchicine analogue MTC) are thermodynamically independent from one another, irrespective of tubulin being bound to GTP or GDP. The fact that the interfacial ligands bind equally well to tubulin dimers or stathmin complexes supports a bent conformation of the free tubulin dimers. It is tempting to speculate that stathmin evolved to recognize curved structures in unassembled and disassembling tubulin, thus regulating microtubule assembly.

Synthesis and complete assignment of the 1H and 13C NMR signals of new acetamido and aminoflavonoid derivatives

The complete (1)H and (13)C NMR assignment of 9 acetamidochalcones, 18 acetamidoflavones, 18 aminoflavones, 9 acetamidoflavonols and 9 aminoflavonols has been performed using one- and two-dimensional NMR techniques including COSY, HMQC and HMBC experiments.