Phosphorylation regulates the microtubule-destabilizing activity of stathmin and its interaction with tubulin

The relationship between stathmin-2 level and metabolic parameters in newly diagnosed type 2 diabetes mellitus patients

BACKGROUND

Stathmin is a phosphoprotein that plays a role in intercellular and intracellular signaling, inflammation, and differentiation. Our aim was to evaluate the stathmin-2 level and its relationship with the metabolic parameters of newly diagnosed type 2 diabetes mellitus (nT2DM) patients.

MATERIAL AND METHOD

This case-control study included 76 patients with nT2DM and 76 healthy individuals with a normal oral glucose tolerance test who were matched for body mass index (BMI), age, and gender. In addition to laboratory and anthropometric measurements related to type 2 diabetes mellitus (T2DM), stathmin-2 levels were determined using an enzyme-linked immunosorbent assay.

RESULTS

We observed significantly higher circulating stathmin-2 levels in subjects with T2DM compared to the control group (6.39±1.60 ng/mL and 4.66±0.80 ng/mL, p<0.0001). In patients with metabolic syndrome, circulating stathmin-2 levels were significantly elevated compared to those without metabolic syndrome in both the T2DM and control groups (T2DM: 7.16±1.24 vs 5.06±1.24 ng/mL, p<0.001; Control: 3.84±1.40 vs 3.82±1.40 ng/mL). In both groups, we observed a positive correlation between stathmin-2 levels and BMI and circumference. Moreover, stathmin-2 showed a positive correlation with high-sensitivity C-reactive protein (hs-CRP), homeostatic model assessment of insulin resistance, insulin, fasting blood glucose, hemoglobin A1c, BMI, low-density lipoprotein cholesterol, and total cholesterol. A negative correlation was observed with stathmin-2 and high-density lipoprotein cholesterol. Stathmin-2 did not show any correlation with age, triglyceride, and lactate dehydrogenase.

CONCLUSIONS

Stathmin-2 levels were found to be elevated in patients with nT2DM and exhibited positive correlations with hyperinsulinaemia, hyperglycaemia, HOMO-IR and hs-CRP levels. These results indicate that stathmin-2 may play a role in T2DM pathogenesis.

The Stathmin-2 membrane-targeting domain is required for axon protection and regulated degradation by DLK signaling

Axon integrity is essential for functional connectivity in the nervous system. The degeneration of stressed or damaged axons is a common and sometimes initiating event in neurodegenerative disorders. Stathmin-2 (Stmn2) is an axon maintenance factor that is depleted in amyotrophic lateral sclerosis, and replenishment of Stmn2 can restore neurite outgrowth in diseased neurons. However, mechanisms responsible for Stmn2-mediated axon maintenance in injured neurons are not known. We used primary sensory neurons to interrogate the role of Stmn2 in the degeneration of severed axons. We discover that membrane association of Stmn2 is critical for its axon-protective activity. Structure-function studies revealed that axonal enrichment of Stmn2 is driven by palmitoylation as well as tubulin interaction. Using live imaging, we discover that another Stmn, Stmn3, comigrates with Stmn2-containing vesicles. We also demonstrate that Stmn3 undergoes regulated degradation through dual leucine zipper kinase (DLK)-c-Jun N-terminal kinase signaling. The Stmn2 membrane-targeting domain is both necessary and sufficient for localization to a specific vesicle population and confers sensitivity to DLK-dependent degradation. Our findings reveal a broader role for DLK in tuning the local abundance of palmitoylated Stmns in axon segments. Moreover, palmitoylation is a critical component of Stmn-mediated axon protection, and defining the Stmn2-containing vesicle population will provide important clues toward mechanisms of axon maintenance.

Multi-omic profiling reveals the ataxia protein sacsin is required for integrin trafficking and synaptic organization

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is a childhood-onset cerebellar ataxia caused by mutations in SACS, which encodes the protein sacsin. Cellular ARSACS phenotypes include mitochondrial dysfunction, intermediate filament disorganization, and progressive death of cerebellar Purkinje neurons. It is unclear why the loss of sacsin causes these deficits or why they manifest as cerebellar ataxia. Here, we perform multi-omic profiling in sacsin knockout (KO) cells and identify alterations in microtubule dynamics and mislocalization of focal adhesion (FA) proteins, including multiple integrins. Deficits in FA structure, signaling, and function can be rescued by targeting PTEN, a negative regulator of FA signaling. ARSACS mice possess mislocalization of ITGA1 in Purkinje neurons and synaptic disorganization in the deep cerebellar nucleus (DCN). The sacsin interactome reveals that sacsin regulates interactions between cytoskeletal and synaptic adhesion proteins. Our findings suggest that disrupted trafficking of synaptic adhesion proteins is a causal molecular deficit in ARSACS.

UNC-45A breaks the microtubule lattice independently of its effects on non-muscle myosin II

In invertebrates, UNC-45 regulates myosin stability and functions. Vertebrates have two distinct isoforms of the protein: UNC-45B, expressed in muscle cells only, and UNC-45A, expressed in all cells and implicated in regulating both non-muscle myosin II (NMII)- and microtubule (MT)-associated functions. Here, we show that, in vitro and in human and rat cells, UNC-45A binds to the MT lattice, leading to MT bending, breakage and depolymerization. Furthermore, we show that UNC-45A destabilizes MTs independent of its C-terminal NMII-binding domain and even in the presence of the NMII inhibitor blebbistatin. These findings identified UNC-45A as a novel type of MT-severing protein with a dual non-mutually exclusive role in regulating NMII activity and MT stability. Because many human diseases, from cancer to neurodegenerative diseases, are caused by or associated with deregulation of MT stability, our findings have profound implications in the biology of MTs, as well as the biology of human diseases and possible therapeutic implications for their treatment.This article has an associated First Person interview with the joint first authors of the paper.

Combining optogenetics with sensitive FRET imaging to monitor local microtubule manipulations

Optogenetic methods for switching molecular states in cells are increasingly prominent tools in life sciences. Förster Resonance Energy Transfer (FRET)-based sensors can provide quantitative and sensitive readouts of altered cellular biochemistry, e.g. from optogenetics. However, most of the light-inducible domains respond to the same wavelength as is required for excitation of popular CFP/YFP-based FRET pairs, rendering the techniques incompatible with each other. In order to overcome this limitation, we red-shifted an existing CFP/YFP-based OP18 FRET sensor (COPY) by employing an sYFP2 donor and mScarlet-I acceptor. Their favorable quantum yield and brightness result in a red-shifted FRET pair with an optimized dynamic range, which could be further enhanced by an R125I point mutation that stimulates intramolecular interactions. The new sensor was named ROPY and it visualizes the interaction between the microtubule regulator stathmin/OP18 and free tubulin heterodimers. We show that through phosphorylation of the ROPY sensor, its tubulin sequestering ability can be locally regulated by photo-activatable Rac1 (PARac1), independent of the FRET readout. Together, ROPY and PARac1 provide spatiotemporal control over free tubulin levels. ROPY/PARac1-based optogenetic regulation of free tubulin levels allowed us to demonstrate that depletion of free tubulin prevents the formation of pioneer microtubules, while local upregulation of tubulin concentration allows localized microtubule extensions to support the lamellipodia.

