Stathmin phosphorylation is regulated in striatal neurons by vasoactive intestinal peptide and monoamines via multiple intracellular pathways

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.

Influence of a genetic variant of the neuronal growth associated protein Stathmin 1 on cognitive and affective control processes: an event-related potential study

Stathmin 1 (STMN1) is a neuronal growth associated protein (NGAP) that is involved in microtubule dynamics and plays an important role in neurite outgrowth and synaptic plasticity. It is highly expressed in the amygdala, but also in different areas of the neocortex including the frontal lobe. Based on previous findings regarding an impact of STMN1 on fear processing, the present study aimed at extending the evidence concerning its functional role to include the domain of executive (frontal lobe) functions. To this end, a group of 59 healthy volunteers stratified for the single-nucleotide polymorphism rs182455 of the STMN1 gene was examined by means of three experimental paradigms probing different aspects of cognitive-affective functioning. Event-related potential measures of cognitive response control, emotional interference processing, and action monitoring were analyzed. STMN1 genotype significantly affected the NoGo-anteriorization (NGA)-a neurophysiological marker of cognitive response control associated with medial prefrontal cortex activation-as well as the modulation of the P300 by the valence of emotional Stroop stimuli. In both cases, carriers of the rs182455 C-allele showed altered cognitive-affective processing; effects appeared to be more pronounced in females. Our findings indicate a functional impact of STMN1 on cognitive and affective control processes, thereby complementing previous evidence on its role in fear processing. Based on these results, an influence of STMN1 should be considered in studies aiming at the etiopathogenesis of a broad range of neuropsychiatric disorders with dysfunctional networking, including neurodegenerative disorders as well as schizophrenia, autism spectrum disorders, anxiety disorders, depression, and ADHD.

Involvement of stathmin 1 in the neurotrophic effects of PACAP in PC12 cells

Rat pheochromocytoma PC12 cells have been widely used to investigate the neurotrophic activities of pituitary adenylate cyclase-activating polypeptide (PACAP). In particular, PACAP has been shown to promote differentiation and to inhibit apoptosis of PC12 cells. In order to identify the mechanisms mediating these effects, we sought for proteins that are phosphorylated upon PACAP treatment. High-performance liquid chromatography and 2D gel electrophoresis analysis, coupled with mass spectrometry, revealed that stathmin 1 is strongly phosphorylated within only 5 min of exposure to PACAP. Western blot experiments confirmed that PACAP induced a robust phosphorylation of stathmin 1 in a time-dependent manner. On the other hand, PACAP decreased stathmin 1 gene expression. Investigations of the signaling mechanisms known to be activated by PACAP revealed that phosphorylation of stathmin 1 was mainly mediated through the protein kinase A and mitogen-activated protein kinase pathways. Blockage of stathmin 1 expression with small interfering RNA did not affect PC12 cell differentiation induced by PACAP but reduced the ability of the peptide to inhibit caspase 3 activity and significantly decreased its neuroprotective action. Taken together, these data demonstrate that stathmin 1 is involved in the neurotrophic effect of PACAP in PC12 cells.

Protein expression profiling identifies maspin and stathmin as potential biomarkers of adenoid cystic carcinoma of the salivary glands

Adenoid cystic carcinoma (ACC) is one of the most common malignant tumors of the salivary glands. It tends to grow slowly but is associated with a poor prognosis compared to other malignant salivary gland tumors. To identify specific markers of ACC, we examined protein expression profiling in ACC xenograft and normal salivary glands (NSG) using fluorescent 2-dimensional differential in-gel electrophoresis (2-D-DIGE), an emerging technique for comparative proteomics, that improves the reproducibility and reliability of differential protein expression analysis between the samples. To identify the proteins, matrix-assisted laser desorption/ionization time-of-flight peptide mass fingerprinting was carried out. Using these strategies, we detected 4 upregulated proteins and 5 downregulated proteins in ACC xenograft. Maspin and stathmin were selected for further analyses. Western blotting and immunohistochemical staining showed a higher expression of these proteins in ACC xenograft and clinical ACC tissue compared to NSG. Furthermore, Expression of these proteins was correlated with the histologic grading of ACC (n = 10). Therefore, our data indicate that maspin and stathmin may be not only useful biomarkers of ACC but also markers of biologic behavior in this tumor.

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.

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.

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.

SCG10-related neuronal growth-associated proteins in neural development, plasticity, degeneration, and aging

Neuronal growth-associated proteins (nGAPs) are in general neuron-specific gene products whose expression correlates tightly with neuronal process outgrowth and/or regeneration, and are mostly good downstream targets of neurotrophin stimulation. Expression of genes encoding nGAPs such as GAP-43, SCG10, and stathmin is upregulated following lesioning of cortical and hippocampal regions of the adult rat brain. In the brains of aged animals, however, the magnitude of the response is reduced, whereas the time course of the response is mostly unchanged when compared with that for brains of young ones. Expression of GAP-43 and stathmin is reduced by aging, and is also changed in age-related neurodegenerative conditions such as Alzheimer's disease in humans. Certain nGAPs are induced during long-term potentiation (LTP) and also during critical periods of song-learning and ocular dominance column formation in birds and cats, respectively. Recent evidence further supports the idea that functional synaptic modulation is often associated with remodeling of synaptic structures. These results suggest that neurotrophin-responsive nGAPs serve as molecular markers of neuronal plasticity during development and aging, and that the neuronal plasticity decreases, at least in certain neuronal circuits, in the aged brain and neurodegenerative diseases. Recent findings on the roles of stathmin and SCG10-related proteins in microtubule destabilization and its functional block by phosphorylation further support the importance of the SCG10 family proteins in neuronal cytoskeletal regulation, particularly as to microtubule dynamics. We summarize here a decade of research on SCG10 and its related molecules with special interests to brain aging and disease.

