The quantitation of tubulin in neuroblastoma cells by radioimmunoassay

Multiscale Mechanobiology in Brain Physiology and Diseases

Increasing evidence suggests that mechanics play a critical role in regulating brain function at different scales. Downstream integration of mechanical inputs into biochemical signals and genomic pathways causes observable and measurable effects on brain cell fate and can also lead to important pathological consequences. Despite recent advances, the mechanical forces that influence neuronal processes remain largely unexplored, and how endogenous mechanical forces are detected and transduced by brain cells into biochemical and genetic programs have received less attention. In this review, we described the composition of brain tissues and their pronounced microstructural heterogeneity. We discuss the individual role of neuronal and glial cell mechanics in brain homeostasis and diseases. We highlight how changes in the composition and mechanical properties of the extracellular matrix can modulate brain cell functions and describe key mechanisms of the mechanosensing process. We then consider the contribution of mechanobiology in the emergence of brain diseases by providing a critical review on traumatic brain injury, neurodegenerative diseases, and neuroblastoma. We show that a better understanding of the mechanobiology of brain tissues will require to manipulate the physico-chemical parameters of the cell microenvironment, and to develop three-dimensional models that can recapitulate the complexity and spatial diversity of brain tissues in a reproducible and predictable manner. Collectively, these emerging insights shed new light on the importance of mechanobiology and its implication in brain and nerve diseases.

Microtubule cytoskeleton-disrupting activity of MWCNTs: applications in cancer treatment

Microtubules and carbon nanotubes (CNTs), and more particularly multi-walled CNTs (MWCNTs), share many mechanical and morphological similarities that prompt their association into biosynthetic tubulin filaments both, in vitro and in vivo. Unlike CNTs, microtubules are highly dynamic protein polymers that, upon interaction with these nanomaterials, display enhanced stability that has critical consequences at the cellular level. Among others, CNTs prompt ectopic (acentrosomal) microtubule nucleation and the disassembly of the centrosome, causing a dramatic cytoskeletal reorganization. These changes in the microtubule pattern trigger the generation of ineffective biomechanical forces that result in migration defects, and ultimately in spindle-assembly checkpoint (SAC) blockage and apoptosis. In this review, we describe the molecular mechanism involved in the intrinsic interference of CNTs with the microtubule dynamics and illustrate the consequences of this effect on cell biomechanics. We also discuss the potential application of these synthetic microtubule-stabilizing agents as synergetic agents to boost the effect of classical chemotherapy that includes spindle poisons (i.e. paclitaxel) or DNA interfering agents (5-fluorouracil)-, and list some of the advantages of the use of MWCNTs as adjuvant agents in preventing cell resistance to chemotherapy.

Microtubules as Sub-Cellular Memristors

Memristors represent the fourth electrical circuit element complementing resistors, capacitors and inductors. Hallmarks of memristive behavior include pinched and frequency-dependent I–V hysteresis loops and most importantly a functional dependence of the magnetic flux passing through an ideal memristor on its electrical charge. Microtubules (MTs), cylindrical protein polymers composed of tubulin dimers are key components of the cytoskeleton. They have been shown to increase solution’s ionic conductance and re-orient in the presence of electric fields. It has been hypothesized that MTs also possess intrinsic capacitive and inductive properties, leading to transistor-like behavior. Here, we show a theoretical basis and experimental support for the assertion that MTs under specific circumstances behave consistently with the definition of a memristor. Their biophysical properties lead to pinched hysteretic current–voltage dependence as well a classic dependence of magnetic flux on electric charge. Based on the information about the structure of MTs we provide an estimate of their memristance. We discuss its significance for biology, especially neuroscience, and potential for nanotechnology applications.

