Cyclin B interaction with microtubule-associated protein 4 (MAP4) targets p34cdc2 kinase to microtubules and is a potential regulator of M-phase microtubule dynamics

n-Butanol induces depolymerization of microtubules in vivo and in vitro

The effects of butanol on microtubules (MTs) were examined by immunofluorescence microscopy. Fragmentation of cortical MTs was induced by n-butanol, but not by s- and t-butanols, in cultured tobacco BY-2 cells. Taxol prevented n-butanol-induced MT fragmentation. Fragmented cortical MTs were still attached to the inner face of the plasma membrane when n-butanol-treated protoplasts were ruptured on the slide glass. Moreover, MTs were depolymerized in the presence of n-butanol in vitro. Therefore, n-butanol is not only an activator of phospholipase D but also an effective MT-depolymerizing agent.

Doublecortin-like kinase controls neurogenesis by regulating mitotic spindles and M phase progression

The mechanisms controlling neurogenesis during brain development remain relatively unknown. Through a differential protein screen with developmental versus mature neural tissues, we identified a group of developmentally enriched microtubule-associated proteins (MAPs) including doublecortin-like kinase (DCLK), a protein that shares high homology with doublecortin (DCX). DCLK, but not DCX, is highly expressed in regions of active neurogenesis in the neocortex and cerebellum. Through a dynein-dependent mechanism, DCLK regulates the formation of bipolar mitotic spindles and the proper transition from prometaphase to metaphase during mitosis. In cultured cortical neural progenitors, DCLK RNAi Lentivirus disrupts the structure of mitotic spindles and the progression of M phase, causing an increase of cell-cycle exit index and an ectopic commitment to a neuronal fate. Furthermore, both DCLK gain and loss of function in vivo specifically promote a neuronal identity in neural progenitors. These data provide evidence that DCLK controls mitotic division by regulating spindle formation and also determines the fate of neural progenitors during cortical neurogenesis.

Microtubule regulation in mitosis: tubulin phosphorylation by the cyclin-dependent kinase Cdk1

The activation of the cyclin-dependent kinase Cdk1 at the transition from interphase to mitosis induces important changes in microtubule dynamics. Cdk1 phosphorylates a number of microtubule- or tubulin-binding proteins but, hitherto, tubulin itself has not been detected as a Cdk1 substrate. Here we show that Cdk1 phosphorylates beta-tubulin both in vitro and in vivo. Phosphorylation occurs on Ser172 of beta-tubulin, a site that is well conserved in evolution. Using a phosphopeptide antibody, we find that a fraction of the cell tubulin is phosphorylated during mitosis, and this tubulin phosphorylation is inhibited by the Cdk1 inhibitor roscovitine. In mitotic cells, phosphorylated tubulin is excluded from microtubules, being present in the soluble tubulin fraction. Consistent with this distribution in cells, the incorporation of Cdk1-phosphorylated tubulin into growing microtubules is impaired in vitro. Additionally, EGFP-beta3-tubulin(S172D/E) mutants that mimic phosphorylated tubulin are unable to incorporate into microtubules when expressed in cells. Modeling shows that the presence of a phosphoserine at position 172 may impair both GTP binding to beta-tubulin and interactions between tubulin dimers. These data indicate that phosphorylation of tubulin by Cdk1 could be involved in the regulation of microtubule dynamics during mitosis.

MAP kinases and cell migration

Recent studies have demonstrated that mitogen-activated protein kinases (MAPKs), including Jun N-terminus kinase (JNK), p38 and Erk, play crucial roles in cell migration. JNK, for example, regulates cell migration by phosphorylating paxillin, DCX, Jun and microtubule-associated proteins. Studies of p38 show that this MAPK modulates migration by phosphorylating MAPK-activated protein kinase 2/3 (MAPKAP 2/3), which appears to be important for directionality of migration. Erk governs cell movement by phosphorylating myosin light chain kinase (MLCK), calpain or FAK. Thus, the different kinases in the MAPK family all seem able to regulate cell migration but by distinct mechanisms.