A growth factor-dependent nuclear kinase phosphorylates p27(Kip1) and regulates cell cycle progression

The cyclin-dependent kinase inhibitor, p27(Kip1), which regulates cell cycle progression, is controlled by its subcellular localization and subsequent degradation. p27(Kip1) is phosphorylated on serine 10 (S10) and threonine 187 (T187). Although the role of T187 and its phosphorylation by Cdks is well-known, the kinase that phosphorylates S10 and its effect on cell proliferation has not been defined. Here, we identify the kinase responsible for S10 phosphorylation as human kinase interacting stathmin (hKIS) and show that it regulates cell cycle progression. hKIS is a nuclear protein that binds the C-terminal domain of p27(Kip1) and phosphorylates it on S10 in vitro and in vivo, promoting its nuclear export to the cytoplasm. hKIS is activated by mitogens during G(0)/G(1), and expression of hKIS overcomes growth arrest induced by p27(Kip1). Depletion of KIS using small interfering RNA (siRNA) inhibits S10 phosphorylation and enhances growth arrest. p27(-/-) cells treated with KIS siRNA grow and progress to S/G(2 )similar to control treated cells, implicating p27(Kip1) as the critical target for KIS. Through phosphorylation of p27(Kip1) on S10, hKIS regulates cell cycle progression in response to mitogens.

Two-dimensional gel electrophoresis in proteomics: old, old fashioned, but it still climbs up the mountains

Proteomics has traditionally used the separating power of two-dimensional electrophoresis for the quantitative analysis of protein amounts in complex extracts. However, the limitations of this approach in terms of throughput and analyzable protein range have elicited the development of other proteomics approaches, based either on peptide separations instead of protein separations, or based on direct protein recognition and selection on dedicated arrays (protein chips). These recent methods seem very promising, and probably look more promising than they will ultimately be, just because their weaknesses are not fully characterized yet. The purpose of this paper is thus to highlight the strengths and weaknesses of all the proteomics approaches proposed to date and to try to deduce the respective niches in proteomics that these approaches will have in the future.

Proteome analysis of rat spermatogonia: reinvestigation of stathmin spatio-temporal expression within the testis

Stathmin is a protein known to be involved in various cell processes including cell proliferation and differentiation. It has already been described in the testis but its recent identification using a proteomic approach in mitotic spermatogenetic stem cells named spermatogonia (Guillaume et al., 2000) has lead us to reinvestigate its expression within the testis. Stathmin and its mRNAs were studied in isolated cells by Western and Northern blots and in situ using immunohistochemistry. We demonstrated that stathmin is indeed expressed in spermatogonia, and that it is also intensively expressed in the meiotic spermatocytes and in the first generations of spermatids. Furthermore, we showed aggregations of the protein in the cytoplasm of the later generations of spermatids preceding its elimination at the time of spermiation. Our Northern blots reveal the presence of two stathmin transcripts of 1.1 and 3.2 kb within the testis from the fetal stage onwards, in spermatogonia, spermatocytes, and spermatids. However, the 3.2 kb RNA transcript was barely detectable in the spermatids. Stathmin expression is known to be associated with microtubule dynamics. Therefore, its expression in the germ line is most probably related to the extremely complex structural cellular rearrangements occurring in germ cells during spermatogenesis. However, the exact role of stathmin and the reason of the existence of two transcripts in the male germ lineage awaits further investigation.

Differences in phosphorylation of human and chicken stathmin by MAP kinase

Stathmin/Op18 is a highly conserved 19 kDa cytosolic phosphoprotein. Human and chicken stathmin share 93% identity with only 11 amino acid substitutions. One of the substituted amino acids is serine 25, which is a glycine in chicken stathmin. In human stathmin, serine 25 is the main phosphorylation site for MAP kinase. In this study, we have compared the phosphorylation of human and chicken stathmin. The proteins were expressed in Sf9 cells using the baculovirus expression system and purified for in vitro phosphorylation assays. Phosphorylation with MAP kinase showed that chicken stathmin was phosphorylated 10 times less than human stathmin. To identify the phosphorylation sites we used liquid chromatography/mass spectrometry (LC/MS/MS). The only amino acid found phosphorylated was serine 38, which corresponds to the minor phosphorylation site in human stathmin. Phosphorylation with p34(cdc2)- and cGMP-dependent protein kinases gave almost identical phosphorylation levels in the two stathmins.