Investigation of the Electrical Properties of Microtubule Ensembles under Cell-Like Conditions

Microtubules are hollow cylindrical polymers composed of the highly negatively-charged (~23e), high dipole moment (1750 D) protein α, β- tubulin. While the roles of microtubules in chromosomal segregation, macromolecular transport, and cell migration are relatively well-understood, studies on the electrical properties of microtubules have only recently gained strong interest. Here, we show that while microtubules at physiological concentrations increase solution capacitance, free tubulin has no appreciable effect. Further, we observed a decrease in electrical resistance of solution, with charge transport peaking between 20-60 Hz in the presence of microtubules, consistent with recent findings that microtubules exhibit electric oscillations at such low frequencies. We were able to quantify the capacitance and resistance of the microtubules (MT) network at physiological tubulin concentrations to be 1.27 × 10^-5 F and 9.74 × 10⁴ Ω. Our results show that in addition to macromolecular transport, microtubules also act as charge storage devices through counterionic condensation across a broad frequency spectrum. We conclude with a hypothesis of an electrically tunable cytoskeleton where the dielectric properties of tubulin are polymerisation-state dependent.

Relationship between Smooth Muscle and Cytoskeleton Antibodies in Neuroblastoma

The previously reported high prevalence of smooth muscle antibodies in neuroblastoma has been found to be associated with a similarly elevated prevalence of anti-cytoskeleton antibodies. The most relevant finding is related to anti-microfilaments (anti-actin) and anti-microtubules antibodies, which were detected with highly significantly different prevalences when compared with a disease control group (p < 0.001 and p < 0.000001, respectively). The correspondence between smooth muscle antibodies and anti-cytoskeleton antibodies is incomplete, and it is more relevant for anti-microfilaments. It is concluded that antigen specificities of smooth muscle antibodies in neuroblastoma are as complex as documented in other diseases. Possible pathogenetic and clinical implications emerging from these data are discussed.

Exosome is a mechanism of intercellular drug transfer: Application of quantitative pharmacology

PURPOSE

Exosomes are small membrane vesicles (30-100nm in diameter) secreted by cells into extracellular space. The present study evaluated the effect of chemotherapeutic agents on exosome production and/or release, and quantified the contribution of exosomes to intercellular drug transfer and pharmacodynamics.

METHODS

Human cancer cells (breast MCF7, breast-to-lung metastatic LM2, ovarian A2780 and OVCAR4) were treated with paclitaxel (PTX, 2-1000nM) or doxorubicin (DOX, 20-1000nM) for 24-48h. Exosomes were isolated from the culture medium of drug-treated donor cells (Donor cells) using ultra-centrifugation, and analyzed for acetylcholinesterase activity, total proteins, drug concentrations, and biological effects (cytotoxicity and anti-migration) on drug-naïve recipient cells (Recipient cells). These results were used to develop computational predictive quantitative pharmacology models.

RESULTS

Cells in exponential growth phase released ~220 exosomes/cell in culture medium. PTX and DOX significantly promoted exosome production and/or release in a dose- and time-dependent manner, with greater effects in ovarian cancer cells than in breast cancer cells. Exosomes isolated from Donor cells contained appreciable drug levels (2-7pmole/10⁶ cells after 24h treatment with 100-1000nM PTX), and caused cytotoxicity and inhibited migration of Recipient cells. Quantitative pharmacology models that integrated cellular PTX pharmacokinetics with PTX pharmacodynamics successfully predicted effects of exosomes on intercellular drug transfer, cytotoxicity of PTX on Donor cells and cytotoxicity of PTX-containing exosomes on Recipient cells. Additional model simulations indicate that within clinically achievable PTX concentrations, the contribution of exosomes to active drug efflux increased with drug concentration and exceeded the p-glycoprotein efflux when the latter was saturated.

CONCLUSIONS

Our results indicate (a) chemotherapeutic agents stimulate exosome production or release, and (b) exosome is a mechanism of intercellular drug transfer that contributes to pharmacodynamics of neighboring cells.

Proteomic data from human cell cultures refine mechanisms of chaperone-mediated protein homeostasis