βPix-d promotes tubulin acetylation and neurite outgrowth through a PAK/Stathmin1 signaling pathway

Microtubules are a major cytoskeletal component of neurites, and the regulation of microtubule stability is essential for neurite morphogenesis. βPix (ARHGEF7) is a guanine nucleotide exchange factor for the small GTPases Rac1 and Cdc42, which modulate the organization of actin filaments and microtubules. βPix is expressed as alternatively spliced variants, including the ubiquitous isoform βPix-a and the neuronal isoforms βPix-b and βPix-d, but the function of the neuronal isoforms remains unclear. Here, we reveal the novel role of βPix neuronal isoforms in regulating tubulin acetylation and neurite outgrowth. At DIV4, hippocampal neurons cultured from βPix neuronal isoform knockout (βPix-NIKO) mice exhibit defects in neurite morphology and tubulin acetylation, a type of tubulin modification which often labels stable microtubules. Treating βPix-NIKO neurons with paclitaxel, which stabilizes the microtubules, or reintroducing either neuronal βPix isoform to the KO neurons overcomes the impairment in neurite morphology and tubulin acetylation, suggesting that neuronal βPix isoforms may promote microtubule stabilization during neurite development. βPix-NIKO neurons also exhibit lower phosphorylation levels for Stathmin1, a microtubule-destabilizing protein, at Ser16. Expressing either βPix neuronal isoform in the βPix-NIKO neurons restores Stathmin1 phosphorylation levels, with βPix-d having a greater effect than βPix-b. Furthermore, we find that the recovery of neurite length and Stathmin1 phosphorylation via βPix-d expression requires PAK kinase activity. Taken together, our study demonstrates that βPix-d regulates the phosphorylation of Stathmin1 in a PAK-dependent manner and that neuronal βPix isoforms promote tubulin acetylation and neurite morphogenesis during neuronal development.

Phosphorylation Time-Course Study of the Response during Adenovirus Type 2 Infection

PTMs such as phosphorylations are usually involved in signal transduction pathways. To investigate the temporal dynamics of phosphoproteome changes upon viral infection, a model system of IMR-90 cells infected with human adenovirus type 2 (Ad2) is used in a time-course quantitative analysis combining titanium dioxide (TiO₂ ) particle enrichment and SILAC-MS. Quantitative data from 1552 phosphorylated sites clustered the highly altered phosphorylated sites to the signaling by rho family GTPases, the actin cytoskeleton signaling, and the cAMP-dependent protein kinase A signaling pathways. Their activation is especially pronounced at early time post-infection. Changes of several phosphorylated sites involved in the glycolysis pathway, related to the activation of the Warburg effect, point at virus-induced energy production. For Ad2 proteins, 32 novel phosphorylation sites are identified and as many as 52 phosphorylated sites on 17 different Ad2 proteins are quantified, most of them at late time post-infection. Kinase predictions highlighted activation of PKA, CDK1/2, MAPK, and CKII. Overlaps of kinase motif sequences for viral and human proteins are observed, stressing the importance of phosphorylation during Ad2 infection.

Perspective: Potential Impact and Therapeutic Implications of Oncogenic PI3K Activation on Chromosomal Instability

Genetic activation of the class I PI3K pathway is very common in cancer. This mostly results from oncogenic mutations in PIK3CA, the gene encoding the ubiquitously expressed PI3Kα catalytic subunit, or from inactivation of the PTEN tumour suppressor, a lipid phosphatase that opposes class I PI3K signalling. The clinical impact of PI3K inhibitors in solid tumours, aimed at dampening cancer-cell-intrinsic PI3K activity, has thus far been limited. Challenges include poor drug tolerance, incomplete pathway inhibition and pre-existing or inhibitor-induced resistance. The principle of pharmacologically targeting cancer-cell-intrinsic PI3K activity also assumes that all cancer-promoting effects of PI3K activation are reversible, which might not be the case. Emerging evidence suggests that genetic PI3K pathway activation can induce and/or allow cells to tolerate chromosomal instability, which-even if occurring in a low fraction of the cell population-might help to facilitate and/or drive tumour evolution. While it is clear that such genomic events cannot be reverted pharmacologically, a role for PI3K in the regulation of chromosomal instability could be exploited by using PI3K pathway inhibitors to prevent those genomic events from happening and/or reduce the pace at which they are occurring, thereby dampening cancer development or progression. Such an impact might be most effective in tumours with clonal PI3K activation and achievable at lower drug doses than the maximum-tolerated doses of PI3K inhibitors currently used in the clinic.

Integration of intracellular signaling: Biological analogues of wires, processors and memories organized by a centrosome 3D reference system

BACKGROUND

Myriads of signaling pathways in a single cell function to achieve the highest spatio-temporal integration. Data are accumulating on the role of electromechanical soliton-like waves in signal transduction processes. Theoretical studies strongly suggest feasibility of both classical and quantum computing involving microtubules.

AIM

A theoretical study of the role of the complex composed of the plasma membrane and the microtubule-based cytoskeleton as a system that transmits, stores and processes information.

METHODS

Theoretical analysis presented here refers to (i) the Penrose-Hameroff theory of consciousness (Orchestrated Objective Reduction; Orch OR), (ii) the description of the centrosome as a reference system for construction of the 3D map of the cell proposed by Regolini, (iii) the Heimburg-Jackson model of the nerve pulse propagation along axons' lipid bilayer as soliton-like electro-mechanical waves.

RESULTS AND CONCLUSION

The ideas presented in this paper provide a qualitative model for the decision-making processes in a living cell undergoing a differentiation process.

OUTLOOK

This paper paves the way for the real-time live-cell observation of information processing by microtubule-based cytoskeleton and cell fate decision making.