Probing the native structure of stathmin and its interaction domains with tubulin. Combined use of limited proteolysis, size exclusion chromatography, and mass spectrometry

Stathmin is a cytosoluble phosphoprotein proposed to be a regulatory relay integrating diverse intracellular signaling pathway. Its interaction with tubulin modulates microtubule dynamics by destabilization of assembled microtubules or inhibition of their polymerization from free tubulin. The aim of this study was to probe the native structure of stathmin and to delineate its minimal region able to interact with tubulin. Limited proteolysis of stathmin revealed four structured domains within the native protein, corresponding to amino acid sequences 22-81 (I), 95-113 (II), 113-128 (III), and 128-149 (IV), which allows us to propose stathmin folding hypotheses. Furthermore, stathmin proteolytic fragments were mixed to interact with tubulin, and those that retained affinity for tubulin were isolated by size exclusion chromatography and identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. The results indicate that, to interact with tubulin, a stathmin fragment must span a minimal core region from residues 42 to 126, which interestingly corresponds to the predicted alpha-helical "interaction region" of stathmin. In addition, an interacting stathmin fragment must include a short N- or C-terminal extension. The functional significance of these interaction constrains is further validated by tubulin polymerization inhibition assays with fragments designed on the basis of the tubulin binding results. The present results will help to optimize further stathmin structural studies and to develop molecular tools to target its interaction with tubulin.

Overexpression of stathmin in breast carcinomas points out to highly proliferative tumours

We recently discovered that stathmin was overexpressed in a subgroup of human breast carcinomas. Stathmin is a cytosolic phosphoprotein proposed to act as a relay integrating diverse cell signalling pathways, notably during the control of cell growth and differentiation. It may also be considered as one of the key regulators of cell division for its ability to destabilize microtubules in a phosphorylation-dependent manner. To assess the significance of stathmin overexpression in breast cancer, we evaluated the correlation of stathmin expression, quantified by reverse transcription polymerase chain reaction, with several disease parameters in a large series of human primary breast cancer (n = 133), obtained in strictly followed up women, whose clinico-pathological data were fully available. In agreement with our preliminary survey, stathmin was found overexpressed in a subgroup of tumours (22%). In addition, overexpression was correlated to the loss of steroid receptors (oestrogen, P = 0.0006; progesterone, P = 0.008), and to the Scarff-Bloom-Richardson histopathological grade III (P= 0.002), this latter being ascribable to the mitotic index component (P= 0.02). Furthermore studies at the DNA level indicated that stathmin is overexpressed irrespective of its genomic status. Our findings raise important questions concerning the causes and consequences of stathmin overexpression, and the reasons of its inability to counteract cell proliferation in the overexpression group.

Cellular and subcellular localization of stathmin during oocyte and preimplantation embryo development

Stathmin is a 19 kDa cytosolic phosphoprotein, proposed to act as a relay integrating diverse intracellular signaling pathways involved in regulation of cell proliferation, differentiation, and function. To gain further information about its significance during early development, we analyzed stathmin expression and subcellular localization in mouse oocytes and preimplantation embryos. RT-PCR analysis revealed a low expression of stathmin mRNA in unfertilized oocytes and a higher expression at the blastocyst stage. A fine cytoplasmic punctuate fluorescent immunoreactive stathmin pattern was detected in the oocyte, while it evolved toward an increasingly speckled pattern in the two-cell and later four- to eight-cell embryo, with even larger speckles at the morula stage. In blastocysts, stathmin immunoreactivity was fine and intense in inner cell mass cells, whereas it was low and variable in trophectodermal cells. Electron microscopic analysis allowed visualization with more detail of two types of stathmin immunolocalization: small clusters in the cytoplasm of oocytes and blastocyst cells, together with loosely arranged clusters around the outer membrane of cytoplasmic vesicles, corresponding to the immunofluorescent speckles in embryos until the morula stage. In conclusion, it appears from our results that maternal stathmin is accumulated in the oocyte and is relocalized within the oocyte and early preimplantation embryonic cell cytoplasm to interact with specific cytoplasmic membrane formations. Probably newly synthesized, embryonic stathmin is expressed in the blastocyst, where it is localized more uniformly in the cytoplasm mostly of inner cell mass (ICM) cells. These expression and localization patterns are probably related to the particular roles of stathmin at the successive steps of oocyte maturation and early embryonic development. They further support the proposed physiologic importance of stathmin in essential biologic regulation.

Overexpression of the stathmin gene in a subset of human breast cancer

Stathmin is a highly conserved cytosolic phosphoprotein that destabilizes microtubules. Stathmin, which has been proposed as a relay protein integrating diverse cell signalling pathways, acts in vitro as a tubulin-sequestering protein, and its activity is dramatically reduced by phosphorylation. Interestingly, stathmin expression and phosphorylation are regulated during the control of cell growth and differentiation, and there is much evidence suggesting that in vivo stathmin plays a role in the control of microtubule dynamics during mitosis. Stathmin may thus be considered as one of the key regulators of cell division. We examined 50 human primary breast tumours for stathmin mRNA and protein expression and screened for abnormalities in the chromosome region harbouring the stathmin gene. Overexpression of stathmin was found in 15 tumours (30%). At the present stage, no clear correlation emerged between stathmin expression and several prognosis markers. Interestingly, perfect matching was observed between stathmin mRNA overexpression, protein overexpression and strong staining for stathmin on paraffin-embedded tumour sections when specimens were available. Furthermore, a tentative link between loss of heterozygosity (LOH) in the 1p32-1pter region and stathmin overexpression was observed. Our results suggest that stathmin might play a role in breast carcinogenesis and that stathmin-overexpressing tumours may represent a new subtype of breast cancer.