In the crowded environment of human cells, folding of nascent polypeptides and refolding of stress-unfolded proteins is error prone. Accumulation of cytotoxic misfolded and aggregated species may cause cell death, tissue loss, degenerative conformational diseases, and aging. Nevertheless, young cells effectively express a network of molecular chaperones and folding enzymes, termed here "the chaperome," which can prevent formation of potentially harmful misfolded protein conformers and use the energy of adenosine triphosphate (ATP) to rehabilitate already formed toxic aggregates into native functional proteins. In an attempt to extend knowledge of chaperome mechanisms in cellular proteostasis, we performed a meta-analysis of human chaperome using high-throughput proteomic data from 11 immortalized human cell lines. Chaperome polypeptides were about 10% of total protein mass of human cells, half of which were Hsp90s and Hsp70s. Knowledge of cellular concentrations and ratios among chaperome polypeptides provided a novel basis to understand mechanisms by which the Hsp60, Hsp70, Hsp90, and small heat shock proteins (HSPs), in collaboration with cochaperones and folding enzymes, assist de novo protein folding, import polypeptides into organelles, unfold stress-destabilized toxic conformers, and control the conformal activity of native proteins in the crowded environment of the cell. Proteomic data also provided means to distinguish between stable components of chaperone core machineries and dynamic regulatory cochaperones.

Paclitaxel-conjugated PAMAM dendrimers adversely affect microtubule structure through two independent modes of action

Paclitaxel (Taxol) is an anticancer drug that induces mitotic arrest via microtubule hyperstabilization but causes side effects due to its hydrophobicity and cellular promiscuity. The targeted cytotoxicity of hydrophilic paclitaxel-conjugated polyamidoamine (PAMAM) dendrimers has been demonstrated in cultured cancer cells. Mechanisms of action responsible for this cytotoxicity are unknown, that is, whether the cytotoxicity is due to paclitaxel stabilization of microtubules, as is whether paclitaxel is released intracellularly from the dendrimer. To determine whether the conjugated paclitaxel can bind microtubules, we used a combination of ensemble and single microtubule imaging techniques in vitro. We demonstrate that these conjugates adversely affect microtubules by (1) promoting the polymerization and stabilization of microtubules in a paclitaxel-dependent manner, and (2) bundling preformed microtubules in a paclitaxel-independent manner, potentially due to protonation of tertiary amines in the dendrimer interior. Our results provide mechanistic insights into the cytotoxicity of paclitaxel-conjugated PAMAM dendrimers and uncover unexpected risks of using such conjugates therapeutically.

A diffusion-based neurite length-sensing mechanism involved in neuronal symmetry breaking

Shootin1, one of the earliest markers of neuronal symmetry breaking, accumulates in the neurite tips of polarizing neurons in a neurite length-dependent manner. Thus, neurons sense their neurites' length and translate this spatial information into a molecular signal, shootin1 concentration.

Quantitative live cell imaging of shootin1 dynamics combined with mathematical modeling analyses reveals that its anterograde transport and retrograde diffusion in neurite shafts account for the neurite length-dependent accumulation of shootin1.

The neurite length-dependent shootin1 accumulation and shootin1-induced neurite outgrowth constitute a positive feedback loop that amplifies stochastic shootin1 signals in neurite tips.

Quantitative mathematical modeling shows that the above positive feedback loop, together with shootin1 upregulation, constitutes a core mechanism for neuronal symmetry breaking.

Cell morphology and size must be properly controlled to ensure cellular function. Although there has been significant progress in understanding the molecular signals that change cell morphology, the manner in which cells monitor their size and length to regulate their morphology is poorly understood. Cultured hippocampal neurons polarize by forming a single long axon and multiple short dendrites (Craig and Banker, 1994; Arimura and Kaibuchi, 2007), and symmetry breaking is the initial step of this process. This symmetry-breaking step reproduces even when the neuronal axon is transected; the longest neurite usually grows rapidly to become an axon after transection, regardless of whether it is the axonal stump or another neurite (Goslin and Banker, 1989). Elongation of an immature neurite by mechanical tension also leads to its axonal specification (Lamoureux et al, 2002). These results suggest that cultured hippocampal neurons can sense neurite length, identify the longest one, and induce its subsequent axonogenesis for symmetry breaking. However, little is known about the mechanism for this process.

Shootin1 is one of the earliest markers of neuronal symmetry breaking (Toriyama et al, 2006). During the symmetry-breaking step, it undergoes a stochastic accumulation in neurite tips, and eventually accumulates predominantly in a single neurite that subsequently grows to become an axon. In this study, we demonstrated that shootin1 accumulates in neurite tips in a neurite length-dependent manner, regardless of whether it is the axonal stump or another neurite (Figure 3A, C–F). Thus, morphological information (neurite length) is translated into a molecular signal (shootin1 concentration in neurite tips).