Exposure to Di-(2-ethylhexyl) Phthalate During Perinatal Period Gender-Specifically Impairs the Dendritic Growth of Pyramidal Neurons in Rat Offspring

Di-(2-ethylhexyl) phthalate (DEHP), as a prevalent xenoestrogen endocrine disrupter, is omnipresent in the environment and commonly used in polyethylene plastic products. Although DEHP has potential adverse effects on multisystem organs, damage to the central nervous system is more significant. However, the consequences and mechanisms of DEHP exposure remain to be explored. The aim of this study was to investigate the effects and related mechanisms of maternal DEHP exposure on dendritic development of hippocampal pyramidal neurons in a rat model. Pregnant Wistar rats were intragastrically administrated either vehicle or DEHP (30, 300, and 750 mg/kg/d) from gestation day 0 to postnatal day (PN) 21. The dendritic length and complexity of dendritic arbors' pattern in pyramidal neurons of the hippocampus were measured using Golgi-Cox staining and Sholl analysis. The expression of dendritic development-related proteins was detected using western blot and immunofluorescence staining. DEHP-treated male but not female pups showed an obvious decrease in the total length and branching numbers of basal dendrites on PN7, PN14, and PN21. The phosphorylation of MAP2c, stathmin, and JNK1 in the male pup hippocampus was significantly decreased in DEHP treatment groups compared to controls. However, protein expression alteration in the hippocampus of female offspring was not observed. In summary, our study indicated that DEHP has a gender-specific negative impact on the dendritic growth of CA1 pyramidal neurons in male offspring of a rat model of DEHP exposure. The adverse impact may be related to the dysregulation of phosphorylated and total MAP2c and stathmin mediated by JNK1.

Superior Therapeutic Efficacy of Nanoparticle Albumin Bound Paclitaxel Over Cremophor-Bound Paclitaxel in Experimental Esophageal Adenocarcinoma

Esophageal adenocarcinoma (EAC) is the fastest growing cancer in the western world and the overall 5 year survival rate of EAC is below 20%. Most patients with EAC present with locally advanced or widespread metastatic disease, where current treatment is largely ineffective. Therefore, new therapeutic approaches are urgently needed. Nanoparticle albumin-bound paclitaxel (nab-paclitaxel) is a novel albumin-stabilized, cremophor-free and water soluble nanoparticle formulation of paclitaxel, and the potential role of nab-paclitaxel has not been tested yet in experimental EAC. Here we tested the antiproliferative and antitumor efficacy with survival advantage of nab-paclitaxel as monotherapy and in combinations in in-vitro, and in murine subcutaneous xenograft and peritoneal metastatic survival models of human EAC. Nab-paclitaxel significantly inhibited in-vitro cell proliferation with higher in-vivo antitumour efficacy and survival benefit compared to paclitaxel or carboplatin treatments both in mono- and combination therapies. Nab-paclitaxel treatment increased expression of mitotic-spindle associated phospho-stathmin, decreased expression of proliferative markers and enhanced apoptosis. This study demonstrates that nab-paclitaxel had stronger antiproliferative and antitumor activity in experimental EAC than the current standard chemotherapeutic agents which supports the rationale for its clinical use in EAC.

Phosphoproteomics reveals network rewiring to a pro-adhesion state in annexin-1-deficient mammary epithelial cells

Background

Annexin-1 (ANXA1) plays pivotal roles in regulating various physiological processes including inflammation, proliferation and apoptosis, and deregulation of ANXA1 functions has been associated with tumorigenesis and metastasis events in several types of cancer. Though ANXA1 levels correlate with breast cancer disease status and outcome, its distinct functional involvement in breast cancer initiation and progression remains unclear. We hypothesized that ANXA1-responsive kinase signaling alteration and associated phosphorylation signaling underlie early events in breast cancer initiation events and hence profiled ANXA1-dependent phosphorylation changes in mammary gland epithelial cells.

Methods

Quantitative phosphoproteomics analysis of mammary gland epithelial cells derived from ANXA1-heterozygous and ANXA1-deficient mice was carried out using stable isotope labeling with amino acids in cell culture (SILAC)-based mass spectrometry. Kinase and signaling changes underlying ANXA1 perturbations were derived by upstream kinase prediction and integrated network analysis of altered proteins and phosphoproteins.

Results

We identified a total of 8110 unique phosphorylation sites, of which 582 phosphorylation sites on 372 proteins had ANXA1-responsive changes. A majority of these phosphorylation changes occurred on proteins associated with cytoskeletal reorganization spanning the focal adhesion, stress fibers, and also the microtubule network proposing new roles for ANXA1 in regulating microtubule dynamics. Comparative analysis of regulated global proteome and phosphoproteome highlighted key differences in translational and post-translational effects of ANXA1, and suggested closely coordinated rewiring of the cell adhesion network. Kinase prediction analysis suggested activity modulation of calmodulin-dependent protein kinase II (CAMK2), P21-activated kinase (PAK), extracellular signal-regulated kinase (ERK), and IκB kinase (IKK) upon loss of ANXA1. Integrative analysis revealed regulation of the WNT and Hippo signaling pathways in ANXA1-deficient mammary epithelial cells, wherein there is downregulation of transcriptional effects of TEA domain family (TEAD) suggestive of ANXA1-responsive transcriptional rewiring.

Conclusions

The phosphoproteome landscape uncovered several novel perspectives for ANXA1 in mammary gland biology and highlighted its involvement in key signaling pathways modulating cell adhesion and migration that could contribute to breast cancer initiation.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-017-0924-4) contains supplementary material, which is available to authorized users.

Stathmin/Op18 depletion induces genomic instability and leads to premature senescence in human normal fibroblasts

Stathmin/oncoprotein18 regulates microtubule dynamics and participates in mitotic entry and exit. We isolated stathmin as a physically interacting partner of KIFC1, a minus-end-directed kinesin functioning in bipolar spindle formation and maintenance. We found that stathmin depletion leads to multipolar spindle formation in IMR-90 normal human fibroblasts. Stathmin-depleted IMR-90 cells showed early mitotic delay but managed to undergo chromosome segregation by forming multiple poles or pseudo-bipoles. Consistent with these observations, lagging chromosomes, and micronuclei were elevated in stathmin-depleted IMR-90 cells, demonstrating that stathmin is essential for maintaining genomic stability during mitosis in human cells. Genomic instability induced by stathmin depletion led to premature senescence without any indication of cell death in normal IMR-90 cells. Double knock-down of both stathmin and p53 also did not induce cell death in IMR-90 cells, while the stathmin knock-down triggered apoptosis in p53-proficient human lung adenocarcinoma cells. Our results suggest that stathmin is essential in bipolar spindle formation to maintain genomic stability during mitosis, and the depletion of stathmin prevents the initiation of chromosome instability by inducing senescence in human normal fibroblasts.