Serine 16 of Stathmin as a Cytosolic Target for Ca2+/Calmodulin-Dependent Kinase II After CD2 Triggering of Human T Lymphocytes

We investigated specific signaling events initiated after T cell triggering through the costimulatory surface receptors CD2 and CD28 as compared with activation via the Ag receptor (TCR/CD3). We therefore followed the phosphorylation of stathmin, a ubiquitous cytoplasmic phosphoprotein proposed as a general relay integrating diverse intracellular signaling pathways through the combinatorial phosphorylation of serines 16, 25, 38, and 63, the likely physiologic substrates for Ca2+/calmodulin (CaM)-dependent kinases, mitogen-activated protein (MAP) kinase, cyclin-dependent kinases (cdks), and protein kinase A, respectively. We addressed the specific protein kinase systems involved in the CD2 pathway of T cell activation through the analysis of stathmin phosphorylation patterns in exponentially growing Jurkat T cells, as revealed by phosphopeptide mapping. Stimulation via CD2 activated multiple signal transduction pathways, resulting in phosphorylation of distinct sites of stathmin, the combination of which only partially overlaps the CD3- and CD28-induced patterns. The partial redundancy of the three T cell activation pathways was evidenced by the phosphorylation of Ser25 and Ser38, substrates of MAP kinases and of the cdk family kinase(s), respectively. Conversely, the phosphorylation of Ser16 of stathmin was observed in response to both CD2 and CD28 triggering, but not CD3 triggering, with a kinetics compatible with the lasting activation of CaM kinase II in response to CD2 triggering. In vitro, Ser16 of recombinant human stathmin was phosphorylated also by purified CaM kinase II, and in vivo, CaM kinase II activity was indeed stimulated in CD2-triggered Jurkat cells. Altogether, our results favor an association of CaM kinase II activity with costimulatory signals of T lymphocyte activation and phosphorylation of stathmin on Ser16.

Phosphorylation of stathmin modulates its function as a microtubule depolymerizing factor

Oncoprotein 18 or stathmin was isolated from bovine brain, characterized and novel features of its function as a microtubule depolymerizing factor were tested. The effect of phosphorylation of stathmin on its function as a microtubule depolymerizing factor has been tested in vitro. Five different protein kinases, protein kinase A, MAP kinase, cdc2 kinase, glycogen synthase kinase 3 and casein kinase 2, were used to modify stathmin, since it is known that these kinases could phosphorylate several residues that are modified in vivo and could have important roles in stathmin function. The residues phosphorylated in vitro by the different protein kinases were identified and in some cases they correspond to those modified in vivo. Recombinant unphosphorylated stathmin and native stathmin, which was previously dephosphorylated with alkaline phosphatase, showed similar microtubule depolymerizing activity. This activity is higher than that of stathmin phosphorylated by protein kinase A, MAP kinase or cdc 2 kinase, whereas phosphorylation of the protein with casein kinase 2 or glycogen synthase kinase 3 resulted in a slight increase of the depolymerizing activity.

Luteinizing hormone-releasing hormone-signal transduction and stathmin phosphorylation in the gonadotrope alphaT3-1 cell line

We have investigated the effects of GnRH (LHRH) and of the protein kinase C (PKC) activator 12-O-tetradecanoylphorbol-13-acetate on stathmin phosphorylation in the gonadotrope alphaT3-1 cell line. Stathmin expression and its phosphorylation were maximal during the exponential phase of cell growth. LHRH stimulated stathmin phosphorylation through a specific receptor in a dose- and time-dependent manner, and TPA induced a similar extensive stathmin phosphorylation. Their effects were inhibited either in PKC-depleted alphaT3-1 cells, or by the PKC inhibitor staurosporine. In the context of the known implication of PKC in LHRH-induced signal transduction, our results show that stathmin phosphorylation is involved in LHRH transduction, either as a result of direct activation of specific PKC isoforms or through a pathway involving kinases downstream to PKC activation.

Characterization of rat brain stathmin isoforms by two-dimensional gel electrophoresis-matrix assisted laser desorption/ionization and electrospray ionization-ion trap mass spectrometry

Stathmin is a regulatory phosphoprotein that is a target for both cell cycle and cell surface receptor-regulated phosphorylation events. There are at least 14 isoforms of stathmin that migrate on two-dimensional electrophoresis (2-DE): two unphosphorylated, and 12 increasingly phosphorylated proteins. Following extracellular stimuli, stathmin is phosphorylated on four serines (Ser16, Ser25, Ser38, and Ser63) by several kinases, such as mitogen-activated protein (MAP), cdc2 kinase, protein kinase A, and Ca2+/calmodulin-dependent kinase-Gr. While all forms of stathmin are derived from the same protein encoded by a single mRNA, the precise nature of the post-translational modifications has not been clear. In this study we have characterized three rat brain stathmin isoforms, #1, #3 and #4, which electrophorese on 2-DE with apparent molecular weight (Mr)/isoelectric point (pI) values of 15,500/6.2, 15,000/6.1, and 15,000/6.0, respectively. The phosphorylation status of these isoforms was determined using a combination of peptide mapping, matrix-assisted laser desorption/ionization mass spectrometry and electrospray-ionization ion trap mass spectrometry. Stathmin isoform #1 was not phosphorylated, stathmin isoform #3 was phosphorylated on Ser38 only, and stathmin isoform #4 was phosphorylated on Ser38; however, the phosphorylation status of Ser63 could not be determined. In addition, three proteins which electrophorese near stathmin were identified in order to more accurately define the Mr/pI locus of this region of the 2-DE gel map. These include: phosphatidyl ethanolamine binding protein (Mr approximately 18,000/pI 6.0), synuclein forms 2 and 3 (Mr approximately 14,000/pI 5.4), and synuclein form 2 (Mr approximately 15,000/pI 5.0).