Mammalian septins regulate microtubule stability through interaction with the microtubule-binding protein MAP4

Mammalian septins constitute a family of at least 12 GTP-binding proteins that can form hetero-oligomers and that are sometimes found in association with actin or microtubule filaments. However, their functions are not understood. Using RNA interference, we found that suppression of septin expression in HeLa cells caused a pronounced increase in microtubule stability. Mass spectroscopic analysis of proteins coprecipitating with Sept6 identified the microtubule-associated protein MAP4 as a septin binding partner. A small, proline-rich region in the C-terminal half of MAP4 bound directly to a Sept 2:6:7 heterotrimer, and to the Sept2 monomer. The trimer blocked the ability of this MAP4 fragment to bind and bundle microtubules in vitro. In intact cells, MAP4 was required for the stabilization of microtubules induced by septin depletion. Moreover, septin depletion increased the number of cells with abnormal nuclei, and this effect was blocked by gene silencing of MAP4. These data identify a novel molecular function for septins in mammalian cells: the modulation of microtubule dynamics through interaction with MAP4.

Truncation of the projection domain of MAP4 (microtubule-associated protein 4) leads to attenuation of microtubule dynamic instability

MAP4, a ubiquitous heat-stable MAP, is composed of an asymmetric structure common to the heat-stable MAPs, consisting of an N-terminal projection (PJ) domain and a C-terminal microtubule (MT)-binding (MTB) domain. Although the MTB domain has been intensively studied, the role of the PJ domain, which protrudes from MT-wall and does not bind to MTs, remains unclear. We investigated the roles of the PJ domain on the dynamic instability of MTs by dark-field microscopy using various PJ domain deletion constructs of human MAP4 (PJ1, PJ2, Na-MTB and KDM-MTB). There was no obvious difference in the dynamic instability between the wtMAP4 and any fragments at 0.1 microM, the minimum concentration required to stabilize MTs. The individual MTs stochastically altered between polymerization and depolymerization phases with similar profiles of length change as had been observed in the presence of MAP2 or tau. We also examined the effects at the increased concentrations of 0.7 microM, and found that in some cases the dynamic instability was almost entirely attenuated. The length of both the polymerization and depolymerization phases decreased and "pause-phases" were occasionally observed, especially in the case of PJ1, PJ2 or Na-MTB. No obvious change was observed in the increased concentration of wtMAP4 and KDM-MTB. Additionally, the profiles of MT length change were quite different in 0.7 microM PJ2. Relatively rapid and long depolymerization phases were sometimes observed among quite slow length changes. Perhaps, this unusual profile could be due to the uneven distribution of PJ2 along the MT lattice. These results indicate that the PJ domain of MAP4 participates in the regulation of the dynamic instability.

CD2AP expression in a renal ischemia/reperfusion injury model and analysis of its related genes using overexpression and RNA interference technique

BACKGROUND

CD2-associated protein (CD2AP) is a ubiquitously expressed 80-kDa intracellular protein, and has been speculated to act as an intracellular signaling pathway between plasma membrane proteins and cytoskeleton proteins. CD2AP expression has been reported in both the glomerulus and tubular epithelium in the kidney, and CD2AP knockout mice exhibit congenital nephrotic syndrome. However, the precise properties and its role in the renal tubules have not been clarified.

METHODS

We used an established rat model of ischemic/reperfusion renal injury (IRI) to examine the expression of CD2AP by real-time PCR, Western blotting, and immunohistochemistry. We also investigated the expression of genes related to apoptosis and cell proliferation in mouse collecting duct-derived cells (M1 cells) transfected with full-length of CD2AP cDNA or short interfering RNA.

RESULTS

CD2AP mRNA and protein expression had significantly increased in the IRI kidney. Real-time PCR indicated that expression of genes regulating apoptosis, such as B-Raf and Caspase-12, and genes regulating cell proliferation factors, CDC2, was decreased in CD2AP-overexpressing M1 cells and increased in CD2AP-interfered M1 cells.

CONCLUSIONS

These results suggest that CD2AP expression was increased following renal ischemia and that CD2AP may be related to the process of cell repair and/or cell differentiation following injury.