Genetic Demonstration of a Role for Stathmin in Adult Hippocampal Neurogenesis, Spinogenesis, and NMDA Receptor-Dependent Memory

Neurogenesis and memory formation are essential features of the dentate gyrus (DG) area of the hippocampus, but to what extent the mechanisms responsible for both processes overlap remains poorly understood. Stathmin protein, whose tubulin-binding and microtubule-destabilizing activity is negatively regulated by its phosphorylation, is prominently expressed in the DG. We show here that stathmin is involved in neurogenesis, spinogenesis, and memory formation in the DG. tTA/tetO-regulated bitransgenic mice, expressing the unphosphorylatable constitutively active Stathmin4A mutant (Stat4A), exhibit impaired adult hippocampal neurogenesis and reduced spine density in the DG granule neurons. Although Stat4A mice display deficient NMDA receptor-dependent memory in contextual discrimination learning, which is dependent on hippocampal neurogenesis, their NMDA receptor-independent memory is normal. Confirming NMDA receptor involvement in the memory deficits, Stat4A mutant mice have a decrease in the level of synaptic NMDA receptors and a reduction in learning-dependent CREB-mediated gene transcription. The deficits in neurogenesis, spinogenesis, and memory in Stat4A mice are not present in mice in which tTA/tetO-dependent transgene transcription is blocked by doxycycline through their life. The memory deficits are also rescued within 3 d by intrahippocampal infusion of doxycycline, further indicating a role for stathmin expressed in the DG in contextual memory. Our findings therefore point to stathmin and microtubules as a mechanistic link between neurogenesis, spinogenesis, and NMDA receptor-dependent memory formation in the DG.

SIGNIFICANCE STATEMENT

In the present study, we aimed to clarify the role of stathmin in neuronal and behavioral functions. We characterized the neurogenic, behavioral, and molecular consequences of the gain-of-function stathmin mutation using a bitransgenic mouse expressing a constitutively active form of stathmin. We found that stathmin plays an important role in adult hippocampal neurogenesis and spinogenesis. In addition, stathmin mutation led to impaired NMDA receptor-dependent and neurogenesis-associated memory and did not affect NMDA receptor-independent memory. Moreover, biochemical analysis suggested that stathmin regulates the synaptic transport of NMDA receptors, which in turn influence CREB-mediated gene transcription machinery. Overall, these data suggest that stathmin is an important molecule for neurogenesis, spinogenesis, and NMDA receptor-dependent learning and memory.

Novel indolyl-chalcones target stathmin to induce cancer cell death

Efficacy of current therapies for advanced and metastatic cancers remains a challenge in clinical practice. We investigated the anti-cancer potency of 3 novel indoly-chalcones (CITs). Our results indicated the lead molecule CIT-026 (Formula = C20H16FNO) induced cell death in prostate and lung cancer cell lines at sub-micromolar concentration. CITs (CIT-026, CIT-214, CIT-223) lead to microtubule destabilization, cell death and low cell proliferation, which in part was dependent on stathmin (STMN1) expression. Knockdown of STMN1 with siRNA against STMN1 in part restored viability of cancer cells in response to CITs. Further, CIT-026 and CIT-223 blocked cancer cell invasion through matrigel-coated chambers. Mechanistically, CITs inhibited phosphorylation of STMN1 leading to STMN1 accumulation and mitotic catastrophe. In summary, we have synthetized novel anti-cancer CIT molecules and defined their mechanism of action in vitro.

Microtubule Dynamics in Neuronal Development, Plasticity, and Neurodegeneration

Neurons are the basic information-processing units of the nervous system. In fulfilling their task, they establish a structural polarity with an axon that can be over a meter long and dendrites with a complex arbor, which can harbor ten-thousands of spines. Microtubules and their associated proteins play important roles during the development of neuronal morphology, the plasticity of neurons, and neurodegenerative processes. They are dynamic structures, which can quickly adapt to changes in the environment and establish a structural scaffold with high local variations in composition and stability. This review presents a comprehensive overview about the role of microtubules and their dynamic behavior during the formation and maturation of processes and spines in the healthy brain, during aging and under neurodegenerative conditions. The review ends with a discussion of microtubule-targeted therapies as a perspective for the supportive treatment of neurodegenerative disorders.

The Fanconi Anemia C Protein Binds to and Regulates Stathmin-1 Phosphorylation

The Fanconi anemia (FA) proteins are involved in a signaling network that assures the safeguard of chromosomes. To understand the function of FA proteins in cellular division events, we investigated the interaction between Stathmin-1 (STMN1) and the FA group C (FANCC) protein. STMN1 is a ubiquitous cytosolic protein that regulates microtubule dynamics. STMN1 activities are regulated through phosphorylation-dephosphorylation mechanisms that control assembly of the mitotic spindle, and dysregulation of STMN1 phosphorylation is associated with mitotic aberrancies leading to chromosome instability and cancer progression. Using different biochemical approaches, we showed that FANCC interacts and co-localizes with STMN1 at centrosomes during mitosis. We also showed that FANCC is required for STMN1 phosphorylation, as mutations in FANCC reduced serine 16- and 38-phosphorylated forms of STMN1. Phosphorylation of STMN1 at serine 16 is likely an event dependent on a functional FA pathway, as it is reduced in FANCA- and FANCD2-mutant cells. Furthermore, FA-mutant cells exhibited mitotic spindle anomalies such as supernumerary centrosomes and shorter mitotic spindles. These results suggest that FA proteins participate in the regulation of cellular division via the microtubule-associated protein STMN1.

Deceivingly dynamic: Learning-dependent changes in stathmin and microtubules

Microtubules, one of the major cytoskeletal structures, were previously considered stable and only indirectly involved in synaptic structure and function in mature neurons. However, recent evidence demonstrates that microtubules are dynamic and have an important role in synaptic structure, synaptic plasticity, and memory. In particular, learning induces changes in microtubule turnover and stability, and pharmacological manipulation of microtubule dynamics alters synaptic plasticity and long-term memory. These learning-induced changes in microtubules are controlled by the phosphoprotein stathmin, whose only known cellular activity is to negatively regulate microtubule formation. During the first eight hours following learning, changes in the phosphorylation of stathmin go through two phases causing biphasic shifts in microtubules stability/instability. These shifts, in turn, regulate memory formation by controlling in the second phase synaptic transport of the GluA2 subunit of AMPA receptors. Improper regulation of stathmin and microtubule dynamics has been observed in aged animals and in patients with Alzheimer's disease and depression. Thus, recent work on stathmin and microtubules has identified new molecular players in the early stages of memory encoding.

Learning-induced and stathmin-dependent changes in microtubule stability are critical for memory and disrupted in ageing

Changes in the stability of microtubules regulate many biological processes, but their role in memory remains unclear. Here we show that learning causes biphasic changes in the microtubule-associated network in the hippocampus. In the early phase, stathmin is dephosphorylated, enhancing its microtubule-destabilizing activity by promoting stathmin-tubulin binding, whereas in the late phase these processes are reversed leading to an increase in microtubule/KIF5-mediated localization of the GluA2 subunit of AMPA receptors at synaptic sites. A microtubule stabilizer paclitaxel decreases or increases memory when applied at the early or late phases, respectively. Stathmin mutations disrupt changes in microtubule stability, GluA2 localization, synaptic plasticity and memory. Aged wild-type mice show impairments in stathmin levels, changes in microtubule stability, and GluA2 localization. Blocking GluA2 endocytosis rescues memory deficits in stathmin mutant and aged wild-type mice. These findings demonstrate a role for microtubules in memory in young adult and aged individuals.

