The effect of multivalent cations and Tau on paclitaxel-stabilized microtubule assembly, disassembly, and structure

TAUCON and TAUCOM: A novel biosensor based on fluorescence resonance energy transfer for detecting tau hyperphosphorylation-associated cellular pathologies

Tauopathies are neurodegenerative diseases characterized by abnormal conformational changes in tau protein. Early hyperphosphorylation-induced conformational changes are considered a hallmark of tauopathy, but real-time tracking methods are lacking. Here, we present two novel fluorescence resonance energy transfer (FRET)-based tau biosensors that detect such changes with high spatiotemporal resolution at the single-cell level. The TAUCON biosensor measures instantaneous conformational changes in hyperphosphorylated tau within 20 min, while the TAUCOM biosensor detects changes in the paper-clip structure of microtubule-associated tau. Our biosensors provide faster and more precise detection than conventional methods and can serve as valuable tools for investigating the initial causes, mechanisms, progression, and treatment of tauopathies.

Spastin Promotes the Migration and Invasion Capability of T98G Glioblastoma Cells by Interacting with Pin1 through Its Microtubule-Binding Domain

Microtubule-severing protein Spastin has been shown to co-localize with actin in migratory glioblastoma cells and is linked to glioblastomas' migration and invasion capacity. However, the effectiveness of Spastin in glioblastoma migration and the molecular mechanism underpinning the orientation of Spastin towards actin filaments remain unknown. Here, we demonstrated that Spastin plays an active role in glioblastoma migration by showing a reduced migratory potential of T98G glioblastoma cells using real-time cell analysis (RTCA) in Spastin-depleted cells. Pull-down assays revealed that a cis-trans isomerase Pin1 interacts with Spastin through binding to the phosphorylated Pin1 recognition motifs in the microtubule-binding domain (MBD), and immunocytochemistry analysis showed that interaction with Pin1 directs Spastin to actin filaments in extended cell regions. Consequently, by utilizing RTCA, we proved that the migration and invasion capacity of T98G glioblastoma cells significantly increased with the overexpression of Spastin, of which the Pin1 recognition motifs in MBD are constitutively phosphorylated, while the overexpression of phospho-mutant form did not have a significant effect on migration and invasion of T98G glioblastoma cells. These findings demonstrate that Pin1 is a novel interaction partner of Spastin, and their interaction drives Spastin to actin filaments, allowing Spastin to contribute to the glioblastomas' migration and invasion abilities.

Asparagus officinalis combined with paclitaxel exhibited synergistic anti-tumor activity in paclitaxel-sensitive and -resistant ovarian cancer cells

PURPOSE

Although paclitaxel is a promising first-line chemotherapeutic drug for ovarian cancer, acquired resistance to paclitaxel is one of the leading causes of treatment failure, limiting its clinical application. Asparagus officinalis has been shown to have anti-tumorigenic effects on cell growth, apoptosis, cellular stress and invasion of various types of cancer cells and has also been shown to synergize with paclitaxel to inhibit cell proliferation in ovarian cancer.

METHODS

Human ovarian cancer cell lines MES and its PTX-resistant counterpart MES-TP cell lines were used and were treated with Asparagus officinalis and paclitaxel alone as well as in combination. Cell proliferation, cellular stress, invasion and DMA damage were investigated and the synergistic effect of a combined therapy analyzed.

RESULTS

In this study, we found that Asparagus officinalis combined with low-dose paclitaxel synergistically inhibited cell proliferation, induced cellular stress and apoptosis and reduced cell invasion in paclitaxel-sensitive and -resistant ovarian cancer cell lines. The combined treatment effects were dependent on DNA damage pathways and suppressing microtubule dynamics, and the AKT/mTOR pathway and microtubule-associated proteins regulated the inhibitory effect through different mechanisms in paclitaxel-sensitive and -resistant cells.

