The effect of stathmin phosphorylation on microtubule assembly depends on tubulin critical concentration

Tubulin Complexity in Cancer and Metastasis

Tubulin plays a fundamental role in cellular function and as the subject for microtubule-active agents in the treatment of ovarian cancer. Microtubule-binding proteins (e.g., tau, MAP1/2/4, EB1, CLIP, TOG, survivin, stathmin) and posttranslational modifications (e.g., tyrosination, deglutamylation, acetylation, glycation, phosphorylation, polyamination) further diversify tubulin functionality and may permit additional opportunities to understand microtubule behavior in disease and to develop microtubule-modifying approaches to combat ovarian cancer. Tubulin-based structures that project from suspended ovarian cancer cells known as microtentacles may contribute to metastatic potential of ovarian cancer cells and could represent an exciting novel therapeutic target.

The C-terminus of stathmin-like proteins governs the stability of their complexes with tubulin

Microtubule dynamics is modulated by many cellular factors including stathmin family proteins. Vertebrate stathmins sequester two αβ-tubulin heterodimers into a tight complex that cannot be incorporated in microtubules. Stathmins are regulated at the expression level during development and among tissues; they are also regulated by phosphorylation. Here, we study the dissociation kinetics of tubulin:stathmin assemblies in presence of different tubulin-binding proteins and identify a critical role of the C-terminus of the stathmin partner. Destabilizing this C-terminal region may represent an additional regulatory mechanism of the interaction with tubulin of stathmin proteins.

Nitrosylation of β2-Tubulin Promotes Microtubule Disassembly and Differentiated Cardiomyocyte Beating in Ischemic Mice

BACKGROUND

Beating cardiomyocyte regeneration therapies have revealed as alternative therapeutics for heart transplantation. Nonetheless, the importance of nitric oxide (NO) in cardiomyocyte regeneration has been widely suggested, little has been reported concerning endogenous NO during cardiomyocyte differentiation.

METHODS

Here, we used P19CL6 cells and a Myocardiac infarction (MI) model to confirm NO-induced protein modification and its role in cardiac beating. Two tyrosine (Tyr) residues of β2-tubulin (Y106 and Y340) underwent nitrosylation (Tyr-NO) by endogenously generated NO during cardiomyocyte differentiation from pre-cardiomyocyte-like P19CL6 cells.

RESULTS

Tyr-NO-β2-tubulin mediated the interaction with Stathmin, which promotes microtubule disassembly, and was prominently observed in spontaneously beating cell clusters and mouse embryonic heart (E11.5d). In myocardial infarction mice, Tyr-NO-β2-tubulin in transplanted cells was closely related with cardiac troponin-T expression with their functional recovery, reduced infarct size and thickened left ventricular wall.

CONCLUSION

This is the first discovery of a new target molecule of NO, β2-tubulin, that can promote normal cardiac beating and cardiomyocyte regeneration. Taken together, we suggest therapeutic potential of Tyr-NO-β2-tubulin, for ischemic cardiomyocyte, which can reduce unexpected side effect of stem cell transplantation, arrhythmogenesis.

Preclinical assessment of synergistic efficacy of MELK and CDK inhibitors in adrenocortical cancer

BACKGROUND

Adrenocortical cancer (ACC) is a rare and aggressive cancer with dismal 5-year survival due to a lack of effective treatments. We aimed to identify a new effective combination of drugs and investigated their synergistic efficacy in ACC preclinical models.

METHODS

A quantitative high-throughput drug screening of 4,991 compounds was performed on two ACC cell lines, SW13 and NCI-H295R, based on antiproliferative effect and caspase-3/7 activity. The top candidate drugs were pairwise combined to identify the most potent combinations. The synergistic efficacy of the selected inhibitors was tested on tumorigenic phenotypes, such as cell proliferation, migration, invasion, spheroid formation, and clonogenicity, with appropriate mechanistic validation by cell cycle and apoptotic assays and protein expression of the involved molecules. We tested the efficacy of the drug combination in mice with luciferase-tagged human ACC xenografts. To study the mRNA expression of target molecules in ACC and their clinical correlations, we analyzed the Gene Expression Omnibus and The Cancer Genome Atlas.

RESULTS

We chose the maternal embryonic leucine zipper kinase (MELK) inhibitor (OTS167) and cyclin-dependent kinase (CDK) inhibitor (RGB-286638) because of their potent synergy from the pairwise drug combination matrices derived from the top 30 single drugs. Multiple publicly available databases demonstrated overexpression of MELK, CDK1/2, and partnering cyclins mRNA in ACC, which were independently associated with mortality and other adverse clinical features. The drug combination demonstrated a synergistic antiproliferative effect on ACC cells. Compared to the single-agent treatment groups, the combination treatment increased G2/M arrest, caspase-dependent apoptosis, reduced cyclins A2, B1, B2, and E2 expression, and decreased cell migration and invasion with reduced vimentin. Moreover, the combination effectively decreased Foxhead Box M1, Axin2, glycogen synthase kinase 3-beta, and β-catenin. A reduction in p-stathmin from the combination treatment destabilized microtubule assembly by tubulin depolymerization. The drug combination treatment in mice with human ACC xenografts resulted in a significantly lower tumor burden than those treated with single-agents and vehicle control groups.

CONCLUSIONS

Our preclinical study revealed a novel synergistic combination of OTS167 and RGB-286638 in ACC that effectively targets multiple molecules associated with ACC aggressiveness. A phase Ib/II clinical trial in patients with advanced ACC is therefore warranted.