The role of the cytoskeleton at the immunological synapse

It has been over 30 years since the reorganization of both the microtubule network and a 'peculiar actin polarization' was reported at the contact area of cytotoxic T lymphocytes interacting with target cells. Since that time, hundreds of studies have been published in an effort to elucidate the structure and function of the microtubule network and the actin cytoskeleton in T-cell activation, migration, and effector function at the interface between a T cell and its cognate antigen-presenting cell or target cell. This interface has become known as the immunological synapse, and this review examines some of the roles played by the cytoskeleton at the synapse.

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.

Expanding applications of chemical genetics in signal transduction

Chemical genetics represents an expanding collection of techniques applied to a variety of signaling processes. These techniques use a combination of chemical reporters and protein engineering to identify targets of a signaling enzyme in a global and non-directed manner without resorting to hypothesis-driven candidate approaches. In the last year, chemical genetics has been applied to a variety of kinases, revealing a much broader spectrum of substrates than had been appreciated. Here, we discuss recent developments in chemical genetics, including insights from our own proteomic screen for substrates of the kinase ERK2. These studies have revealed that many kinases have overlapping substrate specificity, and they often target several proteins in any particular downstream pathway. It remains to be determined whether this configuration exists to provide redundant control, or whether each target contributes a fraction of the total regulatory effect. From a general perspective, chemical genetics is applicable in principle to a broad range of posttranslational modifications (PTMs), most notably methylation and acetylation, although many challenges remain in implementing this approach. Recent developments in chemical reporters and protein engineering suggest that chemical genetics will soon be a powerful tool for mapping signal transduction through these and other PTMs.

Stathmin regulates microtubule dynamics and microtubule organizing center polarization in activated T cells

Polarization of T cells involves reorientation of the microtubule organizing center (MTOC). Because activated ERK is localized at the immunological synapse, we investigated its role by showing that ERK activation is important for MTOC polarization. Suspecting that ERK phosphorylates a regulator of microtubules, we next focused on stathmin, a known ERK substrate. Our work indicates that during T cell activation, ERK is recruited to the synapse, allowing it to phosphorylate stathmin molecules near the immunological synapse. Supporting an important role of stathmin phosphorylation in T cell activation, we showed that T cell activation results in increased microtubule growth rate dependent on the presence of stathmin. The significance of this finding was demonstrated by results showing that CTLs from stathmin(-/-) mice displayed defective MTOC polarization and defective target cell cytolysis. These data implicate stathmin as a regulator of the microtubule network during T cell activation.

Cytoplasmic mislocalization of p27 and cdk2 mediates the anti-migratory and anti-proliferative effects of Nodal in human trophoblast cells

p27Kip1, a cyclin-dependent kinase (CDK) inhibitor, is a multi-functional protein that regulates various cellular activities. Trophoblast proliferation, migration, and invasion are some of the key processes of placental development. We have recently reported that Nodal, a member of the transforming growth factor-β (TGF-β) superfamily, inhibits human trophoblast cell proliferation, migration and invasion. In this study, we investigated the mechanism by which Nodal regulates trophoblast activities. We found that Nodal increased p27 mRNA and protein levels by enhancing their stability. Interestingly, Nodal signaling also induced nuclear export of p27 and cdk2. Cytoplasmic translocation of p27 induced by Nodal requires p27 phosphorylation at S10. In addition, Nodal enhanced the association of p27 with cdk2, cdk5 and a microtubule-destabilizing protein; stathmin, and induced stathmin phosphorylation at S25 and S38. Furthermore, Nodal increased tubulin stability as revealed by immunofluorescent staining of acetylated tubulin. Finally, silencing of p27 reversed the inhibitory effect of Nodal on trophoblast cell proliferation, migration, and invasion. Taken together, our findings revealed a novel function of simultaneous p27 and cdk2 cytoplasmic mislocalization in mediating growth factor-regulated cell proliferation, migration and invasion.

Assessment of downstream effectors of BCR/ABL protein tyrosine kinase using combined proteomic approaches

Leukaemic transformation is frequently associated with the aberrant activity of a protein tyrosine kinase (PTK). As such it is of clinical relevance to be able to map the effects of these leukaemogenic PTKs on haemopoietic cells at the level of phosphorylation modulation. In this paradigm study we have employed a range of proteomic approaches to analyse the effects of one such PTK, BCR/ABL. We have employed phosphoproteome enrichment techniques allied to peptide and protein quantification to identify proteins and pathways involved in cellular transformation. Amongst the proteins shown to be regulated at the post-translational level were cofilin, an actin-severing protein thus linked to altered motility and Cbl an E3 ubiquitin ligase integrally linked to the control of tyrosine kinase signalling (regulated by 5 and 6 PTKs respectively). The major class of proteins identified however were molecular chaperones. We also showed that HSP90 phosphorylation is altered by BCR/ABL action and that HSP90 plays a crucial role in oncogene stability. Further investigation with another six leukaemogenic PTKs demonstrates that this HSP90 role in oncogene stability appears to be a common phenomenon in a range of leukaemias. This opens up the potential opportunity to treat different leukaemias with HSP90 inhibitors.

Quantitative phosphoproteomics of proteasome inhibition in multiple myeloma cells

BACKGROUND

The proteasome inhibitor bortezomib represents an important advance in the treatment of multiple myeloma (MM). Bortezomib inhibits the activity of the 26S proteasome and induces cell death in a variety of tumor cells; however, the mechanism of cytotoxicity is not well understood.

METHODOLOGY/PRINCIPAL FINDINGS

We investigated the differential phosphoproteome upon proteasome inhibition by using stable isotope labeling by amino acids in cell culture (SILAC) in combination with phosphoprotein enrichment and LC-MS/MS analysis. In total 233 phosphoproteins were identified and 72 phosphoproteins showed a 1.5-fold or greater change upon bortezomib treatment. The phosphoproteins with expression alterations encompass all major protein classes, including a large number of nucleic acid binding proteins. Site-specific phosphopeptide quantitation revealed that Ser38 phosphorylation on stathmin increased upon bortezomib treatment, suggesting new mechanisms associated to bortezomib-induced apoptosis in MM cells. Further studies demonstrated that stathmin phosphorylation profile was modified in response to bortezomib treatment and the regulation of stathmin by phosphorylation at specific Ser/Thr residues participated in the cellular response induced by bortezomib.

CONCLUSIONS/SIGNIFICANCE

Our systematic profiling of phosphorylation changes in response to bortezomib treatment not only advanced the global mechanistic understanding of the action of bortezomib on myeloma cells but also identified previously uncharacterized signaling proteins in myeloma cells.