Phosphorylation controls CLIMP-63-mediated anchoring of the endoplasmic reticulum to microtubules

The microtubule-binding 63-kDa cytoskeleton-linking membrane protein (CLIMP-63) is an integral membrane protein that links the endoplasmic reticulum (ER) to microtubules. Here, we tested whether this interaction is regulated by phosphorylation. Metabolic labeling with (32)P showed that CLIMP-63 is a phosphoprotein with increased phosphorylation during mitosis. CLIMP-63 of mitotic cells is unable to bind to microtubules in vitro. Mitotic phosphorylation can be prevented by mutation of serines 3, 17, and 19 in the cytoplasmic domain of CLIMP-63. When these residues are mutated to glutamic acid, and hence mimic mitotic phosphorylation, CLIMP-63 does no longer bind to microtubules in vitro. Overexpression of the phospho-mimicking mitotic form of CLIMP-63 in interphase cells leads to a collapse of the ER around the nucleus, leaving the microtubular network intact. The results suggest that CLIMP-63-mediated stable anchoring of the ER to microtubules is required to maintain the spatial distribution of the ER during interphase and that this interaction is abolished by phosphorylation of CLIMP-63 during mitosis.

Microtubule-associated proteins and their essential roles during mitosis

Microtubules play essential roles during mitosis, including chromosome capture, congression, and segregation. In addition, microtubules are also required for successful cytokinesis. At the heart of these processes is the ability of microtubules to do work, a property that derives from their intrinsic dynamic behavior. However, if microtubule dynamics were not properly regulated, it is certain that microtubules alone could not accomplish any of these tasks. In vivo, the regulation of microtubule dynamics is the responsibility of microtubule-associated proteins. Among these, we can distinguish several classes according to their function: (1) promotion and stabilization of microtubule polymerization, (2) destabilization or severance of microtubules, (3) functioning as linkers between various structures, or (4) motility-related functions. Here we discuss how the various properties of microtubule-associated proteins can be used to assemble an efficient mitotic apparatus capable of ensuring the bona fide transmission of the genetic information in animal cells.

Identification of a Neural Cell Specific Variant of Microtubule-Associated Protein 4

The microtubule-binding domain of MAP4, a ubiquitous microtubule-associated protein, contains a region rich in proline and basic residues (proline-rich region). We searched the bovine adrenal gland for MAP4 isoforms, and identified a novel variant lacking 72 consecutive amino acid residues within the proline-rich region, as compared with the full-length MAP4. The amino acid sequence of the missing region was highly conserved (about 85% identity/similarity) among the corresponding regions of bovine, human, mouse, and rat MAP4, which suggested the functional significance of this region. A comparison of the genomic sequence with the cDNA sequence revealed that the missing region is encoded by a single exon. A MAP4 variant cDNA homologous to the bovine form was also detected in rat cells, suggesting that the new variant can be generated by alternative splicing, not only in bovine but also in other mammalian species. The mRNA expression of the novel isoform was restricted to the brain and the adrenal medulla, suggesting that this isoform is specific to a certain cell type. Using a bacterially expressed fragment corresponding to the microtubule-binding domain of the novel isoform, we analyzed its in vitro characteristics. The fragment induced microtubule assembly and bound to preformed microtubules, but the activities were slightly lower than those of the conventional MAP4 fragment, which carries the full-length proline-rich region. The microtubules assembled in the presence of the fragment failed to be bundled. Instead, a constant spacing between neighboring microtubules was observed.

The cytoskeleton-dependent localization of cdc2/cyclin B in blastomere cortex duringXenopus embryonic cell cycle

In the early development of the frog, Xenopus laevis, blastomeres undergo synchronous divisions at about the 12th cell cycle, followed by asynchronous divisions, which is referred to as mid-blastula transition (MBT). We investigated the distribution of several regulating factors for cell cycles around MBT using immunocytochemistry and confocal fluorescence microscopy. At the 8th cell cycle, most of the cdc2/cyclin B was localized in the cortical cytoplasm throughout the cell cycle, in the centrosomes and the nucleus at interphase and prometaphase, and in the spindles at metaphase and anaphase. Cdc2 was also localized in the chromatins at metaphase and anaphase. Cyclin B1 mRNA was localized in the periphery of the nucleus, but not in the cell cortex. At the 13th cell cycle, the amount of cdc2/cyclin B in the cortical cytoplasm decreased, and the inactive form of cdc2, phosphorylated at tyrosine 15, appeared in the nucleus and the centrosomes at interphase, indicating that the regulation of cdc2 by phosphorylation occurs around MBT. When the blastomeres were treated with nocodazole or latrunculin A at the 8th cell cycle, the amount of cortical cdc2 decreased, but that of cyclin B did not change. The cortical localization of cdc2 is dependent upon both microtubules and microfilaments. Most of the cdc27 was localized in the centrosomes, and in the spindle poles, but no significant difference was observed between the 8th and the 13th cell cycles. It is possible that the cortical MPF activity is regulated by the differential localization between cdc2 and cyclin B.