The Rac-GAP alpha2-Chimaerin Signals via CRMP2 and Stathmins in the Development of the Ocular Motor System

A precise sequence of axon guidance events is required for the development of the ocular motor system. Three cranial nerves grow toward, and connect with, six extraocular muscles in a stereotyped pattern, to control eye movements. The signaling protein alpha2-chimaerin (α2-CHN) plays a pivotal role in the formation of the ocular motor system; mutations in CHN1, encoding α2-CHN, cause the human eye movement disorder Duane Retraction Syndrome (DRS). Our research has demonstrated that the manipulation of α2-chn signaling in the zebrafish embryo leads to ocular motor axon wiring defects, although the signaling cascades regulated by α2-chn remain poorly understood. Here, we demonstrate that several cytoskeletal regulatory proteins-collapsin response mediator protein 2 (CRMP2; encoded by the gene dpysl2), stathmin1, and stathmin 2-bind to α2-CHN. dpysl2, stathmin1, and especially stathmin2 are expressed by ocular motor neurons. We find that the manipulation of dpysl2 and of stathmins in zebrafish larvae leads to defects in both the axon wiring of the ocular motor system and the optokinetic reflex, impairing horizontal eye movements. Knockdowns of these molecules in zebrafish larvae of either sex caused axon guidance phenotypes that included defasciculation and ectopic branching; in some cases, these phenotypes were reminiscent of DRS. chn1 knock-down phenotypes were rescued by the overexpression of CRMP2 and STMN1, suggesting that these proteins act in the same signaling pathway. These findings suggest that CRMP2 and stathmins signal downstream of α2-CHN to orchestrate ocular motor axon guidance and to control eye movements.SIGNIFICANCE STATEMENT The precise control of eye movements is crucial for the life of vertebrate animals, including humans. In humans, this control depends on the arrangement of nerve wiring of the ocular motor system, composed of three nerves and six muscles, a system that is conserved across vertebrate phyla. Mutations in the protein alpha2-chimaerin have previously been shown to cause eye movement disorders (squint) and axon wiring defects in humans. Our recent work has unraveled how alpha2-chimaerin coordinates axon guidance of the ocular motor system in animal models. In this article, we demonstrate key roles for the proteins CRMP2 and stathmin 1/2 in the signaling pathway orchestrated by alpha2-chimaerin, potentially giving insight into the etiology of eye movement disorders in humans.

Synthetic lethality of RB1 and aurora A is driven by stathmin-mediated disruption of microtubule dynamics

RB1 mutational inactivation is a cancer driver in various types of cancer including lung cancer, making it an important target for therapeutic exploitation. We performed chemical and genetic vulnerability screens in RB1-isogenic lung cancer pair and herein report that aurora kinase A (AURKA) inhibition is synthetic lethal in RB1-deficient lung cancer. Mechanistically, RB1^−/− cells show unbalanced microtubule dynamics through E2F-mediated upregulation of the microtubule destabilizer stathmin and are hypersensitive to agents targeting microtubule stability. Inhibition of AURKA activity activates stathmin function via reduced phosphorylation and facilitates microtubule destabilization in RB1^−/− cells, heavily impacting the bipolar spindle formation and inducing mitotic cell death selectively in RB1^−/− cells. This study shows that stathmin-mediated disruption of microtubule dynamics is critical to induce synthetic lethality in RB1-deficient cancer and suggests that upstream factors regulating microtubule dynamics, such as AURKA, can be potential therapeutic targets in RB1-deficient cancer.

Retinoblastoma susceptibility gene (RB1) is frequently mutated in lung cancers. Here the authors perform chemical and genetic vulnerability screens and identify aurora A kinase (AURKA) as a synthetic lethal candidate for RB1-deficient lung cancer cells and that AURKA inhibition sensitizes these cells to mitotic cell death.

Behaviors of individual microtubules and microtubule populations relative to critical concentrations: dynamic instability occurs when critical concentrations are driven apart by nucleotide hydrolysis

The concept of critical concentration (CC) is central to understanding the behavior of microtubules (MTs) and other cytoskeletal polymers. Traditionally, these polymers are understood to have one CC, measured in multiple ways and assumed to be the subunit concentration necessary for polymer assembly. However, this framework does not incorporate dynamic instability (DI), and there is work indicating that MTs have two CCs. We use our previously established simulations to confirm that MTs have (at least) two experimentally relevant CCs and to clarify the behavior of individuals and populations relative to the CCs. At free subunit concentrations above the lower CC (CC_Elongation), growth phases of individual filaments can occur transiently; above the higher CC (CC_NetAssembly), the population’s polymer mass will increase persistently. Our results demonstrate that most experimental CC measurements correspond to CC_NetAssembly, meaning that “typical” DI occurs below the concentration traditionally considered necessary for polymer assembly. We report that [free tubulin] at steady state does not equal CC_NetAssembly, but instead approaches CC_NetAssembly asymptotically as [total tubulin] increases, and depends on the number of stable MT nucleation sites. We show that the degree of separation between CC_Elongation and CC_NetAssembly depends on the rate of nucleotide hydrolysis. This clarified framework helps explain and unify many experimental observations.

Oncoprotein Stathmin Modulates Sensitivity to Apoptosis in Hepatocellular Carcinoma Cells During Hepatitis C Viral Replication

Patients with chronic hepatitis C virus (HCV) infection risk complications of cirrhosis, liver failure, and hepatocellular carcinoma (HCC). Previously, our proteomic examination of hepatocytes carrying a HCV-replicon revealed that deregulation of cytoskeletal dynamics may be a potential mechanism of viral-induced HCC growth. Here, we demonstrate the effect of HCV replication on the microtubule regulator stathmin (STMN1) in HCC cells. We further explore how the altered activity or synthesis of stathmin affects cellular proliferation and sensitivity to apoptosis in control HCC cells (Huh7.5) and experimental HCV-replicon harboring HCC cells (R-Huh7.5). The HCV-replicon harboring HCC cells (R-Huh 7.5) lack viral structural genes/proteins for acute infectivity and thus is the standard model for in vitro chronic infection study. Knockdown of endogenous stathmin reduced sensitivity to apoptosis in replicon cells. Meanwhile, constitutively active stathmin increased sensitivity to apoptosis in replicon cells. In addition, overexpression of constitutively active stathmin reduced cell proliferation in both control and replicon cells. These findings implicate, for the first time, a novel role for stathmin in viral replication–related apoptosis. Stathmin’s potential role in HCV replication and HCC make it a candidate for the future study of viral-induced malignancies.