Stathmin overexpression in 293 cells affects signal transduction and cell growth

Stathmin is a ubiquitous cytoplasmic protein whose phosphorylation state changes markedly in response to extracellular signals, and during the cell cycle. To clarify the function of stathmin, its four phosphorylation sites were mutated to either alanines (4A-stathmin) or glutamates (4E-stathmin). In transfected cells, 4A-stathmin caused a strong G2/M block and also inhibited the responsiveness of a co-transfected fos promoter/ luciferase reporter plasmid to serum stimulation, whereas wild type and 4E-stathmin had relatively minor effects. These results support the idea that stathmin plays a role in multiple cellular processes and indicate that the regulation of the phosphorylation state of stathmin is likely to determine its action.

Upregulation of stathmin (p19) gene expression in adult rat brain during injury-induced synapse formation

Stathmin (p19) is developmentally regulated as a neural-enriched phosphoprotein associated with neurite outgrowth and synaptic formation during cell proliferation and differentiation, and remains highly abundant in adult rat brain. Whether stathmin is involved in injury-induced reactive synaptogenesis in adult rat was examined in this study. Following unilateral cortical lesion, a significant increase in stathmin mRNA expression was found in the cells of contralateral homotypic cortex and in the subventricular zone of the lateral ventricle. This increase coincided in time with the corticostriatal axon sprouting and synaptic remodeling previously found in denervated striatum. Our data suggest that stathmin plays an important role in regulation of reactive synaptogenesis in adult brain.

Brain-derived neurotrophic factor stimulates phosphorylation of stathmin in cortical neurons

We have identified by two-dimensional polyacrylamide gel electrophoresis a protein known as stathmin which is phosphorylated in a time- and concentration-dependent manner in response to brain-derived neurotrophic factor (BDNF) in primary cultures of cortical neurons. We show that stathmin phosphorylation is preceded by the activation of mitogen-activated protein kinase (MAPK) isoforms p44 and p42. Moreover, the MAPK kinase inhibitor PD 098059, which inhibits MAPK activation, also markedly reduces BDNF-stimulated phosphorylation of stathmin, therefore suggesting that phosphorylation of stathmin is triggered by the activation of MAPK. Phosphorylation of stathmin is specific for BDNF since nerve growth factor does not stimulate MAPK and stathmin phosphorylation in cultured cortical neurons. Taken together, these results identify stathmin as a new target protein of BDNF, possibly involved in the development of cortical neurons.

The stathmin/tubulin interaction in vitro

Stathmin is a highly conserved ubiquitous cytoplasmic protein, phosphorylated in response to extracellular signals and during the cell cycle. Stathmin has recently been shown to destabilize microtubules, but the molecular mechanisms of this function remained unclear. We show here that stathmin directly interacts with tubulin. We assessed the conditions of this interaction and determined some its quantitative parameters using plasmon resonance, gel filtration chromatography, and analytical ultracentrifugation. The stathmin/tubulin interaction leads to the formation of a 7.7 S complex with a 60-A Stokes radius, associating one stathmin with two tubulin heterodimer molecules as determined by direct quantification by Western blotting. This interaction is sensitive to pH and ionic environment. Its equilibrium dissociation constant, determined by plasmon resonance measurement of kinetic constants, has an optimum value of 0.5 microM at pH 6.5. The affinity was lowered with a fully "pseudophosphorylated" 4-Glu mutant form of stathmin, suggesting that it is modulated in vivo by stathmin phosphorylation. Finally, analysis of microtubule dynamics by video microscopy shows that, in our conditions, stathmin reduces the growth rate of microtubules with no effect on the catastrophe frequency. Overall, our results suggest that the stathmin destabilizing activity on microtubules is related to tubulin sequestration by stathmin.

The stathmin family -- molecular and biological characterization of novel mammalian proteins expressed in the nervous system

Stathmin is a ubiquitous phosphoprotein proposed to be a relay integrating various intracellular signaling pathways. Its high phylogenetic conservation and the identification of the related molecules, SCG10 in rat and XB3 in Xenopus, suggested the existence of a stathmin-related family. A systematic PCR-based approach allowed the identification of several novel mammalian sequences of which two coded for expressed members of the stathmin family; the translated RB3 sequence shares 88% amino-acid identity with that of XB3 and is thus its rat homologue, and RB3' corresponds to an alternatively spliced product of the same gene, encoding a truncated form. Within their stathmin-like domain, the alpha helix, probably responsible for coiled-coil protein-protein interactions, is conserved, as well as are two consensus phosphorylation sites; in their N-terminal extension domain, two cystein residues most likely responsible for membrane attachment through palmitoylation, are present in RB3/RB3' as in SCG10. The novel identification and characterization of the corresponding proteins showed that all three are associated with the particulate, membrane-containing fraction. They furthermore display several spots of decreasing pI on two-dimensional immunoblots, suggesting that they are phosphorylated in vivo. As for SCG10, RB3 mRNA is detectable only in the nervous system by in situ hybridization, but at similar levels in the newborn and the adult brain as revealed by Northern blots, whereas SCG10 expression decreases in the adult. Furthermore, RB3 mRNA is undetectable in PC12 cells, whereas SCG10 mRNA increases after treatment with nerve growth factor, inducing neuronal differentiation. In conclusion, we demonstrate here the existence of a highly conserved stathmin-related family in mammals, of which each member seems to play specific roles, related to the control of cell proliferation and activities for stathmin and to that of neuronal differentiation for SCG10, the novel RB3/RB3' proteins being rather related to the expression of differentiated neuronal functions.