Regulation of microtubule dynamics through phosphorylation on stathmin by Epstein-Barr virus kinase BGLF4

Stathmin is an important microtubule (MT)-destabilizing protein, and its activity is differently attenuated by phosphorylation at one or more of its four phosphorylatable serine residues (Ser-16, Ser-25, Ser-38, and Ser-63). This phosphorylation of stathmin plays important roles in mitotic spindle formation. We observed increasing levels of phosphorylated stathmin in Epstein-Barr virus (EBV)-harboring lymphoblastoid cell lines (LCLs) and nasopharyngeal carcinoma (NPC) cell lines during the EBV lytic cycle. These suggest that EBV lytic products may be involved in the regulation of stathmin phosphorylation. BGLF4 is an EBV-encoded kinase and has similar kinase activity to cdc2, an important kinase that phosphorylates serine residues 25 and 38 of stathmin during mitosis. Using an siRNA approach, we demonstrated that BGLF4 contributes to the phosphorylation of stathmin in EBV-harboring NPC. Moreover, we confirmed that BGLF4 interacts with and phosphorylates stathmin using an in vitro kinase assay and an in vivo two-dimensional electrophoresis assay. Interestingly, unlike cdc2, BGLF4 was shown to phosphorylate non-proline directed serine residues of stathmin (Ser-16) and it mediated phosphorylation of stathmin predominantly at serines 16, 25, and 38, indicating that BGLF4 can down-regulate the activity of stathmin. Finally, we demonstrated that the pattern of MT organization was changed in BGLF4-expressing cells, possibly through phosphorylation of stathmin. In conclusion, we have shown that a viral Ser/Thr kinase can directly modulate the activity of stathmin and this contributes to alteration of cellular MT dynamics and then may modulate the associated cellular processes.

KIS protects against adverse vascular remodeling by opposing stathmin-mediated VSMC migration in mice

Vascular proliferative diseases are characterized by VSMC proliferation and migration. Kinase interacting with stathmin (KIS) targets 2 key regulators of cell proliferation and migration, the cyclin-dependent kinase inhibitor p27Kip1 and the microtubule-destabilizing protein stathmin. Phosphorylation of p27Kip1 by KIS leads to cell-cycle progression, whereas the target sequence and the physiological relevance of KIS-mediated stathmin phosphorylation in VSMCs are unknown. Here we demonstrated that vascular wound repair in KIS-/- mice resulted in accelerated formation of neointima, which is composed predominantly of VSMCs. Deletion of KIS increased VSMC migratory activity and cytoplasmic tubulin destabilizing activity, but abolished VSMC proliferation through the delayed nuclear export and degradation of p27Kip1. This promigratory phenotype resulted from increased stathmin protein levels, caused by a lack of KIS-mediated stathmin phosphorylation at serine 38 and diminished stathmin protein degradation. Downregulation of stathmin in KIS-/- VSMCs fully restored the phenotype, and stathmin-deficient mice demonstrated reduced lesion formation in response to vascular injury. These data suggest that KIS protects against excessive neointima formation by opposing stathmin-mediated VSMC migration and that VSMC migration represents a major mechanism of vascular wound repair, constituting a relevant target and mechanism for therapeutic interventions.

Stathmin expression during newt retina regeneration

Japanese common newts (Cynops pyrrhogaster) have high ability to regenerate their injured organs including neural tissues, for example, the neural retina belonging to central nervous system. We attempted to clarify the molecular mechanism underlying the formation of a neural network during newt retina regeneration, and focused on the microtubule dynamics controlled by stathmin family proteins. Stathmin is a small cytoplasmic phosphoprotein known to be a microtubule regulator. We isolated a clone encoding stathmin from the newt. The expression level of stathmin is higher in lung and spleen than in the adult intact retina where stathmin was localized on plexiform layers, the ganglion layer and in photoreceptor inner segments. However, in a regenerating process of the retina, stathmin was upregulated from an early regenerating stage until the retinal layered structure was formed. Immunohistochemical analyses revealed that stathmin existed all around the regenerating retina consisting of retinal progenitor cells. These results suggest that stathmin plays important roles in the construction and maintenance of retinal structure and its neural network, by controlling the proliferation of retinal progenitor cells and the microtubule dynamics of retinal neurons. Moreover, stathmin may function in the dedifferentiating process of retinal pigment epithelium cells.

Increased expression and differential phosphorylation of stathmin may promote prostate cancer progression

BACKGROUND

Proteins which regulate normal development may promote tumorigenesis, tumor progression, or metastasis through dysregulation of these functions. We postulate that proteins, which regulate prostate growth also promote prostate cancer (PCa) progression.

METHODS

Two Dimensional Gel Electrophoresis was utilized to compare patterns of protein expression in 12T-7f prostates (LPB-Tag mouse model for PCa) during tumor development and progression with those of normal developing and adult wild type CD-1 prostates. Stathmin expression and phosphorylation patterns were analyzed in mouse and human PCa cell lines as well as in human PCa tissue arrays.

RESULTS

Stathmin was identified by two-dimensional gel electrophoresis and mass spectrometry. Stathmin levels increase early during normal mouse prostate development and again during prostate tumor development and progression. In human prostate adenocarcinoma, stathmin increases in Gleason pattern 5. Further, stathmin is differentially phosphorylated in androgen-dependent LNCaP cells compared to androgen-independent PC-3 and DU145 cells. This differential phosphorylation is modulated by androgen and anti-androgen treatment.

CONCLUSION

Stathmin expression is highest when the prostate is undergoing morphogenesis or tumorigenesis and these processes may be regulated through differential phosphorylation. Furthermore, modulation of stathmin phosphorylation may correlate with the development of androgen-independent PCa.

Reversal of stathmin-mediated resistance to paclitaxel and vinblastine in human breast carcinoma cells

Antimicrotubule agents are commonly used chemotherapy drugs for the treatment of breast and other cancers. However, these agents have variable activity partly because of microtubule regulatory proteins. Stathmin, an 18-kDa phosphoprotein that promotes microtubule depolymerization, was found to be frequently overexpressed in breast cancer. We previously identified stathmin-mediated mechanisms of resistance to antimicrotubule agents, including altered drug binding and delayed transit from G(2) into M phase, where these agents are effective in disrupting microtubule dynamics. We hypothesized that by reversing stathmin-mediated depolymerization of microtubules or by promoting entry into mitosis, this could increase sensitivity to antimicrotubule agents in human breast cancer cells overexpressing stathmin. We found that targeting stathmin or wee-1 expression with RNA interference can induce microtubule polymerization and promote G(2)/M progression, respectively, and sensitize stathmin-overexpressing breast cancer cells to paclitaxel and vinblastine. Furthermore, targeting wee-1 led to the phosphorylation of stathmin, which is known to attenuate its activity. Therefore, these data suggest a novel approach to improving the efficacy of certain antimicrotubule agents against breast cancer by regulating the function of stathmin.

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.