Microtubules-associated Rac regulation of endothelial barrier: a role of Asef in acute lung injury

The endothelial barrier function regulated by the cytoskeletal reorganizations has been implicated in the pathogenesis of multiple lung diseases including asthma, sepsis, edema, and acute respiratory distress syndrome. The extensive studies have established that activation of small GTPase Rac is a key mechanism in endothelial barrier protection but the role of microtubules-associated Rac in the endothelial functions remains poorly understood. With the emerging evidences that microtubules disassembly also plays a critical role in actin cytoskeleton remodeling leading to endothelial permeability, the knowledge on microtubules-mediated regulation of endothelial barrier is imperative to better understand the etiology of lung injuries as well as to develop novel therapeutics against these disorders. In this regard, our recent studies have revealed some novel aspects of microtubules-mediated regulation of endothelial barrier functions and unraveled a putative role of Rac-specific guanine nucleotide exchange factor Asef in mediating the barrier protective effects of hepatocyte growth factor. In this review, we will discuss the role of this novel Rac activator Asef in endothelial barrier protection and its regulation by microtubules.

Model of Growth Cone Membrane Polarization via Microtubule Length Regulation

We present a mathematical model of membrane polarization in growth cones. We proceed by coupling an active transport model of cytosolic proteins along a two-dimensional microtubule (MT) network with a modified Dogterom-Leibler model of MT growth. In particular, we consider a Rac1-stathmin-MT pathway in which the growth and catastrophe rates of MTs are regulated by cytosolic stathmin, while the stathmin is regulated by Rac1 at the membrane. We use regular perturbation theory and numerical simulations to determine the steady-state stathmin concentration, the mean MT length distribution, and the resulting distribution of membrane-bound proteins. We thus show how a nonuniform Rac1 distribution on the membrane generates a polarized distribution of membrane proteins. The mean MT length distribution and hence the degree of membrane polarization are sensitive to the precise form of the Rac1 distribution and parameters such as the catastrophe-promoting constant and tubulin association rate. This is a consequence of the fact that the lateral diffusion of stathmin tends to weaken the effects of Rac1 on the distribution of mean MT lengths.

Stathmin 1/2-triggered microtubule loss mediates Golgi fragmentation in mutant SOD1 motor neurons

BACKGROUND

Pathological Golgi fragmentation represents a constant pre-clinical feature of many neurodegenerative diseases including amyotrophic lateral sclerosis (ALS) but its molecular mechanisms remain hitherto unclear.

RESULTS

Here, we show that the severe Golgi fragmentation in transgenic mutant SOD1(G85R) and SOD1(G93A) mouse motor neurons is associated with defective polymerization of Golgi-derived microtubules, loss of the COPI coat subunit β-COP, cytoplasmic dispersion of the Golgi tether GM130, strong accumulation of the ER-Golgi v-SNAREs GS15 and GS28 as well as tubular/vesicular Golgi fragmentation. Data mining, transcriptomic and protein analyses demonstrate that both SOD1 mutants cause early presymptomatic and rapidly progressive up-regulation of the microtubule-destabilizing proteins Stathmins 1 and 2. Remarkably, mutant SOD1-triggered Golgi fragmentation and Golgi SNARE accumulation are recapitulated by Stathmin 1/2 overexpression but completely rescued by Stathmin 1/2 knockdown or the microtubule-stabilizing drug Taxol.

CONCLUSIONS

We conclude that Stathmin-triggered microtubule destabilization mediates Golgi fragmentation in mutant SOD1-linked ALS and potentially also in related motor neuron diseases.

Feedback mechanism for microtubule length regulation by stathmin gradients

We formulate and analyze a theoretical model for the regulation of microtubule (MT) polymerization dynamics by the signaling proteins Rac1 and stathmin. In cells, the MT growth rate is inhibited by cytosolic stathmin, which, in turn, is inactivated by Rac1. Growing MTs activate Rac1 at the cell edge, which closes a positive feedback loop. We investigate both tubulin sequestering and catastrophe promotion as mechanisms for MT growth inhibition by stathmin. For a homogeneous stathmin concentration in the absence of Rac1, we find a switchlike regulation of the MT mean length by stathmin. For constitutively active Rac1 at the cell edge, stathmin is deactivated locally, which establishes a spatial gradient of active stathmin. In this gradient, we find a stationary bimodal MT-length distribution for both mechanisms of MT growth inhibition by stathmin. One subpopulation of the bimodal length distribution can be identified with fast-growing and long pioneering MTs in the region near the cell edge, which have been observed experimentally. The feedback loop is closed through Rac1 activation by MTs. For tubulin sequestering by stathmin, this establishes a bistable switch with two stable states: one stable state corresponds to upregulated MT mean length and bimodal MT length distributions, i.e., pioneering MTs; the other stable state corresponds to an interrupted feedback with short MTs. Stochastic effects as well as external perturbations can trigger switching events. For catastrophe-promoting stathmin, we do not find bistability.

Quantitative phosphoproteomic analyses of the inferior parietal lobule from three different pathological stages of Alzheimer's disease

Alzheimer's disease (AD), the most common age-related neurodegenerative disorder, is clinically characterized by progressive neuronal loss resulting in loss of memory and dementia. AD is histopathologically characterized by the extensive distribution of senile plaques and neurofibrillary tangles, and synapse loss. Amnestic mild cognitive impairment (MCI) is generally accepted to be an early stage of AD. MCI subjects have pathology and symptoms that fall on the scale intermediately between 'normal' cognition with little or no pathology and AD. A rare number of individuals, who exhibit normal cognition on psychometric tests but whose brains show widespread postmortem AD pathology, are classified as 'asymptomatic' or 'preclinical' AD (PCAD). In this study, we evaluated changes in protein phosphorylation states in the inferior parietal lobule of subjects with AD, MCI, PCAD, and control brain using a 2-D PAGE proteomics approach in conjunction with Pro-Q Diamond phosphoprotein staining. Statistically significant changes in phosphorylation levels were found in 19 proteins involved in energy metabolism, neuronal plasticity, signal transduction, and oxidative stress response. Changes in the disease state phosphoproteome may provide insights into underlying mechanisms for the preservation of memory with expansive AD pathology in PCAD and the progressive memory loss in amnestic MCI that escalates to the dementia and the characteristic pathology of AD brain.