KIS is a protein kinase with an RNA recognition motif

Protein phosphorylation is involved at multiple steps of RNA processing and in the regulation of protein expression. We present here the first identification of a serine/threonine kinase that possesses an RNP-type RNA recognition motif: KIS. We originally isolated KIS in a two-hybrid screen through its interaction with stathmin, a small phosphoprotein proposed to play a general role in the relay and integration of diverse intracellular signaling pathways. Determination of the primary sequence of KIS shows that it is formed by the juxtaposition of a kinase core with little homology to known kinases and a C-terminal domain that contains a characteristic RNA recognition motif with an intriguing homology to the C-terminal motif of the splicing factor U2AF. KIS produced in bacteria has an autophosphorylating activity and phosphorylates stathmin on serine residues. It also phosphorylates in vitro other classical substrates such as myelin basic protein and synapsin but not histones that inhibit its autophosphorylating activity. Immunofluorescence and biochemical analyses indicate that KIS overexpressed in HEK293 fibroblastic cells is partly targetted to the nucleus. Altogether, these results suggest the implication of KIS in the control of trafficking and/or splicing of RNAs probably through phosphorylation of associated factors.

Neurofibrillary tangle-associated alteration of stathmin in Alzheimer's disease

Stathmin (p19), a 19-kDa cytosolic phosphorotein, plays a key role in converting extracellular signals into intracellular biochemical changes. Antibodies and cDNA specific for stathmin were used to study its levels and localization in normal and Alzheimer's disease (AD) brain tissue. The stathmin protein concentration was reduced in AD neocortex as assessed by Western blotting, whereas the concentration of its mRNA detected by both in situ hybridization and slot blot were increased in AD. The alteration of the stathmin protein concentration was negatively correlated with neurofibrillary tangle numbers but not with plaque numbers. Immunoreactivity was evenly localized to the cytoplasm of neurons in control cortical sections, whereas in AD it was preferentially localized to some of the neurofibrillary tangle-bearing neurons. Numbers of stathmin-positive neurons were inversely correlated with tangle numbers but not with plaque numbers in the frontal cortex of AD patients.

Down-regulation but not phosphorylation of stathmin is associated with induction of HL60 cell growth arrest and differentiation by physiological agents

Stathmin is a cytosolic phosphoprotein that has an important but, as yet, undefined role in cell proliferation and differentiation. Induction of growth arrest and differentiation of HL60 cells to monocytes by phorbol 12-myristate 13-acetate is associated with rapid phosphorylation of the protein. Stathmin phosphorylation was not seen when HL60 cells were induced to differentiate to monocytes, by 1 alpha, 25-dihydroxyvitamin D3, and to neutrophils, by all-trans retinoic acid and granulocyte colony stimulating factor. In all the above instances, stathmin expression was down-regulated. Thus, increased stathmin phosphorylation is not required for cell growth arrest or differentiation or down-regulation of stathmin expression.

Stathmin interaction with a putative kinase and coiled-coil-forming protein domains

Stathmin is a ubiquitous, cytosolic 19-kDa protein, which is phosphorylated on up to four sites in response to many regulatory signals within cells. Its molecular characterization indicates a functional organization including an N-terminal regulatory domain that bears the phosphorylation sites, linked to a putative alpha-helical binding domain predicted to participate in coiled-coil, protein-protein interactions. We therefore proposed that stathmin may play the role of a relay integrating diverse intracellular regulatory pathways; its action on various target proteins would be a function of its combined phosphorylation state. To search for such target proteins, we used the two-hybrid screen in yeast, with stathmin as a "bait." We isolated and characterized four cDNAs encoding protein domains that interact with stathmin in vivo. One of the corresponding proteins was identified as BiP, a member of the hsp70 heat-shock protein family. Another is a previously unidentified, putative serine/threonine kinase, KIS, which might be regulated by stathmin or, more likely, be part of the kinases controlling its phosphorylation state. Finally, two clones code for subdomains of two proteins, CC1 and CC2, predicted to form alpha-helices participating in coiled-coil interacting structures. Their isolation by interaction screening further supports our model for the regulatory function of stathmin through coiled-coil interactions with diverse downstream targets via its presumed alpha-helical binding domain. The molecular and biological characterization of KIS, CC1, and CC2 proteins will give further insights into the molecular functions and mechanisms of action of stathmin as a relay of integrated intracellular regulatory pathways.