The MAP2/Tau family of microtubule-associated proteins

Microtubule-associated proteins (MAPs) of the MAP2/Tau family include the vertebrate proteins MAP2, MAP4, and Tau and homologs in other animals. All three vertebrate members of the family have alternative splice forms; all isoforms share a conserved carboxy-terminal domain containing microtubule-binding repeats, and an amino-terminal projection domain of varying size. MAP2 and Tau are found in neurons, whereas MAP4 is present in many other tissues but is generally absent from neurons. Members of the family are best known for their microtubule-stabilizing activity and for proposed roles regulating microtubule networks in the axons and dendrites of neurons. Contrary to this simple, traditional view, accumulating evidence suggests a much broader range of functions, such as binding to filamentous (F) actin, recruitment of signaling proteins, and regulation of microtubule-mediated transport. Tau is also implicated in Alzheimer's disease and other dementias. The ability of MAP2 to interact with both microtubules and F-actin might be critical for neuromorphogenic processes, such as neurite initiation, during which networks of microtubules and F-actin are reorganized in a coordinated manner. Various upstream kinases and interacting proteins have been identified that regulate the microtubule-stabilizing activity of MAP2/Tau family proteins.

Fertilization and preimplantation development of mouse oocytes after prolonged incubation with caffeine

Background and Aims:  Previous studies have shown that caffeine might cause artificial dephosphorylation at threonine-14 and tyrosine-15 of the p34cdc2 catalytic subunit of maturation-promoting factor (MPF), elevate MPF activity in metaphase II oocytes cultured for a prolonged period, and that caffeine decreases fragmentation in oocytes cultured for up to 96 h. Methods:  Studies were carried out on: (i) the effect of caffeine on the morphological status of oocytes cultured for 96 h; (ii) the parthenogenetic activation and the fertilization of oocytes incubated in a medium that contained caffeine, and (iii) the fertilization and preimplantation development ability of zona-intact and zona-free oocytes by in vitro fertilization (IVF) and by intracytoplasmic sperm injection. Results:  In parthenogenetic activation, the incidence of diploid parthenotes in 24-h caffeine-treated oocytes was significantly higher than 24-h non-treated oocytes. For fertilizability of these oocytes, a significant increase in the fertilization rate resulted from IVF after 12-h caffeine incubation. Although no fertilized eggs were observed after intracytoplasmic sperm injection in 24-h non-treated oocytes, fertilized eggs were observed in caffeine-treated oocytes. MPF activation occurs in relation to nuclear/spindle position, and mitotic spindles and actin filaments determine the site of cleavage during cytokinesis. Spindle disruption does not cause cytofragmentation, but does induce cell cycle arrest. Conclusion:  Based on our results, although caffeine might increase MPF activity, prolonged time in any incubation causes some disruption of cytoskeletal filaments, which might be responsible for the poor development of caffeine-treated oocytes. (Reprod Med Biol 2004; 3: 245-251).

Coupling of posterior cytoskeletal morphogenesis to the G1/S transition in the Trypanosoma brucei cell cycle

The expression levels of four Cdc2-related kinases (CRK1, 2, 4, and 6) in the procyclic form of Trypanosoma brucei were knocked down in pairs using the RNA interference (RNAi) technique. A double knockdown of CRK1 and CRK2 resulted in arrested cell growth in the G1 phase accompanied by an apparent cessation of nuclear DNA synthesis. The arrested cells became elongated at the posterior end like the G1-phase cells generated by knockdown of CycE1/CYC2 in a previous study. However, approximately 5% of the G1 cells in the current study also possessed multiply branched posterior ends, which have not previously been observed in T. brucei. DAPI and immunofluorescence staining showed a single nucleus, kinetoplast, basal body, and flagellum in the anterior portion of each G1 cell. The split and grossly extended posterior ends were heavily stained with antibodies to tyrosinated alpha-tubulin, suggesting an accumulation of newly synthesized microtubules. A significant population of anucleate cells (zoids), apparently derived from kinetoplast-dictated cytokinesis and cell division of the G1 cells, also had extended and branched posterior ends filled with newly synthesized microtubules. This continued posterior extension of microtubules in the G1 cells and zoids suggests that CRK1 and CRK2 exert a coordinated control on G1/S passage and the limited growth of the microtubule corset toward the posterior end. This connection may provide a new insight into the mechanism of morphological maintenance of an ancient protist during its cell cycle progression.