Phosphorylation of the tubulin-binding protein, stathmin, by Cdk5 and MAP kinases in the brain

Regulation of cytoskeletal dynamics is essential to neuronal plasticity during development and adulthood. Dysregulation of these mechanisms may contribute to neuropsychiatric and neurodegenerative diseases. The neuronal protein kinase, cyclin-dependent kinase 5 (Cdk5), is involved in multiple aspects of neuronal function, including regulation of cytoskeleton. A neuroproteomic search identified the tubulin-binding protein, stathmin, as a novel Cdk5 substrate. Stathmin was phosphorylated by Cdk5 in vitro at Ser25 and Ser38, previously identified as mitogen-activated protein kinase (MAPK) and p38 MAPKdelta sites. Cdk5 predominantly phosphorylated Ser38, while MAPK and p38 MAPKdelta predominantly phosphorylated Ser25. Stathmin was phosphorylated at both sites in mouse brain, with higher levels in cortex and striatum. Cdk5 knockout mice exhibited decreased phospho-Ser38 levels. During development, phospho-Ser25 and -Ser38 levels peaked at post-natal day 7, followed by reduction in total stathmin. Inhibition of protein phosphatases in striatal slices caused an increase in phospho-Ser25 and a decrease in total stathmin. Interestingly, the prefrontal cortex of schizophrenic patients had increased phospho-Ser25 levels. In contrast, total and phospho-Ser25 stoichiometries were decreased in the hippocampus of Alzheimer's patients. Thus, microtubule regulatory mechanisms involving the phosphorylation of stathmin may contribute to developmental synaptic pruning and structural plasticity, and may be involved in neuropsychiatric and neurodegenerative disorders.

Expression of stathmin family genes in the murine uterus during early pregnancy

Stathmin, a cytosolic phosphoprotein that regulates microtubule dynamics during cell-cycle progression, is abundantly expressed at embryo implantation sites in rats. Here, we characterized the expression of stathmin and its family genes in the murine uterus during the peri-implantation period. Stathmin protein was expressed in the glandular and luminal epithelium, blood vessels, and stromal cells on day 3 of pregnancy. On the day of implantation (day 5), stathmin was mainly localized in blood vessels in the endometrium. On day 7, intense stathmin expression was limited to capillary vessels and secondary decidual cells. Stathmin expression was higher at implantation sites than at uterine segments between implantation sites and increased during oil-induced decidualization. Although the artificially-induced deciduoma weights and number of implantation sites were similar between stathmin-knockout (KO) and wild-type (WT) mice, the stathmin-KO mice had fewer newborn pups (reduced by 30%). The expression of alkaline phosphatase, desmin, and cyclin D3 was attenuated in decidual zones of stathmin-KO mice. Messenger RNA level of the stathmin family gene, SCG10, was high at the time of decidualization in WT and stathmin-KO mice. In contrast, the others of stathmin family members, SCLIP and RB3 were highly expressed in stathmin-KO mice compared to WT mice. These results suggest that stathmin and stathmin family genes are expressed in the murine endometrium with enhanced expression in the implantation or the decidualization process.

A proteome analysis of the anterior cingulate cortex gray matter in schizophrenia

The Anterior Cingulate Cortex (ACC, Brodmans Area 24) is implicated in the pathogenesis of schizophrenia due to its normal functions and connectivity together with reports of structural, morphological and neurotransmitter aberrations within this brain area in the disease state. Two-dimensional gel electrophoresis (2DE) was employed to scan and compare the ACC gray matter proteomes between schizophrenia (n = 10) and control (n = 10) post-mortem human tissue. This proteomic approach has detected 42 protein spots with altered levels in the schizophrenia cohort, which to our knowledge is the first proteomic analysis of the ACC in schizophrenia. Thirty nine of these proteins were subsequently identified using mass spectrometry and functionally classified into metabolism and oxidative stress, cytoskeletal, synaptic, signalling, trafficking and glial-specific groups. Some of the identified proteins have previously been implicated in the disease pathogenesis and some offer new insights into schizophrenia. Investigating these proteins, the genes encoding these proteins, their functions and interactions may shed light on the molecular mechanisms underlying the heterogeneous symptoms characteristic of schizophrenia.

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.

Identification of Op18/stathmin as a potential target of ASK1-p38 MAP kinase cascade

Apoptosis signal-regulating kinase 1 (ASK1) is a mitogen-activated protein (MAP) kinase kinase kinase that activates the JNK and p38 MAP kinase cascades and has a broad range of biological activities including cell differentiation and stress-induced apoptosis. However, effector molecules of ASK1-MAP kinase cascades that exert such activities have not been fully identified. Here we have identified oncoprotein 18 (Op18)/stathmin as a potential target of the ASK1-p38 cascade. By two-dimensional electrophoresis, phosphorylation of Op18/stathmin was found to be increased upon the expression of constitutively active ASK1 (ASK1DeltaN) in PC12 cells. The ASK1-dependent increase in the phosphorylation of Op18/stathmin was attenuated by the treatment with SB203580, suggesting that p38alpha and/or p38beta contribute to the phosphorylation of Op18/stathmin. Consistently, we found that all four isoforms of p38 directly phosphorylated Op18/stathmin primarily at serine 25 in vitro. Taken together with the quantitative RT-PCR data indicating that p38alpha was the dominantly expressed isoform in PC12 cells, ASK1-induced phosphorylation of Op18/stathmin appears to be mediated mainly through p38alpha in these cells. Given that the microtubule-destabilizing activity of Op18/stathmin is regulated by its phosphorylation, the ASK1-p38 cascade may regulate microtubule dynamics through Op18/stathmin.

Stathmin and its phosphoprotein family: general properties, biochemical and functional interaction with tubulin

Stathmin, also referred to as Op18, is a ubiquitous cytosolic phosphoprotein, proposed to be a small regulatory protein and a relay integrating diverse intracellular signaling pathways involved in the control of cell proliferation, differentiation and activities. It interacts with several putative downstream target and/or partner proteins. One major action of stathmin is to interfere with microtubule dynamics, by inhibiting the formation of microtubules and/or favoring their depolymerization. Stathmin (S) interacts directly with soluble tubulin (T), which results in the formation of a T2S complex which sequesters free tubulin and therefore impedes microtubule formation. However, it has been also proposed that stathmin's action on microtubules might result from the direct promotion of catastrophes, which is still controversial. Phosphorylation of stathmin regulates its biological actions: it reduces its affinity for tubulin and hence its action on microtubule dynamics, which allows for example progression of cells through mitosis. Stathmin is also the generic element of a protein family including the neural proteins SCG10, SCLIP and RB3/RB3'/RB3". Interestingly, the stathmin-like domains of these proteins also possess a tubulin binding activity in vitro. In vivo, the transient expression of neural phosphoproteins of the stathmin family leads to their localization at Golgi membranes and, as previously described for stathmin and SCG10, to the depolymerization of interphasic microtubules. Altogether, the same mechanism for microtubule destabilization, that implies tubulin sequestration, is a common feature likely involved in the specific biological roles of each member of the stathmin family.