Stathmin is a major substrate for mitogen-activated protein kinase during heat shock and chemical stress in HeLa cells

Stathmin is a ubiquitous, highly conserved 19-kDa cytoplasmic protein whose expression and phosphorylation are regulated in relation to cell proliferation, differentiation or activation, in many biological systems. In this report, we show that stathmin undergoes major phosphorylation in HeLa cells submitted to heat or chemical stress. Heat-shock-induced stathmin phosphorylation was very rapid, as maximal incorporation of phosphate was observed at 5 min. Phosphorylation of stathmin might, therefore, occur as a very early step in the intracellular response to heat shock. The sites of phosphorylation of stathmin involved during the stress response were identified as mostly Ser25 and, to a lesser extent, Ser38. These sites are both followed by a proline residue, and known to be good substrates in vitro for mitogen-activated protein kinase (MAP-kinase) and p34cdc2 kinase, respectively. In lysates from heat-shocked cells, an increased stathmin-kinase activity, distinct from the histone-H1-kinase activity, was found to phosphorylate stathmin mostly on Ser25, the main site for MAP-kinase in vitro. This stathmin-kinase coeluted quantitatively with the stress-activated MAP-kinase from an FPLC MonoQ column. Furthermore, a stathmin kinase activity was precipitated from lysates of heat-shocked HeLa cells by an anti-(MAP-kinase) serum. Together, these results indicate that the phosphorylation of stathmin by MAP-kinase is likely to be a significant component of the signalling array controlling the cellular response to stress, and they further underline the general involvement of stathmin in intracellular signalling.

SCG10 mRNA localization in the hippocampus: comparison with other mRNAs encoding neuronal growth-associated proteins (nGAPs)

SCG10 is a nerve growth factor (NGF)-inducible, neuron-specific protein whose expression is tightly correlated with axonal and/or dendritic growth. We have recently shown that the mRNA encoding SCG10 is expressed at significant levels in certain subsets of neurons in the adult rat brain, while its expression is undetectable or negligible in other non-neuronal tissues. Here we show that regional SCG10 mRNA expression in the adult mouse brain is comparable to that in the rat, however, in the hippocampus its expression profile is distinct. In the mouse, SCG10 mRNA is expressed at high levels in pyramidal cells of CA3-CA4 sub-fields of Ammon's horn and at low levels in the CA1-CA2 sub-fields, while it is found rather uniformly throughout the pyramidal cell layer of the rat hippocampus. SCG10 mRNA is not detectable in the dentate gyrus of the mouse hippocampus, although it is expressed in the rat dentate gyrus. Comparison with other mRNAs encoding neuronal growth-associated proteins (nGAPs) such as GAP-43, MAP2, alpha 1-tubulin and stathmin suggests that dentate granule cells express a different repertoire of neuronal growth-associated genes in mouse and rat.

Differential localization of SCG10 and p19/stathmin messenger RNAs in adult rat brain indicates distinct roles for these growth-associated proteins

SCG10 is a developmentally regulated, growth-associated protein (GAP) that was isolated as a neuronal marker of the neural crest. It was recently found that SCG10 shares an amino acid sequence similarity with a phosphoprotein named stathmin or p19 of which phosphorylation is induced by nerve growth factor and vasoactive intestinal peptide in PC12 cells and striatal neurons, respectively. While expression of SCG10 messenger RNA dramatically decreases during postnatal development, significant levels of expression still persist into adulthood. To examine possible roles of SCG10 in the adult brain, we examined the distribution of messenger RNAs encoding SCG10 and p19/stathmin as well as GAP-43 in adult rat brain sections by northern blot, RNase protection and in situ hybridization. SCG10 transcripts are found at high levels in long-distance projecting neurons and neurons with extensive dendritic arbors, while p19/stathmin messenger RNA was weakly distributed over most brain areas. Both messenger RNAs are expressed in neuronal subpopulations but not in glia, although the overall distribution of the transcripts of these two structurally related genes is distinct. The spatial and temporal expression profiles of SCG10 messenger RNA is comparable to that of GAP-43, another neuronal GAP, in the developing nervous system, however the expression of SCG10 messenger RNA in the adult brain is distinct from that of GAP-43, especially in the hippocampus and brain stem, where the dentate granule cells and sensory and motor neurons of brainstem express SCG10 but not GAP-43. These results suggest that SCG10 may have a unique role in the neuronal growth-response of subsets of mature neurons, and that SCG10 plays a stathmin-like function at nerve terminals, to which it may be rapidly transported by means of membrane attachment due to a hydrophobic domain present in SCG10 but not in p19/stathmin. This suggests that SCG10 may play a role in structural plasticity in the adult brain.

Molecular characterization of human stathmin expressed in Escherichia coli: site-directed mutagenesis of two phosphorylatable serines (Ser-25 and Ser-63)

Stathmin, a probable relay protein possibly integrating multiple intracellular regulatory signals [reviewed in Sobel (1991) Trends Biochem. Sci. 16, 301-305], was expressed in Escherichia coli at levels as high as 20% of total bacterial protein. Characterization of the purified recombinant protein revealed that it had biochemical properties very similar to those of the native protein. It is a good substrate for both cyclic AMP-dependent protein kinase (PKA) and p34cdc2, on the same four sites as the native eukaryotic protein. As shown by m.s., the difference in isoelectric points from the native protein is probably due to the absence of acetylation of the protein produced in bacteria. C.d. studies indicate that stathmin probably contains about 45% of its sequence in an alpha-helical conformation, as also predicted for the sequence between residues 47 and 124 by computer analysis. Replacement of Ser-63 by alanine by in vitro mutagenesis resulted in a ten times less efficient phosphorylation of stathmin by PKA which occurred solely on Ser-16, confirming that Ser-63 is the major target of this kinase. Replacement of Ser-25, the major site phosphorylated by mitogen-activated protein kinase in vitro and in vivo, by the charged amino acid glutamic acid reproduced, in conjunction with the phosphorylation of Ser-16 by PKA, the mobility shift on SDS/polyacrylamide gels induced by the phosphorylation of Ser-25. This result strongly suggests that glutamic acid in position 25 is able to mimic the putative interactions of phosphoserine-25 with phosphoserine-16, as well as the resulting conformational changes that are probably also related to the functional regulation of stathmin.