Unequal cell division regulated by the contents of germinal vesicles

Fertilization occurs during meiosis in many animals, when maternal centrosomes participate in the formation of spindles at the animal pole, which results in polar body formation. Paternal centrosomes do not participate in cell division during oocyte maturation. After meiosis, they form the spindles while the maternal centrosomes are discarded. It is unknown why paternal centrosomes do not form spindles during meiosis. Here, we show that the artificial incorporation of sperm at the animal pole of immature starfish oocytes causes unequal cell division and the formation of polar body-like fragments. The removal of germinal vesicles from the animal pole blocks the formation of polar body-like fragments. Furthermore, translocation of germinal vesicles to the vegetal pole by centrifugation induces the extrusion of polar body-like fragments from the vegetal pole, where sperm penetration is prerequisite. After germinal vesicle breakdown, cyclin B is localized in the maternal and paternal asters and spindles near the germinal vesicle. These results suggest that germinal vesicle components such as the cdc2-cyclin B complex interact with asters and spindles and can induce unequal cell division. During normal fertilization, paternal centrosomes are likely kept away from the germinal vesicle components, resulting in the inhibition of unequal paternal centrosome-dependent cell division.

Serum-dependent phosphorylation of human MAP4 at Ser696 in cultured mammalian cells

In the previous paper (Ookata et al., (1997) Biochemistry, 36: 249-259), we identified two mitotic cdc2 kinase phosphorylation sites (Ser696 and Ser787) in the proline-rich region of human MAP4. One (Ser696) of them was also phosphorylated during interphase. A protein kinase responsible for interphase phosphorylation of Ser696 could necessarily be distinct from cdc2/cyclin B kinase. To get insights into a physiological role for Ser696 phosphorylation, we searched for a Ser696 kinase and for cellular conditions under which Ser696 is dephosphorylated. Because Ser696 conforms to the MAP kinase phosphorylation consensus motif (PXSP), MAP kinase was tested as a possible kinase phosphorylating Ser696. MAP kinase, in fact, did phosphorylate Ser696 in MTB3, the carboxy-terminal half of human MAP4 in vitro. Phosphorylation of Ser696 in HeLa cell extract was suppressed by a MAP kinase inhibitor, DBTM-0004. Also consistent with the notion that Ser696 is a MAP kinase site were the fact that serum-starvation induced dephosphorylation of Ser696 in HeLa cells, TIG-3 and MRC-5-30 human fibroblasts, while readdition of serum recovered Ser696 phosphorylation, albeit after a surprisingly long interval. Thus, phosphorylation of Ser696 of MAP4, most likely carried out by MAP kinase, may play a role in modulation of MAP4 activity in proliferating versus quiescent cells.

Functional analysis of microtubule-binding domain of bovine MAP4

Bovine microtubule-associated protein 4 (MAP4) consists of an amino-terminal projection domain and a carboxyl-terminal microtubule-binding domain. The carboxyl-terminal domain of MAP4 is further divided into three subdomains: a region rich in proline and basic residues (Pro-rich region), a region containing four repeats of an assembly-promoting (AP) sequence, which consists of 22 amino acid residues (AP sequence region), and a hydrophobic tail region (Tail region). The subdomain structure of MAP4 microtubule binding domain is similar to those of other MAPs (MAP2 and tau). In order to study the function of each subdomain per se of bovine MAP4 microtubule-binding domain, we purified a series of truncated fragments of MAP4, expressed in Escherichia coil. Binding affinity of the PA4T fragment (containing the Pro-rich region, the AP sequence region and the Tail region) is only four times higher than that of the A4T fragment (containing the AP sequence region and the Tail region), while the microtubule nucleating activity of the PA4T fragment is far greater. We propose that the Pro-rich region promotes the nucleation of microtubule assembly. The A4 fragment (corresponding to the AP sequence region) stimulated the assembly of tubulin into coldstable amorphous aggregates. The AP sequence region of MAP4 failed to promote microtubule assembly. On the other hand, the fragment has an activity to stimulate microtubule elongation. The function of the MAP4 Tail region is not clear at present. The A4T fragment (containing the AP sequence region and the Tail region) promote both microtubule nucleation and elongation step, but the A4 fragment only promotes microtubule elongation, suggesting that the Tail region is indispensable for the nucleation step. However, the fragment containing only the Tail region could not bind to microtubule. Although MAP4 was considered to be long, thin and flexible molecule, never the Tail region may contribute to be the proper folding of MAP4, and/or may interact with other molecules. We concluded that both the Pro-rich region and the AP sequence region take part in the promotion of tubulin polymerization, and that the former is important for the lateral protofilament-protofilament interaction, and the latter is important for the longitudinal affinity between each tubulin dimer in a protofilament.