Microtubule-Binding Proteins as Promising Biomarkers of Paclitaxel Sensitivity in Cancer Chemotherapy

Microtubules, tirelessly animated and highly dynamic structures, are vital for most cellular processes and their intricacies are still being revealed even after a century since their discovery. The importance of microtubules as chemotherapeutic targets cannot be overstated, and their clinical role is unlikely to abate in the near future. Indeed, improved understanding of microtubule biology could herald a new epoch of anticancer drug design by permitting fine-tuning of microtubule-targeting agents, the clinical utility of which is presently often limited by primary or acquired resistance. Paclitaxel, one such agent belonging to the taxane family, has proven a resoundingly successful treatment for many cancer patients; however, for too many others with paclitaxel-refractory tumors, the drug has offered nothing but side effects. Accumulating evidence suggests that microtubule-binding proteins (MBPs) can regulate paclitaxel sensitivity in a wide range of cancer types. Improved understanding of how these proteins can be assayed to predict treatment responses or manipulated pharmacologically to improve clinical outcomes could transform modern chemotherapy and is urgently awaited.

Mechanism for the catastrophe-promoting activity of the microtubule destabilizer Op18/stathmin

Regulation of microtubule dynamic instability is crucial for cellular processes, ranging from mitosis to membrane transport. Stathmin (also known as oncoprotein 18/Op18) is a prominent microtubule destabilizer that acts preferentially on microtubule minus ends. Stathmin has been studied intensively because of its association with multiple types of cancer, but its mechanism of action remains controversial. Two models have been proposed. One model is that stathmin promotes microtubule catastrophe indirectly, and does so by sequestering tubulin; the other holds that stathmin alters microtubule dynamics by directly destabilizing growing microtubules. Stathmin's sequestration activity is well established, but the mechanism of any direct action is mysterious because stathmin binds to microtubules very weakly. To address these issues, we have studied interactions between stathmin and varied tubulin polymers. We show that stathmin binds tightly to Dolastatin-10 tubulin rings, which mimic curved tubulin protofilaments, and that stathmin depolymerizes stabilized protofilament-rich polymers. These observations lead us to propose that stathmin promotes catastrophe by binding to and acting upon protofilaments exposed at the tips of growing microtubules. Moreover, we suggest that stathmin's minus-end preference results from interactions between stathmin's N terminus and the surface of α-tubulin that is exposed only at the minus end. Using computational modeling of microtubule dynamics, we show that these mechanisms could account for stathmin's observed activities in vitro, but that both the direct and sequestering activities are likely to be relevant in a cellular context. Taken together, our results suggest that stathmin can promote catastrophe by direct action on protofilament structure and interactions.

Mechanisms for axon maintenance and plasticity in motoneurons: alterations in motoneuron disease

In motoneuron disease and other neurodegenerative disorders, the loss of synapses and axon branches occurs early but is compensated by sprouting of neighboring axon terminals. Defective local axonal signaling for maintenance and dynamics of the axonal microtubule and actin cytoskeleton plays a central role in this context. The molecular mechanisms that lead to defective cytoskeleton architecture in two mouse models of motoneuron disease are summarized and discussed in this manuscript. In the progressive motor neuropathy (pmn) mouse model of motoneuron disease that is caused by a mutation in the tubulin-specific chaperone E gene, death of motoneuron cell bodies appears as a consequence of axonal degeneration. Treatment with bcl-2 overexpression or with glial-derived neurotrophic factor prevents loss of motoneuron cell bodies but does not influence the course of disease. In contrast, treatment with ciliary neurotrophic factor (CNTF) significantly delays disease onset and prolongs survival of pmn mice. This difference is due to the activation of Stat-3 via the CNTF receptor complex in axons of pmn mutant motoneurons. Most of the activated Stat-3 protein is not transported to the nucleus to activate transcription, but interacts locally in axons with stathmin, a protein that destabilizes microtubules. This interaction plays a major role in CNTF signaling for microtubule dynamics in axons. In Smn-deficient mice, a model of spinal muscular atrophy, defects in axonal translocation of β-actin mRNA and possibly other mRNA species have been observed. Moreover, the regulation of local protein synthesis in response to signals from neurotrophic factors and extracellular matrix proteins is altered in motoneurons from this model of motoneuron disease. These findings indicate that local signals are important for maintenance and plasticity of axonal branches and neuromuscular endplates, and that disturbances in these signaling mechanisms could contribute to the pathophysiology of motoneuron diseases.

Local axonal function of STAT3 rescues axon degeneration in the pmn model of motoneuron disease

Axonal maintenance, plasticity, and regeneration are influenced by signals from neighboring cells, in particular Schwann cells of the peripheral nervous system. Schwann cells produce neurotrophic factors, but the mechanisms by which ciliary neurotrophic factor (CNTF) and other neurotrophic molecules modify the axonal cytoskeleton are not well understood. In this paper, we show that activated signal transducer and activator of transcription-3 (STAT3), an intracellular mediator of the effects of CNTF and other neurotrophic cytokines, acts locally in axons of motoneurons to modify the tubulin cytoskeleton. Specifically, we show that activated STAT3 interacted with stathmin and inhibited its microtubule-destabilizing activity. Thus, ectopic CNTF-mediated activation of STAT3 restored axon elongation and maintenance in motoneurons from progressive motor neuronopathy mutant mice, a mouse model of motoneuron disease. This mechanism could also be relevant for other neurodegenerative diseases and provide a target for new therapies for axonal degeneration.

Expression and phosphorylation of stathmin correlate with cell migration in esophageal squamous cell carcinoma

Microtubules play extensive roles in cellular processes, including cell motility. Stathmin is an important protein which destabilizes microtubules. The essential function of stathmin is closely associated with its phosphorylation status. Stathmin is overexpressed in many human cancers and has a significant relationship with clinical characteristics such as grade, tumor size and prognosis. We demonstrated that stathmin was overexpressed in ESCC tissues using both 2-DE and immunohistochemistry analysis. In addition, overexpression of stathmin was significantly correlated with histological grade in ESCC. However, no correlation was found with age, gender and lymph node metastasis. Knockdown of stathmin with siRNA impaired cell migration in KYSE30 and KYSE410 cells. When EC0156 cells were treated with paclitaxel, stathmin was stably phosphorylated and migration was impaired. These observations suggest that stathmin may have a more important function in ESCC development and migration. The present study provides further understanding of the importance of stathmin in ESCC therapy or diagnosis.