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.

Role of the microtubule destabilizing proteins SCG10 and stathmin in neuronal growth

The related proteins SCG10 and stathmin are highly expressed in the developing nervous system. Recently it was discovered that they are potent microtubule destabilizing factors. While stathmin is expressed in a variety of cell types and shows a cytosolic distribution, SCG10 is neuron-specific and membrane-associated. It contains an N-terminal targeting sequence that mediates its transport to the growing tips of axons and dendrites. SCG10 accumulates in the central domain of the growth cone, a region that also contains highly dynamic microtubules. These dynamic microtubules are known to be important for growth cone advance and responses to guidance cues. Because overexpression of SCG10 strongly enhances neurite outgrowth, SCG10 appears to be an important factor for the dynamic assembly and disassembly of growth cone microtubules during axonal elongation. Phosphorylation negatively regulates the microtubule destabilizing activity of SCG10 and stathmin, suggesting that these proteins may link extracellular signals to the rearrangement of the neuronal cytoskeleton. A role for these proteins in axonal elongation is also supported by their growth-associated expression pattern in nervous system development as well as during neuronal regeneration.

Cytoskeletal dynamics and transport in growth cone motility and axon guidance

Recent studies indicate the actin and microtubule cytoskeletons are a final common target of many signaling cascades that influence the developing neuron. Regulation of polymer dynamics and transport are crucial for the proper growth cone motility. This review addresses how actin filaments, microtubules, and their associated proteins play crucial roles in growth cone motility, axon outgrowth, and guidance. We present a working model for cytoskeletal regulation of directed axon outgrowth. An important goal for the future will be to understand the coordinated response of the cytoskeleton to signaling cascades induced by guidance receptor activation.

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

Stathmin is a phosphorylation-regulated tubulin-binding protein. In vitro and in vivo studies using nonphosphorylatable and pseudophosphorylated mutants of stathmin have questioned the view that stathmin might act only as a tubulin-sequestering factor. Stathmin was proposed to effectively regulate microtubule dynamic instability by increasing the frequency of catastrophe (the transition from steady growth to rapid depolymerization), without interacting with tubulin. We have used a noninvasive method to measure the equilibrium dissociation constants of the T(2)S complexes of tubulin with stathmin, pseudophosphorylated (4E)-stathmin, and diphosphostathmin. At both pH 6.8 and pH 7.4, the relative sequestering efficiency of the different stathmin variants depends on the concentration of free tubulin, i.e. on the dynamic state of microtubules. This control is exerted in a narrow range of tubulin concentration due to the highly cooperative binding of tubulin to stathmin. Changes in pH affect the stability of tubulin-stathmin complexes but do not change stathmin function. The 4E-stathmin mutant mimics inactive phosphorylated stathmin at low tubulin concentration and sequesters tubulin almost as efficiently as stathmin at higher tubulin concentration. We propose that stathmin acts solely by sequestering tubulin, without affecting microtubule dynamics, and that the effect of stathmin phosphorylation on microtubule assembly depends on tubulin critical concentration.

Regulation and subcellular localization of the microtubule-destabilizing stathmin family phosphoproteins in cortical neurons

Stathmin is a ubiquitous cytosolic phosphoprotein, preferentially expressed in the nervous system, and the generic element of a protein family that includes the neural-specific proteins SCG10, SCLIP, and RB3 and its splice variants, RB3' and RB3". All phosphoproteins of the family share with stathmin its tubulin binding and microtubule (MT)-destabilizing activities. To understand better the specific roles of these proteins in neuronal cells, we performed a comparative study of their expression, regulation, and intracellular distribution in embryonic cortical neurons in culture. We found that stathmin is highly expressed ( approximately 0.25% of total proteins) and uniformly present in the various neuronal compartments (cell body, dendrites, axon, growth cones). It appeared mainly unphosphorylated or weakly phosphorylated on one site, and antisera to specific phosphorylated sites (serines 16, 25, or 38) did not reveal a differential regulation of its phosphorylation among neuronal cell compartments. However, they revealed a subpopulation of cells in which stathmin was highly phosphorylated on serine 16, possibly by CaM kinase II also active in a similar subpopulation. The other proteins of the stathmin family are expressed about 100-fold less than stathmin in partially distinct neuronal populations, RB3 being detected in only about 20% of neurons in culture. In contrast to stathmin, they are each mostly concentrated at the Golgi apparatus and are also present along dendrites and axons, including growth cones. Altogether, our results suggest that the different members of the stathmin family have complementary, at least partially distinct functions in neuronal cell regulation, in particular in relation to MT dynamics.

Regulation of microtubule-associated proteins

Microtubule-associated proteins (MAPs) function to regulate the assembly dynamics and organization of microtubule polymers. Upstream regulation of MAP activities is the major mechanism used by cells to modify and control microtubule assembly and organization. This review summarizes the functional activities of MAPs found in animal cells and discusses how these MAPs are regulated. Mechanisms controlling gene expression, isoform-specific expression, protein localization, phosphorylation, and degradation are discussed. Additional regulatory mechanisms include synergy or competition between MAPs and the activities of cofactors or binding partners. For each MAP it is likely that regulation in vivo reflects a composite of multiple regulatory mechanisms.

Thrombopoietin: a pan-hematopoietic cytokine

The recent discovery of thrombopoietin has enhanced our understanding of both hematopoiesis and platelet production. Thrombopoietin supports hematopoietic stem cell survival and expansion as well as promoting all aspects of megakaryocyte development. The hormone displays many structural similarities to other members of the hematopoietic cytokine family and some notable differences, and regulation of its expression requires both receptor-mediated removal and other mechanisms. Thrombopoietin induces receptor dimerization and tyrosine phosphorylation, and a series of signaling events including activation of JAK/STAT, Shc/Ras/MAPK and PI3K/Akt; these pathways overlap with those induced by other cytokines, but the differences that lead to the unique biological effects of the hormone are gradually being uncovered. Our growing appreciation of how cytokine signaling pathways are translated into megakaryocyte development is discussed.

Inhibition of heat-induced phosphorylation of stathmin by the bioflavonoid quercetin

Effects of quercetin on heat-induced phosphorylation of stathmin in JURKAT cells were examined. Two-dimensional electrophoresis of stathmin showed that heat shock increases mono- and diphosphorylation of stathmin. Monophosphorylation induced by heat shock was inhibited by the presence of 0.1 mM quercetin, but not by the presence of 0.1 microM staurosporine. Immunoblot analysis of phosphorylated stathmin showed that heat-induced phosphorylation at Ser-38 was inhibited by quercetin but not by staurosporine. Quercetin enhanced heat-induced tyrosine phosphorylation of MAP kinase. These observations indicate that quercetin inhibits heat-induced phosphorylation at Ser-38 of stathmin but mitogen-activated protein (MAP) kinase is not involved in its phosphorylation.