Regulation of microtubule organization during interphase and M phase

Microtubule (MT) dynamics and organization change markedly during interphase-M phase transition of the cell cycle. This mini review focuses first on p220, a ubiquitous MT-associated protein of Xenopus. p220 is phosphorylated by p34cdc2 kinase and MAP kinase in M phase, and concomitantly loses its MT-binding and MT-stabilizing activities. A cDNA encoding p220 was cloned, which identified p220 as a Xenopus homolog of MAP4, and p220 was therefore termed XMAP4. To examine the physiological relevance of XMAP4 phosphorylation during mitosis, Xenopus A6 cells were transfected with cDNA encoding wild-type or various XMAP4 mutants fused with a green fluorescent protein (GFP). Mutations of serine and threonine within potential phosphorylation sites for p34cdc2 kinase to nonphosphorylatable alanine interfered with mitosis-associated reduction in MT-affinity of XMAP4 and their overexpression affected chromosome movement during anaphase A. These results indicated that phosphorylation of XMAP4 by p34cdc2 kinase is responsible for the decrease in its MT-binding and MT-stabilizing activities during mitosis which are important for chromosome movement during anaphase A. The second focus is on a novel monoclonal antibody W8C3, which recognizes alpha-tubulin. W8C3 stained spindle MTs but not interphase MTs of Xenopus A6 cells, although tubulin dimers in M phase and interphase were equally recognized by this antibody. The difference in MT staining pattern may be because the W8C3-recognition site on alpha-tubulin is sterically hidden in interphase MTs but not in spindle MTs.

The ATP-dependent helicase RUVBL1/TIP49a associates with tubulin during mitosis

RUVBL1/TIP49a/Pontin52 is a recently identified multi-functional protein with 2 ATP binding (WALKER) sites, which is essential for cell proliferation. We recovered and identified RUVBL1/TIP49a as a tubulin-binding protein from Triton X-100 lysates of U937 promonocytic cells by protein affinity chromatography and tryptic peptide microsequencing. Performing co-immunoprecipitation using newly generated RUVBL1/TIP49a-specific antibodies (mAb and rabbit polyclonal Ab) and RUVBL1/TIP49a-GST fusion protein-pull down assays we demonstrate co-precipitation of alpha- and gamma tubulin with RUVBL1/TIP49a. Confocal immunoflourescence microscopy reveals that RUVBL1/TIP49a was present not only in the nucleus, as expected, but was also concentrated at the centrosome and at the mitotic spindle in colocalization with tubulin. The topology of RUVBL1/TIP49a at the mitotic spindle varied, depending on the mitotic stage. The protein was localized at the centrosome and at the polar and astral microtubules in metaphase, and was detectable at the zone of polar tubule interdigitation in anaphase B and telophase. During cytokinesis the protein reappeared at the area of decondensing chromosomes. Whereas preincubation of U937 cells with colcemid resulted in inhibition of mitotic spindle formation with subsequent loss of RUVBL1/TIP49a mitotic spindle staining, no relevant influence of colcemid on RUVBL1/TIP49a-tubulin binding was observed. An agonistic effect of RUVBL1/TIP49a on in vitro tubulin assembly is demonstrated. Our results reveal a new functional aspect of RUVBL1/TIP49a.