Efficiency of organelle capture by microtubules as a function of centrosome nucleation capacity: general theory and the special case of polyspermia

Transport of organelles along microtubules is essential for the cell metabolism and morphogenesis. The presented analysis derives the probability that an organelle of a given size comes in contact with the microtubule aster. The question is asked how this measure of functionality of the microtubule aster is controlled by the centrosome. A quantitative model is developed to address this question. It is shown that for the given set of cellular parameters, such as size and total tubulin content, a centrosome nucleation capacity exists that maximizes the probability of the organelle capture. The developed general model is then applied to the capture of the female pronucleus by microtubules assembled on the sperm centrosome, following physiologically polyspermic fertilization. This application highlights an unintuitive reflection of nonlinearity of the nucleated polymerization of the cellular pool of tubulin. The prediction that the sperm centrosome should lower its nucleation capacity in the face of the competition from the other sperm is a stark illustration of the new optimality principle. Overall, the model calls attention to the capabilities of the centrosomal pathway of regulation of the transport-related functionality of the microtubule cytoskeleton. It establishes a quantitative and conceptual framework that can guide experiment design and interpretation.

Drosophila stathmins bind tubulin heterodimers with high and variable stoichiometries

In vertebrates, stathmins form a family of proteins possessing two tubulin binding repeats (TBRs), which each binds one soluble tubulin heterodimer. The stathmins thus sequester two tubulins in a phosphorylation-dependent manner, providing a link between signal transduction and microtubule dynamics. In Drosophila, we show here that a single stathmin gene (stai) encodes a family of D-stathmin proteins. Two of the D-stathmins are maternally deposited and then restricted to germ cells, and the other two are detected in the nervous system during embryo development. Like in vertebrates, the nervous system-enriched stathmins contain an N-terminal domain involved in subcellular targeting. All the D-stathmins possess a domain containing three or four predicted TBRs, and we demonstrate here, using complementary biochemical and biophysical methods, that all four predicted TBR domains actually bind tubulin. D-stathmins can indeed bind up to four tubulins, the resulting complex being directly visualized by electron microscopy. Phylogenetic analysis shows that the presence of regulated multiple tubulin sites is a conserved characteristic of stathmins in invertebrates and allows us to predict key residues in stathmin for the binding of tubulin. Altogether, our results reveal that the single Drosophila stathmin gene codes for a stathmin family similar to the multigene vertebrate one, but with particular tubulin binding properties.

Regulation of microtubule dynamic instability in vitro by differentially phosphorylated stathmin

Stathmin is an important regulator of microtubule polymerization and dynamics. When unphosphorylated it destabilizes microtubules in two ways, by reducing the microtubule polymer mass through sequestration of soluble tubulin into an assembly-incompetent T2S complex (two alpha:beta tubulin dimers per molecule of stathmin), and by increasing the switching frequency (catastrophe frequency) from growth to shortening at plus and minus ends by binding directly to the microtubules. Phosphorylation of stathmin on one or more of its four serine residues (Ser(16), Ser(25), Ser(38), and Ser(63)) reduces its microtubule-destabilizing activity. However, the effects of phosphorylation of the individual serine residues of stathmin on microtubule dynamic instability have not been investigated systematically. Here we analyzed the effects of stathmin singly phosphorylated at Ser(16) or Ser(63), and doubly phosphorylated at Ser(25) and Ser(38), on its ability to modulate microtubule dynamic instability at steady-state in vitro. Phosphorylation at either Ser(16) or Ser(63) strongly reduced or abolished the ability of stathmin to bind to and sequester soluble tubulin and its ability to act as a catastrophe factor by directly binding to the microtubules. In contrast, double phosphorylation of Ser(25) and Ser(38) did not affect the binding of stathmin to tubulin or microtubules or its catastrophe-promoting activity. Our results indicate that the effects of stathmin on dynamic instability are strongly but differently attenuated by phosphorylation at Ser(16) and Ser(63) and support the hypothesis that selective targeting by Ser(16)-specific or Ser(63)-specific kinases provides complimentary mechanisms for regulating microtubule function.

SCLIP is crucial for the formation and development of the Purkinje cell dendritic arbor

Cerebellar Purkinje cells elaborate one of the most complex dendritic arbors among neurons to integrate the numerous signals they receive from the cerebellum circuitry. Their dendritic differentiation undergoes successive, tightly regulated phases of development involving both regressive and growth events. Although many players regulating the late phases of Purkinje cell dendritogenesis have been identified, intracellular factors controlling earlier phases of dendritic development remain mostly unknown. In this study, we explored the biological properties and functions of SCLIP, a protein of the stathmin family, in Purkinje cell dendritic differentiation and cerebellum development. Unlike the other stathmins, SCLIP is strongly expressed in Purkinje cells during cerebellar development and accumulates in their dendritic processes at a critical period of their formation and outgrowth. To reveal SCLIP functions, we developed a lentiviral-mediated approach on cerebellar organotypic cultures to inhibit or increase its expression in Purkinje cells in their tissue environment. Depletion of SCLIP promoted retraction of the Purkinje cell primitive process and then prevented the formation of new dendrites at early stages of postnatal development. It also prevented their elongation and branching at later phases of differentiation. Conversely, SCLIP overexpression promoted dendritic branching and development. Together, our results demonstrate for the first time that SCLIP is crucial for both the formation and proper development of Purkinje cell dendritic arbors. SCLIP appears thus as a novel and specific factor that controls the early phases of Purkinje cell dendritic differentiation during cerebellum development.

