Protein phosphorylation in response to the tumor promoter TPA is dependent on the state of differentiation of muscle cells

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.

Cell and molecular biology of myoblast fusion

In organisms from Drosophila to mammals, the musculature is comprised of an elaborate array of distinct fibers that are generated by the fusion of committed myoblasts. These muscle fibers differ from each other in features that include location, pattern of innervation, site of attachment, and size. The sizes of the newly formed muscles of an embryo are controlled in large part by the number of cells that form the syncitial fiber. Over the past few decades, an extensive body of literature has described the process of myoblast fusion in vertebrates, relying primarily on the strengths of tissue culture model systems. More recently, genetic studies in Drosophila embryos have provided new insights into the process. Together, these studies define the steps necessary for myoblast differentiation, the acquisition of fusion competence, the recognition and adhesion between myoblasts, and the fusion of two lipid bilayers into one. In this review, we have attempted to combine insights from both Drosophila and vertebrate studies to trace the processes and molecules involved in myoblast fusion. Implicit in this approach is the assumption that fundamental aspects of myoblast fusion will be similar, independent of the organism in which it is occurring.

Involvement of protein phosphatase-1-mediated MARCKS translocation in myogenic differentiation of embryonic muscle cells

Myristoylated alanine-rich C kinase substrate (MARCKS) translocates from the cytosol to the plasma membrane while mononucleated myoblasts fuse to form multinucleated myotubes. Here, we show that protein phosphatase-1-mediated dephosphorylation of MARCKS largely influences its subcellular localization and the fusion process. Treatment with okadaic acid or tautomycin, which are potent inhibitors of protein phosphatases and cell fusion, was found to reversibly block the MARCKS translocation. Moreover, the dephosphorylating activity against MARCKS markedly increased during myogenesis, and this increase was closely correlated with the membrane fusion of the cells. In addition, protein phosphatase-1 was identified as a major enzyme that is responsible for dephosphorylation of MARCKS. Furthermore, a mutation preventing MARCKS phosphorylation and thus facilitating MARCKS translocation resulted in promotion of the cell fusion. In contrast, overexpression of MARCKS carrying a mutation that blocks myristoylation and thus prevents the MARCKS translocation impaired the myoblast fusion. Together with the fact that MARCKS regulates the cytoskeleton dynamics by crosslinking the actin filaments in the plasma membrane and that myoblast fusion accompanies massive cytoskeleton reorganization, these results suggest that protein phosphatase-1-mediated MARCKS localization at the membrane is required for the fusion of embryonic muscle cells.

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.

Production of an endogenous inhibitor of protein kinase C by embryonic myoblasts undergoing differentiation

The current studies were undertaken to determine whether embryonic myoblasts or myogenic satellite cells undergoing differentiation and fusion contained endogenous modulators of protein kinase C (PKC). Clonal-derived turkey embryonic myoblast and satellite cell cultures were harvested at confluency and at approximately 40% fusion (embryonic myoblasts) or 75% fusion (satellite cells). PKC activity in cystosolic preparations of the cells and myotubes was undetectable. Cytosolic extracts (0.065 mg protein) of confluent and fused satellite cell cultures and confluent embryonic myoblasts had no effect on control PKC activity (control: 14.9 pmol/min, control + cytosols: 15.2, 13.9 and 13.5 pmol/min, respectively). Cytosolic preparations (0.065 mg protein) of embryonic myoblast-derived myotubes inhibited control PKC activity (4.0 pmol/min). In a time-course study, PKC-inhibitory activity was present in embryonic myoblasts at the earliest time point examined (30% fusion). Additionally, protein phosphatase activity correlated with PKC inhibitory activity. Thus, PKC-inhibitory activity appears as embryonic myoblasts begin to undergo fusion to form myotubes, but is not present in differentiating satellite cells.

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.

Induction of stathmin expression during liver regeneration

Stathmin is a 19 kDa cytoplasmic phosphoprotein proposed to act as a relay for signals activating diverse intracellular regulatory pathways. After two-thirds partial hepatectomy, the concentration of stathmin reached a peak between 48 and 72 hours, comparable to the levels observed in neonatal liver, at about 10 times the basal adult level. Stathmin then decreased to basal levels within 7 days, more rapidly than during postnatal tissue development (7 weeks), with no detectable change in its phosphorylation state. Interestingly, the mRNA for stathmin reached a peak much earlier than the protein, at 24 hours posthepatectomy, and decreased to a still detectable level until 96 hours after hepatectomy. Altogether, the present results further support the generatility of the implication of stathmin in regulatory pathways of cell proliferation and differentation during normal tissue development and posttraumatic regeneration.