We previously reported that shootin1 is transported from the cell body to neurite tips as discrete boluses and diffuses back to the cell body (Toriyama et al, 2006). The boluses containing variable amounts of shootin1 traveled repeatedly but irregularly along neurites, and their arrival caused large stochastic fluctuations in shootin1 concentration in the neurite tips. To understand the mechanism of length-dependent shootin1 accumulation, we performed quantitative live cell imaging of the anterograde transport and retrograde diffusion of shootin1 and fitted the obtained data into mathematical models of the anterograde transport and retrograde diffusion. The parameters of these two models were derived entirely from quantitative experimental data, without any adjustment. Shootin1 concentration at neurite tips, calculated by integrating the two models, was neurite length dependent (Figure 3B) and showed good agreement with the experimental data (Figure 3A). These results suggest that the neurite length-dependent accumulation of shootin1 is quantitatively explained by its anterograde transport and retrograde diffusion.

This length-dependent shootin1 accumulation constitutes a positive feedback interaction with the previously reported shootin1-induced neurite outgrowth (Shimada et al, 2008). To analyze the functional role of this feedback loop, we quantified shootin1 upregulation (Toriyama et al, 2006) and shootin1-induced neurite outgrowth, and integrated them, together with the above model of length-dependent shootin1 accumulation, into a model neuron (Figure 7A). Furthermore, the parameters of the model components were chosen to give the best fit to the quantitative experimental data without any adjustment. Integrating the three components into a model neuron resulted in spontaneous symmetry breaking (Figure 7B and C). Furthermore, there are a total of 15 agreements between the model predictions and the experimental data, including the neurite length-dependent axon specification and regeneration (Goslin and Banker, 1989; Lamoureux et al, 2002). These data suggest that the three components in our model—namely, diffusion-based neurite length sensing system, shootin1-induced neurite outgrowth and shootin1 upregulation—are sufficient to induce neuronal symmetry breaking.

Bolus-like transport of shootin1 caused large stochastic fluctuations in shootin1 concentration in neurite tips. Interestingly, the generation of continuous shootin1 transport in our model neuron impaired the symmetry-breaking process (Figure 7D). This is consistent with theoretical models in which feedback amplification of fluctuations in signaling can give rise to robust patterns (Turing, 1952; Meinhardt and Gierer, 2000; Kondo, 2002), and underscores the importance of the stochastic fluctuating signals in spontaneous neuronal symmetry breaking.

The combination of quantitative experimentation and mathematical modeling is regarded as a powerful strategy for attaining a profound understanding of biological systems (Hodgkin and Huxley, 1952b; Lewis, 2008; Ferrell, 2009). By focusing on a simple system involving one of the earliest markers of neuronal symmetry breaking, shootin1, we were able to evaluate here the core components of neuronal symmetry breaking on the basis of quantitative experimental data. The present model may thus provide a core mechanism of neuronal symmetry breaking, to which other possible mechanisms can be added to increase the model's complexity in future studies.

Although there has been significant progress in understanding the molecular signals that change cell morphology, mechanisms that cells use to monitor their size and length to regulate their morphology remain elusive. Previous studies suggest that polarizing cultured hippocampal neurons can sense neurite length, identify the longest neurite, and induce its subsequent outgrowth for axonogenesis. We observed that shootin1, a key regulator of axon outgrowth and neuronal polarization, accumulates in neurite tips in a neurite length-dependent manner; here, the property of cell length is translated into shootin1 signals. Quantitative live cell imaging combined with modeling analyses revealed that intraneuritic anterograde transport and retrograde diffusion of shootin1 account for its neurite length-dependent accumulation. Our quantitative model further explains that the length-dependent shootin1 accumulation, together with shootin1-dependent neurite outgrowth, constitutes a positive feedback loop that amplifies stochastic fluctuations of shootin1 signals, thereby generating an asymmetric signal for axon specification and neuronal symmetry breaking.