CONCLUSION

These findings suggest that the combination of Asparagus officinalis and paclitaxel have potential clinical implications for development as a novel ovarian cancer treatment strategy.

Microtubule-associated proteins (MAPs) in microtubule cytoskeletal dynamics and spermatogenesis

The microtubule (MT) cytoskeleton in Sertoli cells, a crucial cellular structure in the seminiferous epithelium of adult mammalian testes that supports spermatogenesis, was studied morphologically decades ago. However, its biology, in particular the involving regulatory biomolecules and the underlying mechanism(s) in modulating MT dynamics, are only beginning to be revealed in recent years. This lack of studies in delineating the biology of MT cytoskeletal dynamics undermines other studies in the field, in particular the plausible therapeutic treatment and management of male infertility and fertility since studies have shown that the MT cytoskeleton is one of the prime targets of toxicants. Interestingly, much of the information regarding the function of actin-, MT- and intermediate filament-based cytoskeletons come from studies using toxicant models including some genetic models. During the past several years, there have been some advances in studying the biology of MT cytoskeleton in the testis, and many of these studies were based on the use of pharmaceutical/toxicant models. In this review, we summarize the results of these findings, illustrating the importance of toxicant/pharmaceutical models in unravelling the biology of MT dynamics, in particular the role of microtubule-associated proteins (MAPs), a family of regulatory proteins that modulate MT dynamics but also actin- and intermediate filament-based cytoskeletons. We also provide a timely hypothetical model which can serve as a guide to design functional experiments to study how the MT cytoskeleton is regulated during spermatogenesis through the use of toxicants and/or pharmaceutical agents.

Effect of Divalent Cations on the Structure and Mechanics of Vimentin Intermediate Filaments

Divalent cations behave as effective cross-linkers of intermediate filaments (IFs) such as vimentin IF (VIF). These interactions have been mostly attributed to their multivalency. However, ion-protein interactions often depend on the ion species, and these effects have not been widely studied in IFs. Here, we investigate the effects of two biologically important divalent cations, Zn²⁺ and Ca²⁺, on VIF network structure and mechanics in vitro. We find that the network structure is unperturbed at micromolar Zn²⁺ concentrations, but strong bundle formation is observed at a concentration of 100 μM. Microrheological measurements show that network stiffness increases with cation concentration. However, bundling of filaments softens the network. This trend also holds for VIF networks formed in the presence of Ca²⁺, but remarkably, a concentration of Ca²⁺ that is two orders higher is needed to achieve the same effect as with Zn²⁺, which suggests the importance of salt-protein interactions as described by the Hofmeister effect. Furthermore, we find evidence of competitive binding between the two divalent ion species. Hence, specific interactions between VIFs and divalent cations are likely to be an important mechanism by which cells can control their cytoplasmic mechanics.

Phosphorylation of Tau and α-Synuclein Induced Neurodegeneration in MPTP Mouse Model of Parkinson's Disease

PURPOSE

Parkinson's disease (PD) is the second most common neurodegenerative disease. The α-Synuclein is a major component of Lewy bodies and Lewy neurites, the pathologic hallmark of PD. It is known that α-Synuclein is phosphorylated (p-α-Synuclein) in PD and tau-hyperphosphorylation (p-Tau) is also a pathologic feature of PD. However, the relationship between p-Synuclein and p-Tau in PD is not clear, in particular in the MPTP model of PD. The purpose of this study was to reveal their relationship in the mouse MPTP model.

METHODS

Firstly, the p-α-Synuclein, α-Synuclein, p-Tau and Tau protein levels were analyzed. Then, GSK3β activation was determined using immunoblot and immunohistochemical staining. Finally, the dopaminergic neurodegeneration was assessed using Tyrosine Hydroxylase (TH) staining and retrograde labeling and microglial marker were labeled. Microglial activation and nigrostriatal pathway degeneration were observed.