Stathmin: cellular localization of a major phosphoprotein in the adult rat and human CNS

Stathmin is a ubiquitous, 19 kDa cytoplasmic protein the phosphorylation of which is associated with many cellular signaling pathways. It is particularly abundant in neurons and reaches a peak of expression in the neonatal period, although it remains highly expressed in the adult brain. In order to determine whether this abundant expression is associated with discrete cellular populations that are still at an immature stage during adulthood, as suggested by others, the cellular localization of stathmin was investigated in the adult rat and human central nervous system. Western blotting with a specific antiserum indicated that stathmin was ubiquitous in the brain and spinal cord but that its relative concentration varied up to 2.6 times between regions. To characterize the distribution of stathmin within the brain, its cellular localization was analyzed by immunocytochemistry. Highly immunoreactive neurons and oligodendrocytes were observed, and stathmin immunoreactivity was localized to the perikaryon and all processes, but not the nucleus. Most brain and spinal cord cell groups showed stathmin immunoreactivity, although the extent and intensity of labeling differed largely from one place to another. Particularly numerous stathmin-immunoreactive neuronal cell bodies were found in the pyriform, cingulate, and neocortex, as well as in many cholinergic nuclei of the basal forebrain and brainstem, in the medial thalamus, in various brainstem nuclei, in the dorsalmost layers of the spinal cord, and in brain areas lacking a blood-brain barrier to macromolecules. In addition to neuronal populations, stathmin-antibodies intensely labeled choroid plexuses. Many other brain regions exhibited moderate neuronal immunostaining. The distribution of stathmin-immunoreactive processes was in some areas relatively heterogeneous. Intense immunoreactivity was observed in some fiber tracts (corpus callosum, anterior commissure, inferior cerebellar peduncle, etc.) but was missing in others (internal capsule, posterior commissure, etc.). Some brain areas rich in immunoreactive neurons also displayed an intense immunoreactivity of the neuropile, whereas others contained either immunoreactive cells or fibers. In the human brain, stathmin immunostaining occurred in many areas, corresponding to those identified in the rat, with the exception of the cerebral cortex, the hippocampal fascia dentata, and the substantia nigra. The present results support our suggestion that, in addition to its involvement in cell proliferation and differentiation, stathmin may also be related to regulation of differentiated cell functions, as it appears to be a major signaling protein in widespread areas of the adult brain in both rat and human.

The phosphorylation of stathmin by MAP kinase

Stathmin, a ubiquitous cytosolic phosphoprotein which may play a role in integrating the effects of diverse signals regulating proliferation, differentiation and other cell functions, was found to be phosphorylated rapidly and stoichiometrically by mitogen-activated protein (MAP) kinase in vitro. Ser-25 was identified as the major site and Ser-38 as a minor site of phosphorylation, while the p42 and p44 isoforms of MAP kinase were the only significant stathmin kinases detected in PC12 cells after stimulation by nerve growth factor (NGF). The results suggest that MAP kinases are the enzymes responsible for increasing the level of phosphorylation of Ser-25, which has been observed previously in PC12 cells following stimulation by NGF.

Vasoactive intestinal peptide: role of calmodulin and catalytic antibodies

In the accompanying article Paul and Ebadi [1993 (Neurochem Int. 23, 197-214)] review recent observations that the neuropeptide vasoactive intestinal peptide (VIP) and the related peptide growth hormone releasing factor (GRF) bind to two novel protein molecules unrelated to their G-protein coupled receptors. VIP and GRF have been reported to bind with high affinity to calmodulin, leading to modulation of biological activities of the regulatory protein. VIP has also been reported to bind to autoantibodies isolated from both normal and asthmatic subjects. Several of these autoantibodies have been found to catalyze the hydrolysis of the native VIP molecule. The biological consequences of the binding of VIP and GRF to these macromolecular "receptors" are considered.

Expression of transfected stathmin cDNA reveals novel phosphorylated forms associated with developmental and functional cell regulation

Stathmin is a ubiquitous, highly conserved phosphoprotein, which most likely acts as an intracellular relay integrating various transduction pathways triggered by extracellular signals. Two post-translational isoforms (alpha and beta) have been previously identified whose increasingly phosphorylated forms migrate as a set of isoelectric variant spots (molecular mass 19 kDa; pI 6.2-5.6) on two-dimensional electrophoretic gels. In parallel with the phosphorylation of these forms of stathmin, two sets of three proteins migrating with slightly higher apparent molecular masses (21 and 23 kDa respectively) also incorporated radioactive phosphate in response to cell regulation through various transduction pathways. These phosphoproteins, previously referred to as proteins '16' and '17', share several biochemical properties with stathmin and are recognized by antibodies directed to stathmin or to stathmin peptides. Furthermore, when rat stathmin cDNA was transfected into mouse myogenic C2 cells, it directed the expression of protein sets 16 and 17 together with the 19 kDa forms of stathmin, as detected with a species-specific anti-stathmin antiserum. Proteins 16 and 17 are thus novel phosphorylated derivatives of stathmin, encoded by the same cDNA as its previously identified 19 kDa forms. These results increase the known complexity and diversity of stathmin patterns, which may yield the molecular support for its proposed role as a relay integrating various signals which regulate the proliferation, differentiation and functions of cells during development and adult life.