Repair of peroxynitrite damage to tubulin by the thioredoxin reductase system

Cumulative oxidative damage to proteins coupled with a decrease in repair has been implicated in the pathology of several neurodegenerative diseases. Herein we report that peroxynitrite-induced disulfides in porcine brain tubulin are repaired by the thioredoxin reductase system composed of rat liver thioredoxin reductase, human or Escherichia coli thioredoxin, and NADPH. Disulfide bonds between the alpha-tubulin and the beta-tubulin subunits were repaired by thioredoxin reductase as determined by Western blot under nonreducing conditions. Total disulfide repair by thioredoxin reductase was assessed using a sulfhydryl-specific labeling reagent, 5-iodoacetamido-fluorescein. Treatment of tubulin with 1.0 mM peroxynitrite anion decreased 5-iodoacetamido-fluorescein labeling by 48%; repair of peroxynitrite-damaged tubulin with thioredoxin reductase restored sulfhydryl labeling to control levels. Tubulin disulfide reduction by thioredoxin reductase restored tubulin polymerization activity that was lost after peroxynitrite was added. The extent of activity restored by thioredoxin reductase and by the nonspecific disulfide-reducing agent tris(2-carboxyethyl)phosphine hydrochloride was identical; however, activity was not restored to control levels. Tyrosine nitration of tubulin was detected at all concentrations of peroxynitrite tested; thus, tubulin nitration may be responsible for the fraction of activity that could not be restored. Thiol-disulfide exchange between tubulin and thioredoxin was detected by Western blot, thereby providing further support for our observations that optimal repair of tubulin disulfides required thioredoxin.

Mechanisms blocking microtubule minus end assembly: evidence for a tubulin dimer-binding protein

We have characterized an activity in sea urchin eggs which prevents microtubule assembly at minus ends. Using Chlamydomonas axoneme fragments to nucleate the assembly of plus and minus end microtubules, we find robust assembly at microtubule plus ends with negligible assembly at minus ends. The minus end assembly inhibitor does not co-pellet with microtubules when assembly is stimulated with DMSO while the resulting pellet of tubulin and microtubule associated proteins readily assembles from both plus and minus ends of axoneme fragments. Addition of increasing concentrations of porcine bran tubulin to the tubulin and MAP-depleted fraction eventually saturates the minus end inhibitory activity. Compared to purified tubulin, cytosolic fractions both increase the minus end critical concentration approximately 3 fold and decrease the plus end critical concentration. The inhibitory activity is removed by heating, trypsin, or by co-immunoprecipitation with tubulin. We hypothesize that a tubulin dimer binding protein is responsible for preventing assembly onto minus ends in our in vitro assays and speculate that this protein functions in vivo to prevent spontaneous nucleation, thus limiting assembly to nucleation sites.

Adenine and guanine nucleotide content of Triton-extracted cytoskeletal fractions of nonmuscle cells

Determination of the adenine and guanine nucleotides in Triton X-100-extracted cytoskeletal fractions was utilized to estimate the actin and tubulin content of the assembled cytoskeletons in nonmuscle cells. Results with stable cell lines (i.e., rat pheochromocytoma PC12 and neuroblastoma NB41A3) and with primary cultures (i.e., human foreskin fibroblasts and chick embryonic dorsal root ganglion neurons) exhibited levels of cytoskeletal fraction ADP and GDP consistent with their assembly-induced nucleoside-5'-triphosphatase activities only previously analyzed in vitro. Likewise, estimates of actin and tubulin content fall in the range of values obtained by other experimental approaches. In contrast, analysis of whole cell nucleotides showed high [ATP]/[ADP] and [GTP]/[GDP] ratios, suggesting there is little, if any, contamination of the cytoskeletal nucleotide pool by other cellular nucleotides.