RESULTS

The results showed that p-α-Synuclein, α-Synuclein, p-Tau and Tau were upregulated in both hippocampus and substantia nigra of the PD mouse model. Furthermore, p-α-Synuclein and p-Tau were localized in the same regions of substantial nigra (SN) and dentate gyrus (DG) of hippocampus (Hippo). The activated form of GSK3β (phosphor GSK3β Y216) was increased in multiple brain areas. The GSK3β inhibitor AZD1080 injected in MPTP mice suppressed the expression of p-Tau and p-GSK3β and improved motor functions.

CONCLUSION

These findings revealed that p-α-Synuclein and p-Tau proteins are key pathological events leading to neurodegeneration and motor dysfunctions in the mouse MPTP model of PD. Our data suggest that the interference with the GSK3β activity may be an effective approach for the treatment of PD.

Reversible Control of Spacing in Charged Lamellar Membrane Hydrogels by Hydrophobically Mediated Tethering with Symmetric and Asymmetric Double-End-Anchored Poly(ethylene glycol)s

Complex materials often achieve their remarkable functional properties by hierarchical assembly of building blocks via competing and/or synergistic interactions. Here, we describe the properties of new double-end-anchored poly(ethylene glycol)s (DEA-PEGs)-macromolecules designed to impart hydrophobically mediated tethering attractions between charged lipid membranes. We synthesized DEA-PEGs (MW 2000 (2K) and 4.6K) with two double-tail (symmetric) or a double-tail and a single-tail (asymmetric) hydrophobic end anchors and characterized their equilibrium and kinetic properties using small-angle X-ray scattering. Control multilayer membranes without and with PEG lipid (i.e., single-end-anchored PEG) swelled continuously, with the interlayer spacing increasing between 30 and 90 wt % water content due to electrostatic as well as, in the case of PEG lipid, steric repulsion. In contrast, interlayer spacings in lamellar membrane hydrogels containing DEA-PEGs expanded over a limited water dilution range and reached a "locked" state, which displayed a near constant membrane wall-to-wall spacing (δw) with further increases in water content. Remarkably, the locked state displays a simple relation to the PEG radius of gyration δw ≈ 1.6 RG for both 2K and 4.6K PEG. Nevertheless, δw being considerably less than the physical size of PEG (2(5/3)1/2 RG) is highly unexpected and implies that, compared to free PEG, anchoring of the PEG tether at both ends leads to a considerable distortion of the PEG conformation confined between layers. Significantly, the lamellar hydrogel may be designed to reversibly transition from a locked to an unlocked (membrane unbinding) state by variations in the DEA-PEG concentration, controlling the strength of the interlayer attractions due to bridging conformations. The findings with DEA-PEGs have broad implications for hydrophobic-mediated assembly of lipid- or surfactant-coated building blocks with distinct shape and size, at predictable spacing, in aqueous environments.

Identification of proteins associated with paclitaxel resistance of epithelial ovarian cancer using iTRAQ-based proteomics