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

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

CDK1/cyclin B regulation during oocyte maturation in two closely related lugworm species, Arenicola marina and Arenicola defodiens

The molecular mechanisms underlying oocyte maturation in the annelid polychaetes Arenicola marina and Arenicola defodiens were investigated. In both species, a hitherto unidentified hormone triggers synchronous and rapid transition from prophase to metaphase, a maturation process which can be easily reproduced in vitro. Activation of a roscovitine- and olomoucine-sensitive M-phase-specific histone, H1 kinase, occurs during oocyte maturation. Using affinity chromatography on immobilized p9CKShs1, we purified CDK1 and cyclin B from oocyte extracts prepared from both phases and both species. In prophase, CDK1 is present both as an inactive, but Thr161-phosphorylated monomer, and as an inactive (Tyr15-phosphorylated) heterodimer with cyclin B. Prophase to metaphase transition is associated with complete tyrosine dephosphorylation of the cyclin B-associated CDK1, with phosphorylation of cyclin B, and with dramatic activation of the kinase activity of the CDK1/cyclin B complex. We propose that Arenicola oocytes may provide an ideal model system to investigate the acquisition of the ability of oocytes to be fertilized that occurs as oocyte shift from prophase to metaphase, an important physiological event, probably regulated by active CDK1/cyclin B.

Mechanisms governing maintenance of Cdk1/cyclin B1 kinase activity in cells infected with human cytomegalovirus

Previous work has demonstrated dysregulation of key cell cycle components in human cytomegalovirus (HCMV)-infected human fibroblasts, resulting in cell cycle arrest (F. M. Jault, J.-M. Jault, F. Ruchti, E. A. Fortunato, C. L. Clark, J. Corbeil, D. D. Richman, and D. H. Spector, J. Virol. 69:6697-6704, 1995). The activation of the mitotic kinase Cdk1/cyclin B, which was detected as early as 8 h postinfection (p.i.) and maintained throughout the time course, was particularly interesting. To understand the mechanisms underlying the induction of this kinase activity, we have examined the pathways that regulate the activation of Cdk1/cyclin B1 complexes. The accumulation of the cyclin B1 subunit in HCMV-infected cells is the result of increased synthesis and reduced degradation of the protein. In addition, the catalytic subunit, Cdk1, accumulates in its active form in virus-infected cells. The decreased level of the Tyr15-phosphorylated form of Cdk1 in virus-infected fibroblasts is due in part to the down-regulation of the expression and activity of the Cdk1 inhibitory kinases Myt1 and Wee1. Increased degradation of Wee1 via the proteasome also accounts for its absence at 24 h p.i. At late times, we observed accumulation of the Cdc25 phosphatases that remove the inhibitory phosphates from Cdk1. Interestingly, biochemical fractionation studies revealed that the active form of Cdk1, a fraction of total cyclin B1, and the Cdc25 phosphatases reside predominantly in the cytoplasm of infected cells. Collectively, these data suggest that the maintenance of Cdk1/cyclin B1 activity observed in HCMV-infected cells can be explained by three mechanisms: the accumulation of cyclin B1, the inactivation of negative regulatory pathways for Cdk1, and the accumulation of positive factors that promote Cdk1 activity.

Interphase and monoastral-mitotic phenotypes of overexpressed MAP4 are modulated by free tubulin concentrations

The microtubule-associated protein 4 (MAP4) has recently been shown to counteract destabilization of interphase microtubules caused by catastrophe promotion but not by tubulin sequestering. To address how MAP4 discriminates between destabilization of microtubules by these two mechanisms, we have evaluated the combined phenotypes of MAP4 coexpressed with Op18/stathmin family member derivatives with either catastrophe-promoting or sequestering activities. This approach relies on the finding that overexpression of MAP4 alone stabilizes microtubules during all phases of the cell cycle in human leukemia cells, and causes a potent mitotic block and a dramatic, previously unobserved, phenotype characterized by large monoastral spindles. Coexpression of either catastrophe-promoting or tubulin-sequestration-specific Op18 derivatives was found to modulate the activity of ectopic MAP4 during mitosis, but with differential functional outcome. Interestingly, the tubulin-sequestering derivative suppressed the monoastral mitotic phenotype of MAP4 (i.e. coexpression facilitated the formation of functional spindles). To evaluate whether this phenotypic suppression could be explained by tubulin-sequestration-dependent modulation of MAP4 activity, a plasma-membrane-targeted, tubulin-sequestering chimera was constructed to decrease the cytosolic free tubulin concentration substantially. This chimera likewise suppressed the monoastral phenotype caused by overexpression of MAP4, suggesting a direct downregulation of MAP4 activity by reduced free tubulin concentrations.

G2/M transition of pig oocytes: How do oocytes initiate maturation?