High expression of stathmin in multipotential teratocarcinoma and normal embryonic cells versus their early differentiated derivatives

Stathmin is a ubiquitous cytoplasmic protein, phosphorylated in response to agents regulating the proliferation, the differentiation and the specialized functions of cells, in a way possibly integrating the actions of diverse concomitant regulatory signals. Its expression is also regulated in relation with cell proliferation and differentiation and reaches a peak at the neonatal stage. To assess the possible role of stathmin at earlier stages of development, we examined its expression and regulation in embryonal carcinoma (EC) and derived cell lines as well as in the early mouse embryo. Interestingly, stathmin is highly abundant in the undifferentiated, multipotential cells of the F9, 1003 and 1009 EC cell lines. Its high expression markedly decreased, both at the protein and mRNA levels, when F9 cells were induced to differentiate into endodermal-like cells with retinoic acid and dibutyryl-cAMP. Stathmin was also much less abundant in differentiated cell lines such as the trophectodermal line TDM-1, as well as in several F9- and 1003-derived cell lines committed to differentiate towards the mesodermal and neuroectodermal lineages but still proliferating. Therefore, the observed decrease of stathmin expression is not related to the reduced proliferation rate but rather to the differentiation of the multipotential EC cells. The immunocytochemical pattern of stathmin expression during early mouse development indicated that stathmin is also highly abundant in the multipotential cells of the inner cell mass of the blastula, whereas it is much lower in the differentiated trophectodermal cells. These results confirm the physiological relevance of the observations with EC cells, and suggest that stathmin, in addition to its high expression at later stages of development and in the adult nervous system, may be considered as a new marker of the multipotential cells of the early mouse embryo.

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

Stathmin is a ubiquitous soluble protein whose phosphorylation is associated with the intracellular mechanisms involved in the regulations of cell proliferation, differentiation, and functions by extracellular effectors. It is present in the various tissues and cell types as at least two distinct isoforms in their unphosphorylated (Mr approximately 19,000; pI approximately 6.2-6.0) and increasingly phosphorylated forms. Stathmin is particularly abundant in brain, mostly because of its high concentration in neurons, where the protein is a major phosphorylation substrate. In intact striatal neurons grown in primary culture, the cyclic AMP-increasing drug forskolin and the protein kinase C-activating agent 12-O-tetradecanoylphorbol 13-acetate (TPA) induced a potent phosphorylation of stathmin. Their actions were at least partially additive, appearing actually most likely "sequential" on various phosphorylated states of stathmin. Vasoactive intestinal peptide (VIP) reproduced the forskolin-like stimulation but stimulated also other, TPA, and/or Ca2(+)-like protein phosphorylations. These actions of VIP were already maximal after 5 min and were long lasting, still important after 2 h. In addition, concentrations as low as 1 nM were enough to obtain a significant effect, on both cyclic AMP-dependent and independent phosphorylations. Dopamine and the beta-adrenergic agonist isoproterenol were also able to stimulate stathmin phosphorylation, but only with a forskolin-like pattern. Their actions were not additive to those of VIP, confirming previous results on the colocalization of both dopamine D1 and noradrenaline beta 1 receptors with VIP receptors on striatal neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Stathmin: a relay phosphoprotein for multiple signal transduction?

Stathmin is a ubiquitous, phylogenetically conserved protein present in the cytoplasm of cells in a variety of unphosphorylated and phosphorylated forms. Its expression and phosphorylation are regulated throughout development and in response to extracellular signals regulating cell proliferation, differentiation and functions. The overall pattern of its molecular forms reflects the activation of corresponding second messenger pathways. This phosphoprotein is therefore a good candidate as a general relay in signal transduction, possibly integrating diverse signals of the cell's environment.

Okadaic acid blocks membrane fusion of chick embryonic myoblasts in culture

Okadaic acid was found to block membrane fusion of chick embryonic myoblasts in culture. It also induced morphological change of the cells from bipolar to spherical shape. These effects were dose-dependent, and could be reversed upon removal of the drug from the culture medium. It showed, however, no effect on the induction of muscle specific proteins including tropomyosin and creatine kinase. When okadaic acid was treated to the cell lysates, the phosphorylation state of many proteins significantly increased. These results suggest that the inhibition of myoblast fusion by okadaic acid may be mediated by the increase in the phosphorylation of certain, unknown protein(s) that regulate the fusion process.