Measurements of stathmin-tubulin interaction in solution

Stathmin is an important phosphorylation-controlled regulator of microtubule dynamics and plays a crucial role in cell division and cell proliferation. In its non-phosphorylated form, stathmin is the protein that interacts the most tightly with tubulin, in a 2:1 tubulin-stathmin (T2S) complex that does not participate in microtubule assembly. The importance of stathmin at different levels of phosphorylation in different steps of mitosis This article is a short overview of the different methods that have been or could be used to monitor the kinetic and thermodynamic parameters of tubulin-stathmin interaction and to evaluate the effects of phosphorylation. The author has tried to emphasize how hydrodynamic and spectroscopic methods measuring direct binding of stathmin to tubulin can be complemented by methods that make use of linked functions, measuring how the change in a functional property of tubulin upon binding stathmin provides information on binding parameters.

Structure and thermodynamics of the tubulin-stathmin interaction

Oncoprotein 18/stathmin (stathmin) is a phosphorylation-controlled key regulator of microtubule dynamics. In recent years, substantial efforts were undertaken to characterize the complex formed between tubulin and the intrinsically disordered stathmin molecule. Here, I summarize and illustrate the current structural and thermodynamic studies on the tubulin-stathmin interaction. Based on these and on functional information I formulate an updated molecular mechanism on how tubulin-binding by stathmin regulates microtubule dynamics.

Control of intrinsically disordered stathmin by multisite phosphorylation

Stathmin is an intrinsically disordered protein implicated in the regulation of microtubule dynamics and in the development of cancer. The microtubule destabilizing activity of stathmin is down-regulated by phosphorylation of four serine residues, Ser16, Ser25, Ser38, and Ser63. Here we have used calorimetric and spectroscopic methods, including nuclear magnetic resonance to analyze the properties of seven stathmin phosphoisoforms to bind tubulin and inhibit microtubule formation. We found that stathmin phosphorylation results in a substantial loss in hydration entropy upon tubulin-stathmin complex formation. Remarkably, a linear correlation between the free energy change of complex formation and the microtubule inhibition activities of stathmin phosphoisoforms was observed. This finding provides a biophysical basis for understanding the mechanism by which local stathmin activity gradients important for promoting localized microtubule growth are established. We further found that phosphorylation of Ser16 and Ser63 disrupts the formation of a tubulin-interacting beta-hairpin and a helical segment, respectively, explaining the dominant role of these residues in regulating cell cycle progression. The insight into the tubulin-stathmin interaction offers a molecular basis for understanding the nature and the factors that control intrinsically disordered protein systems in general.

Bcl-x(L)-mediated changes in metabolic pathways of breast cancer cells: from survival in the blood stream to organ-specific metastasis

Bcl-x(L) protein plays a role in breast cancer dormancy, promoting survival of cells in metastatic foci by counteracting the proapoptotic signals in the microenvironment. The aim of this study was to identify phenotypes mediated by Bcl-x(L) in breast cancer cells that enhance in vivo survival of clinical metastases. 435/Bcl-x(L) or 435/Neo human breast cancer cells were injected into the inguinal mammary gland of nude mice, and tumors, metastases in lymph node, lung, and bone, and bloodstream surviving cells were examined. Proteomic analysis identified 17 proteins that were overexpressed (more than twofold) or underexpressed (less than twofold) in metastases. A protein interaction program allowed us to functionally associate peroxiredoxin 3, peroxiredoxin 2, carbonyl reductase 3, and enolase 1, suggesting a role for cellular responses to oxidative stress in metastasis organ selection. The prediction included proteins involved in redox systems, kinase pathways, and the ATP synthase complex. Furthermore, the interaction of redox proteins with enolase 1 suggests a connection between glycolysis and antioxidant pathways, enabling achievement of a high metastatic activity. In conclusion, Bcl-x(L) mediates a phenotype in which redox pathways and glycolysis are coupled to protect breast cancer metastatic cells during transit from the primary tumor to the metastatic state.

N-Terminal Stathmin-like Peptides Bind Tubulin and Impede Microtubule Assembly†

Microtubules are major cytoskeletal components involved in numerous cellular functions such as mitosis, cell motility, or intracellular traffic. These cylindrical polymers of alphabeta-tubulin assemble in a closely regulated dynamic manner. We have shown that the stathmin family proteins sequester tubulin in a nonpolymerizable ternary complex, through their stathmin-like domains (SLD) and thus contribute to the regulation of microtubule dynamics. We demonstrate here that short peptides derived from the N-terminal part of SLDs impede tubulin polymerization with various efficiencies and that phosphorylation of the most potent of these peptides reduces its efficiency as in full-length stathmin. To understand the mechanism of action of these peptides, we undertook a NMR-based structural analysis of the peptide-tubulin interaction with the most efficient peptide (I19L). Our results show that, while disordered when free in solution, I19L folds into a beta-hairpin upon binding to tubulin. We further identified, by means of saturation transfer difference NMR, hydrophobic residues located on the beta2-strand of I19L that are involved in its tubulin binding. These structural data were used together with tubulin atomic coordinates from the tubulin/RB3-SLD crystal structure to model the I19L/tubulin interaction. The model agrees with I19L acting through an autonomous tubulin capping capability to impede tubulin polymerization and provides information to help understand the variation of efficiency against tubulin polymerization among the peptides tested. Altogether these results enlighten the mechanism of tubulin sequestration by SLDs, while they pave the way for the development of protein-based compounds aimed at interfering with tubulin polymerization.

Distinct and sequential upregulation of genes regulating cell growth and cell cycle progression during hepatic ischemia-reperfusion injury