Tubulin assembly probed with antibodies to synthetic peptides

Antibodies to synthetic peptides from the alpha and beta-tubulin sequences were employed to study zones of this protein active in microtubule assembly. In purified calf brain tubulin, six short sequences, selected according to their hydrophilicity and conservation, were found to be accessible to their affinity-purified immunoglobulin G (IgG) antibodies, in a competition radioimmunoassay performed under non-assembly native conditions. This indicated that the six sequences are exposed on the surface of the tubulin alpha beta heterodimer. IgG antibodies to the alpha(430-443) and beta(412-431) sequences perturbed substoichiometrically the assembly of purified tubulin, inducing microtubule bundling and the formation of opened up structures. These positions, which are close to the C termini, were accessible to the anti-peptide antibodies in taxol-induced microtubules, Zn2(+)-induced tubulin sheets, Mg2(+)-induced tubulin rings and in PtK2 cell microtubules. This, together with the comparison of the sizes and gross shapes of the antibody probes and microtubules, suggested that these sequences might be located at the protruding parts of the protofilaments. Antibodies to positions alpha(155-168) did not react with microtubules, while the equivalent zone beta(153-165) was accessible. The alpha(214-226) and beta(241-256) sequences were antigenically occluded in the taxol microtubules, Zn2(+)-induced sheets and Mg2(+)-induced ring arrays, as well as in native microtubules from PtK2 cells, though they became reactive by fixation. This result strongly suggested that these two zones are close to tubulin-tubulin contact sites. A working model is proposed in which the positions alpha(214-226) and beta(241-256) are close to the axial contacts between heterodimers, which lead to protofilament formation, while the positions alpha(241-256) and beta(214-226) are suggested to be related to the alpha-beta binding interface within the heterodimer.

An estimation of the soluble tubulin content in Tetrahymena cells of normal and of size-altered phenotype

The amount of soluble tubulin in a temperature-sensitive (ts) size mutant of the ciliate Tetrahymena was measured in a variety of physiological conditions. For this purpose a competitive ELISA assay for tubulin was set up. The assay is based on an antiserum against Tetrahymena axonemal tubulin. Characterization of the antiserum shows its mono-specificity towards tubulin as well as its potential to recognize tubulin from a wide variety of cellular sources and organisms. After fractionation of the cells into soluble material, cold-labile and cold-resistant structures, we found very little tubulin soluble (less than 20% of the total), while most of the tubulin is polymerized, especially into cortical structures. Prolonged starvation does not alter the tubulin content. During the culture growth cycle the percentage of the soluble tubulin increases. Growing the ts mutant at high temperature to a large cell size will also increase the pool of soluble tubulin to a large extent. Only under this condition is the amount of soluble tubulin about equal to that fixed in cilia. The tubulins in the three different compartments are polymorphic and have a different metabolism. This is indicated by the much higher specific activity of soluble tubulin compared with the structurally bound material. In agreement, the half-life of the soluble tubulin is shorter than that of the cortical tubulin.

Altered tubulin distribution in the hypothalamus of aging female C57BL/6J mice

The effects of age and estradiol on hypothalamic content and distribution of tubulin and calmodulin were examined by radioimmunoassay in ovariectomized C57BL/6J female mice. A small (14%) increase in particulate tubulin, but not soluble tubulin, was found in the hypothalamus of reproductively senescent mice (20 months) compared to young (8 months) controls. This alteration was limited to tubulin; calmodulin content was unaffected by age. Post-castration serum LH levels were lower in old, ovariectomized controls relative to young controls, but physiologic levels of estradiol, achieved by subcutaneous implants, suppressed LH levels in both age groups. In contrast to LH and uterine weight, hypothalamic tubulin and calmodulin were unaffected by estradiol treatment. These results suggest that the negative feedback effect of estradiol on LH secretion is exerted by a mechanism other than redistribution of hypothalamic tubulin or calmodulin, or that changes are restricted to a discrete sub-population of neurons.

Changes in some cytoskeletal proteins during neuroblastoma cell differentiation

After in vitro microtubule assembly of mouse neuroblastoma crude extracts, six protein species migrate in the tubulin region of two-dimensional electrophoregrams. The evolution of these forms after morphological cell differentiation of the clone NIE115 shows two major modifications. Form 5 decreased drastically while form 6 increases during neurite formation. Peptide mapping analysis reveals that forms 5 and 6 are vimentin, a component of intermediate filaments, and beta-tubulin subunit, respectively. Sodium butyrate treatment of NIE115 cells or serum starvation of NIA103 cells, conditions blocking cell division and failing to induce morphological differentiation, prevent any modifications in the relative proportion of these proteins. It is concluded that the changes in the distribution of the tubulin isoforms and vimentin are directly related to neurite formation.