Altered protein phosphorylation in murine muscular dystrophy

Protein phosphorylation has been studied in the dydy murine muscular dystrophy, both in intact muscle cells and in various membrane fractions derived from them. The results obtained showed that several polypeptides were more heavily phosphorylated in dystrophic myotubes in culture as well as in dystrophic muscle fibers isolated from tibialis anterior. In vitro phosphorylation studies revealed that a large polypeptide of apparent molecular weight of 170,000-150,000 was phosphorylated under basal conditions (3 mM EGTA) in dydy microsomal membranes. The phosphorylation of this polypeptide was not stimulated further by cAMP, calmodulin, cGMP or 12-O-tetradecanoylphorbol 13-acetate (TPA). Under no condition was the corresponding polypeptide phosphorylated at an appreciable rate in normal microsomal membranes. An antibody raised against the voltage-dependent calcium channel reacted, in an immunoblot assay, with a polypeptide, present in both normal and dydy microsomes, which had migration characteristics identical to the phosphorylated 170-150 kDa polypeptide after one- or two-dimensional gel electrophoresis. Additional differences were identified in the phosphorylation of smaller polypeptides of microsomal membranes. When sarcolemmal membranes of normal and dydy muscle were phosphorylated in vitro, no major differences were observed. These results show the existence of an alteration of protein phosphorylation in dystrophic muscle cells in vitro and in vivo, leading to abnormal phosphorylation of the voltage-dependent calcium channel. The possible causes and consequences of this alteration are discussed.

Stathmin is a major phosphoprotein and cyclic AMP-dependent protein kinase substrate in mouse brain neurons but not in astrocytes in culture: regulation during ontogenesis

Stathmin is a ubiquitous soluble protein (Mr approximately 19,000; pI approximately 6.2-5.5) whose phosphorylation is associated with the intracellular mechanisms involved in the regulations of cell differentiation and functions by extracellular effectors. It is present in various tissues and cell types and has several nonphosphorylated and increasingly phosphorylated forms, and it is particularly abundant in brain. Very high concentrations of stathmin were also detected in mouse embryo striatal neurons grown in primary culture, whereas stathmin was barely detectable in astrocytes from the same source. Stathmin appeared in neurons as a major substrate for protein phosphorylation and, in particular, for the cyclic AMP (cAMP)-dependent protein kinase, because its phosphorylation was stimulated by cAMP in cell-free preparations and in intact cells by forskolin, a potent activator of adenylate cyclase. During brain ontogenesis, stathmin was first detected at embryonic day 12; its concentration increased until birth and then decreased from postnatal day 10 to adulthood. In parallel, its molecular forms shifted from the least phosphorylated to the more phosphorylated ones. This result may reflect the evolution of the activity of stathmin during development and the subsequent maturation of the brain. In conclusion, our results substantiate the likely role of stathmin as an intracellular relay of extracellular regulations, as they point out its specific importance related to neuronal functions and brain differentiation.

Protein kinase C activity and phorbol ester binding to rat myogenic cells during growth and differentiation

Phorbol esters have been reported to induce opposite responses in fetal myoblasts and in satellite cells isolated from adult skeletal muscles. We examined the possibility that different levels of protein kinase C (PKC) activity and different phorbol ester binding characteristics account for these responses. For this purpose, the subcellular distributions of PKC were compared in primary cultures of myogenic cells from fetal and adult rat muscles and in the L6 cell line. Cells were used at the proliferative stage or after differentiation into myotubes. Binding of phorbol dibutyrate (PDBu) was assayed. In all three cell types, the levels of PKC specific activity were comparable at the proliferating and the differentiated stages, and partial translocation of PKC activity from the membrane to the cytosolic compartment was observed after differentiation into myotubes. PDBu binding, which had a Kd of 6 to 13 nM in proliferative cells, rose to between 30 and 52 nM in myotubes. Simultaneously, a small increase was observed in the total number of PDBu binding sites. These results suggest that the role of PKC might change with the stage of differentiation. They also imply that the difference described by others between the sensitivity to phorbol esters of fetal myoblasts and satellite cells is not connected with the phorbol ester receptor (i.e., PKC), but might be caused by events subsequent to PKC activation.