Ischemia-reperfusion injury (IRI) in liver and other organs is manifested as an injury phase followed by recovery and resolution. Control of cell growth and proliferation is essential for recovery from the injury. We examined the expression of three related regulators of cell cycle progression in liver IRI: spermidine/spermine N-acetyltransferase (SSAT), p21 (a cyclin-dependent kinase inhibitor), and stathmin. Mice were subjected to hepatic IRI, and liver tissues were harvested at timed intervals. The expression of SSAT, the rate-limiting enzyme in the polyamine catabolic pathway, had increased fivefold 6 h after IRI and correlated with increased putrescine levels in the liver, consistent with increased SSAT enzymatic activity in IRI. The expression of p21, which is transactivated by p53, was undetectable in sham-operated animals but was heavily induced at 12 and 24 h of reperfusion and declined to undetectable baseline levels at 72 h of reperfusion. The interaction of the polyamine pathway with the p53-p21 pathway was shown in vitro, where activation of SSAT with polyamine analog or the addition of putrescine to cultured hepatocytes induced the expression of p53 and p21 and decreased cell viability. The expression of stathmin, which is under negative transcriptional regulation by p21 and controls cell proliferation and progression through mitosis, remained undetectable at 6, 12, and 24 h of reperfusion and was progressively and heavily induced at 48 and 72 h of reperfusion. Double-immunofluorescence labeling with antibodies against stathmin and PCNA, a marker of cell proliferation, demonstrated colocalization of stathmin and PCNA at 48 and 72 h of reperfusion in hepatocytes, indicating the initiation of cell proliferation. The distinct and sequential upregulation of SSAT, p21, and stathmin, along with biochemical activation of the polyamine catabolic pathway in IRI in vivo and the demonstration of p53-p21 upregulation by SSAT and putrescine in vitro, points to the important role of regulators of cell growth and cell cycle progression in the pathophysiology and/or recovery in liver IRI. The data further suggest that SSAT may play a role in the initiation of injury, whereas p21 and stathmin may be involved in the resolution and recovery after liver IRI.

A synergistic relationship between three regions of stathmin family proteins is required for the formation of a stable complex with tubulin

Stathmin is a ubiquitous 17 kDa cytosolic phosphoprotein proposed to play a general role in the integration and relay of intracellular signalling pathways. It is believed to regulate microtubule dynamics by sequestering tubulin in a complex made of two tubulin heterodimers per stathmin molecule (T2S complex). The other proteins of the stathmin family can also bind two tubulin heterodimers through their SLD (stathmin-like domain), but the different tubulin:SLD complexes display varying stabilities. In this study, we analysed the relative influence of three regions of SLDs on the interaction with tubulin and the mechanistic processes that lead to its sequestration. Tubulin-binding properties of fragments and chimaeras of stathmin and RB3(SLD) were studied in vitro by tubulin polymerization, size-exclusion chromatography and surface plasmon resonance assays. Our results show that the N-terminal region of SLDs favours the binding of the first tubulin heterodimer and that the second C-terminal tubulinbinding site confers the specific stability of a given tubulin:SLD complex. Our results highlight the molecular processes by which tubulin co-operatively interacts with the SLDs. This knowledge may contribute to drug development aimed at disturbing microtubules that could be used for the treatment of cancer.

Modulation of the Stathmin-like Microtubule Destabilizing Activity of RB3, a Neuron-specific Member of the SCG10 Family, by Its N-terminal Domain

RB3 is a neuron-specific homologue of the SCG10/stathmin family proteins, possessing a unique N-terminal membrane-associated domain and the stathmin-like domain at the C terminus, which promotes microtubule (MT) catastrophe and/or tubulin sequestering. We examined herein the contribution of the N-terminal subdomain of RB3 to the regulation of MT dynamics. To begin with, we determined the effects of full-length (RB3-f) and short truncated (RB3-s) forms of RB3 on the polymerization of MT in vitro. RB3-s had a deletion of amino acids 1-75 from the N terminus, leaving the so-called stathmin-like domain, consisting of residues 76-217. Although both RB3-f and RB3-s exhibited MT-depolymerizing activity, RB3-f was less effective. The binding affinity for tubulin was also lower in RB3-f. Direct observation of the dynamics of individual MTs using dark field microscopy revealed that RB3-s slowed MT elongation velocity, increased catastrophes, and reduced rescues. This effect is almost identical to that by stathmin/oncoprotein 18. On the other hand, the MT elongation rate increased at lower concentrations of RB3-f. In addition, RB3-f, indicated higher rescue frequency than control as well as the catastrophe in a dose-dependent manner. The functionality of RB3-f indicated that full-length RB3 has not only stathmin-like MT destabilizing activity but also MT-associated protein-like MT stabilizing activity. Possibly, the balance of these activities is altered in a concentration-dependent manner in vitro. This interesting regulatory role of the unique N-terminal domain of RB3 in MT dynamics would contribute to the physiological regulation of neuronal morphogenesis.

Stathmin-tubulin interaction gradients in motile and mitotic cells

The spatial organization of the microtubule cytoskeleton is thought to be directed by steady-state activity gradients of diffusible regulatory molecules. We visualized such intracellular gradients by monitoring the interaction between tubulin and a regulator of microtubule dynamics, stathmin, using a fluorescence resonance energy transfer (FRET) biosensor. These gradients were observed both during interphase in motile membrane protrusions and during mitosis around chromosomes, which suggests that a similar mechanism may contribute to the creation of polarized microtubule structures. These interaction patterns are likely to reflect phosphorylation of stathmin in these areas.

Regulation of microtubule destabilizing activity of Op18/stathmin downstream of Rac1

In the leading edge of migrating cells, a subset of microtubules exhibits net growth in a Rac1- and p21-activated kinase-dependent manner. Here, we explore the possibility of whether phosphorylation and inactivation of the microtubule-destabilizing protein Op18/stathmin could be a mechanism regulating microtubule dynamics downstream of Rac1 and p21-activated kinases. We find that, in vitro, Pak1 phosphorylates Op18/stathmin specifically at serine 16 and inactivates its catastrophe promoting activity in biochemical and time lapse microscopy microtubule assembly assays. Furthermore, phosphorylation of either serine 16 or 63 is sufficient to inhibit Op18/stathmin in vitro. In cells, the microtubule-destabilizing effect of an excess of Op18/stathmin can be partially overcome by expression of constitutively active Rac1(Q61L), which is dependent on Pak activity, suggesting that the microtubule cytoskeleton can be regulated through inactivation of Op18/stathmin downstream of Rac1 and Pak in vivo. However, in vivo, Pak1 activity alone is not sufficient to phosphorylate Op18, indicating that additional pathways downstream of Rac1 are required for Op18 regulation.