Chemotherapy is an important adjuvant therapy for epithelial ovarian cancer (EOC). The main cause of chemotherapy failure in EOC is paclitaxel resistance. The present study aimed to identify novel biomarkers to predict chemosensitivity to paclitaxel and improve our understanding of the molecular mechanisms underlying paclitaxel resistance in EOC. In the present study, the heterogeneity of EOC was evaluated by adenosine triphosphate-tumor chemosensitivity assay (ATP-TCA) in vitro. Fresh samples were collected from 54 EOC cases during cytoreductive surgery. Tumor cells were isolated, cultured, and tested for sensitivity to paclitaxel. Proteins that were differentially expressed between paclitaxel-resistant tissues and paclitaxel-sensitive tissues were identified via isobaric tags for relative and absolute quantitation (iTRAQ)-based proteomic analysis. Two upregulated proteins, plexin domain containing 2 (Plxdc2) and cytokeratin 7 (CK7), were selected to verify the iTRAQ method using western blot analysis in EOC tissues with different chemosensitivities (sensitive, weakly sensitive and resistant). There was notable heterogeneity of chemosensitivity in the EOC specimens. Highly to mildly-differentiated or early-stage (I/II) EOC specimens had decreased sensitivity to paclitaxel compared with specimens with low differentiation (P<0.05) or an advanced stage (III; P<0.05), respectively. A total of 496 significantly differentially expressed proteins, including 263 that were downregulated (P<0.05) and 233 that were upregulated (P<0.05) in paclitaxel-resistant tissues compared with paclitaxel-sensitive tissues, were identified using iTRAQ in combination with LC-MS/MS. The expression levels of two proteins associated with paclitaxel resistance, Plxdc2 and CK7, were further validated by western blotting, which revealed that they were upregulated in the paclitaxel-resistant tissues. The present study determined candidate proteins associated with paclitaxel resistance in EOC. Plxdc2 and CK7 may be potential makers for distinguishing patients with paclitaxel-resistant EOC from those with paclitaxel-sensitive EOC.

Synchrotron small-angle X-ray scattering and electron microscopy characterization of structures and forces in microtubule/Tau mixtures

Tau, a neuronal protein known to bind to microtubules and thereby regulate microtubule dynamic instability, has been shown recently to not only undergo conformational transitions on the microtubule surface as a function of increasing microtubule coverage density (i.e., with increasing molar ratio of Tau to tubulin dimers) but also to mediate higher-order microtubule architectures, mimicking fascicles of microtubules found in the axon initial segment. These discoveries would not have been possible without fine structure characterization of microtubules, with and without applied osmotic pressure through the use of depletants. Herein, we discuss the two primary techniques used to elucidate the structure, phase behavior, and interactions in microtubule/Tau mixtures: transmission electron microscopy and synchrotron small-angle X-ray scattering. While the former is able to provide striking qualitative images of bundle morphologies and vacancies, the latter provides angstrom-level resolution of bundle structures and allows measurements in the presence of in situ probes, such as osmotic depletants. The presented structural characterization methods have been applied both to equilibrium mixtures, where paclitaxel is used to stabilize microtubules, and also to dissipative nonequilibrium mixtures at 37°C in the presence of GTP and lacking paclitaxel.

Molecular basis of the microtubule-regulating activity of microtubule crosslinking factor 1

The variety of microtubule arrays observed across different cell types should require a diverse group of proteins that control microtubule organization. Nevertheless, mainly because of the intrinsic propensity of microtubules to easily form bundles upon stabilization, only a small number of microtubule crosslinking proteins have been identified, especially in postmitotic cells. Among them is microtubule crosslinking factor 1 (MTCL1) that not only interconnects microtubules via its N-terminal microtubule-binding domain (N-MTBD), but also stabilizes microtubules via its C-terminal microtubule-binding domain (C-MTBD). Here, we comprehensively analyzed the assembly structure of MTCL1 to elucidate the molecular basis of this dual activity in microtubule regulation. Our results indicate that MTCL1 forms a parallel dimer not only through multiple homo-interactions of the central coiled-coil motifs, but also the most C-terminal non-coiled-coil region immediately downstream of the C-MTBD. Among these homo-interaction regions, the first coiled-coil motif adjacent to N-MTBD is sufficient for the MTCL1 function to crosslink microtubules without affecting the dynamic property, and disruption of this motif drastically transformed MTCL1-induced microtubule assembly from tight to network-like bundles. Notably, suppression of the homo-interaction of this motif inhibited the endogenous MTCL1 function to stabilize Golgi-associated microtubules that are essential for Golgi-ribbon formation. Because the microtubule-stabilizing activity of MTCL1 is completely attributed to C-MTBD, the present study suggests possible interplay between N-MTBD and C-MTBD, in which normal crosslinking and accumulation of microtubules by N-MTBD is essential for microtubule stabilization by C-MTBD.