Thermodynamics of the Op18/stathmin-tubulin interaction

Op18/stathmin (stathmin) is an intrinsically disordered protein involved in the regulation of the microtubule filament system. One function of stathmin is to sequester tubulin dimers into assembly incompetent complexes, and recent studies revealed two tubulin binding sites per stathmin molecule. Using high sensitivity isothermal titration calorimetry, we document that at 10 degrees C and under the conditions of 80 mM PIPES, pH 6.8, 1 mM EGTA, 1 mM MgCl2, 1 mM GTP these two binding sites are of equal affinity with an equilibrium binding constant of K0 = 6.0 x 10(6) m(-1). The obtained large negative molar heat capacity change of deltaCp0 = -860 cal mol(-1) K(-1) (referring to tubulin) for the tubulin-stathmin binding equilibrium suggests that the hydrophobic effect is the major driving force of the binding reaction. Replacing GTP by GDP on beta-tubulin had no significant effect on the thermodynamic parameters of the tubulin-stathmin binding equilibrium. The proposed pH-sensitive dual function of stathmin was further evaluated by circular dichroism spectroscopy and nuclear magnetic resonance. At low temperatures, stathmin was found to be extensively helical but devoid of any stable tertiary structure. However, in complex with two tubulin subunits stathmin adopts a stable conformation. Both the stability and conformation of the individual proteins and complexes were not significantly affected by small changes in pH. A 4-fold decrease in affinity of stathmin for tubulin was revealed at pH 7.5 compared with pH 6.8. This decrease could be attributed to a weaker binding of the C terminus of stathmin. These findings do not support the view that stathmin works as a pH-sensitive protein.

Fluorescence correlation spectroscopy analysis of the dynamics of tubulin interaction with RB3, a stathmin family protein

We have used fluorescence correlation spectroscopy to analyze the interaction of GTP-tubulin with rhodamine-labeled RB3, a neural protein of the stathmin family, and to determine the kinetic pathway of the association process. RB3 displayed slow association-dissociation kinetics with tubulin depending on the square of the tubulin concentration. The values of the apparent association and dissociation rate constants of the complex of two tubulin dimers and RB3 are determined to be (3.52+/-0.14)x10(-3) micro;M(-2)/s and (1.9+/-0.6)x10(-3) s(-1) respectively. The value of the equilibrium dissociation constant for the first tubulin-RB3 interaction is estimated to be >or=7 microM at 20 degrees C.

Transcripts in pretreatment biopsies from a three-arm randomized trial in metastatic non-small-cell lung cancer

Non-small-cell lung cancer patients with locally advanced or metastatic disease at the time of diagnosis show marginal response to chemotherapy in terms of tumor shrinkage, time to progression and median survival. The identification and implementation of predictive genetic markers of response-specific cytotoxic drugs is a priority of current research and future trials. In this study, we have used quantitative PCR to analyse expression of beta-tubulin III, stathmin, RRM1, COX-2 and GSTP1 in mRNA isolated from paraffin-embedded tumor biopsies of 75 nonsmall-cell lung cancer patients treated as part of a large randomized trial. In total, 22 patients were treated with gemcitabine/cisplatin, 25 with vinorelbine/cisplatin and 28 with paclitaxel/carboplatin. There were no differences in clinical characteristics and transcript levels in the pretreatment biopsies according to treatment arm. Patients with low beta-tubulin III levels had better response in the paclitaxel/carboplatin arm (P=0.05), and those with low RRM1 levels showed a tendency to better response in the gemcitabine/cisplatin arm. Time to progression was influenced by beta-tubulin III (P=0.03) and stathmin (P=0.05) levels in the vinorelbine/cisplatin arm, and there was a tendency toward correlation between beta-tubulin III levels and time to progression in the paclitaxel/carboplatin arm. RRM1 levels influenced time to progression (P=0.05) and even more so, survival (P=0.0028) in the gemcitabine/cisplatin arm. The predictive value of beta-tubulin III, stathmin and RRM1 should be tested in prospective customized chemotherapy trials, the results of which will help tailor chemotherapy to improve patient survival.

Enhanced expression of uterine stathmin during the process of implantation and decidualization in rats

We used the library subtraction technique to identify genes specifically expressed in the rat uterus during early pregnancy. One such gene was that for stathmin, a factor that is associated with tubulin binding and the destabilization of microtubules. Stathmin was expressed at higher levels in implantation sites than in interimplantation sites on d 6 and 7 of pregnancy; the levels on d 6 and 7 were higher in implantation sites than in the entire uterus on d 3-5 of pregnancy or in nonpregnant uteri. Intense expression of stathmin mRNA was primarily limited to the subluminal stromal cells at the implantation site. Expression was also detected in the decidual zones and was accentuated during the period of decidualization (d 7-12). In the delayed implantation pregnant rat model, uterine stathmin expression was low, but increased after implantation induced by administration of 17beta-estradiol to the progesterone-primed animal. Further, decidualization in the pseudopregnant rat, induced by intrauterine infusion of oil, enhanced stathmin expression. Stathmin expression clearly increases in the uterus when stimulated by embryo implantation and decidualization and may play a role in the early stages of pregnancy.

Expression of stathmin family genes in human tissues: non-neural-restricted expression for SCLIP

The stathmin family consists of phosphoproteins highly conserved in vertebrates and thought to be implicated in the development and functional regulation of various organs, most notably the nervous system. This family includes stathmin, SCG10, SCLIP, and RB3, phosphoproteins that are related by structural and functional homologies. They all sequester tubulin and interfere with microtubule dynamics, a property due to their shared stathmin-like domain. Little is known about the expression of the stathmin gene family in humans. Herein, we describe for the first time, for a collection of human tissues, the expression of each member of this family, using real-time quantitative RT-PCR. We found that stathmin is ubiquitously expressed, whereas SCG10 and RB3 are neural enriched, expression patterns similar to those reported for other mammals. Surprisingly, SCLIP, whose expression is thought to be neural-specific, exhibits a broader tissue distribution. Analyses of the SCLIP gene (approved symbol STMN3) show that it contains several NRSE-like elements that display low or no affinity for the cognate binding protein NRSF. The substantial expression of SCLIP in most tissues points out a novel function for this protein outside the nervous system and raises the possibility that its coexpression with stathmin could provide some degree of functional redundancy.