In the ovary, mammalian oocytes resume meiosis and mature to the second metaphase when they are stimulated with gonadotrophins. Similarly, oocytes can mature in vitro when they are liberated from ovarian follicles and cultured under appropriate conditions. Early in the process of maturation, oocytes undergo dramatic but well-ordered changes at the G2/M transition in the cell cycle including: (i) chromosome condensation; (ii) nucleolus disassembly; (iii) germinal vesicle breakdown (GVBD); and (iv) spindle formation in the first metaphase (MI-spindle). These events have been thought to be induced by MPF (maturation-promoting factor or M-phase promoting factor), now known as Cdc2 kinase or Cdk1 kinase, which consists of a catalytic subunit, Cdc2, and a cyclin B regulatory subunit. In fact, nuclear lamins are phosphorylated by Cdc2 kinase, and nuclear membrane breakdown occurs concomitantly with the activation of Cdc2 kinase in the M-phase of both somatic cells and oocytes. Based on the classical and recent studies of the pig oocyte, however, the chromosomes start to condense and the nucleolus disassembles before full activation of Cdc2 kinase, and the MI-spindle is formed after activation of both Cdc2 kinase and MAP kinase; another kinase known to become activated during oocyte maturation. These findings suggest that chromosome condensation and nucleolus disassembly in oocytes are induced by either some kinase(s) other than Cdc2 kinase and MAP kinase or some phosphatase(s). The accumulation of new results regarding the molecular nature of oocyte maturation is important for improving the reproductive technologies in domestic animals as well as in humans. (Reprod Med Biol 2003; 2: 91-99).

M phase phosphoprotein 1 is a human plus-end-directed kinesin-related protein required for cytokinesis

The human M phase phosphoprotein 1 (MPP1), previously identified through a screening of a subset of proteins specifically phosphorylated at the G2/M transition (Matsumoto-Taniura, N., Pirollet, F., Monroe, R., Gerace, L., and Westendorf, J. M. (1996) Mol. Biol. Cell 7, 1455-1469), is characterized as a plus-end-directed kinesin-related protein. Recombinant MPP1 exhibits in vitro microtubule-binding and microtubule-bundling properties as well as microtubule-stimulated ATPase activity. In gliding experiments using polarity-marked microtubules, MPP1 is a slow molecular motor that moves toward the microtubule plus-end at a 0.07 microm/s speed. In cycling cells, MPP1 localizes mainly to the nuclei in interphase. During mitosis, MPP1 is diffuse throughout the cytoplasm in metaphase and subsequently localizes to the midzone to further concentrate on the midbody. MPP1 suppression by RNA interference induces failure of cell division late in cytokinesis. We conclude that MPP1 is a new mitotic molecular motor required for completion of cytokinesis.

Different protofilament-dependence of the microtubule binding between MAP2 and MAP4

To see a molecular basis of the difference in the microtubule binding between MAP2 and MAP4, we compared the binding of them onto microtubule and Zinc-sheet in the presence of various concentrations of NaCl. The Zinc-sheet is the lateral association of protofilaments arranged in an antiparallel fashion with alternatively exposed opposite surfaces, so that binding requiring adjacent protofilaments is restricted. While the salt-dependence of the MAP2 desorption was not altered between these tubulin polymers, MAP4 dissociated from Zinc-sheet at lower concentrations of NaCl than from microtubule. These results suggest that single protofilament is sufficient for microtubule binding of MAP2 as observed by Al-Bassam et al. [J. Cell Biol. 157 (2002) 1187], but MAP4 appeared to interact with adjacent protofilaments during microtubule-binding. Weakened binding on Zinc-sheets was also observed in the projection domain-deletion mutants of MAP4, so that the difference in the protofilament-dependence would lie in the relatively conserved microtubule-binding domain.

The developmental expression of cell cycle regulators in Xenopus laevis

Several cyclins and cdks have been cloned in Xenopus, but their developmental expression has not been thoroughly examined. We have analyzed the temporal and spatial expression of cdk1, cdk2, cdk4 and cyclins D1, D2, E, A1, A2 and B1 by in situ hybridization. The transcripts of these cyclins and cdks exhibit striking tissue-restricted expression patterns very early in development that cannot be strictly correlated with proliferation. While the cdks and their activating cyclins are expressed in somewhat overlapping patterns, they are not precisely coincident. Additionally, maternal and zygotic cyclin forms demonstrate markedly different expression patterns.

Cytoskeleton of human mononuclear cells as a possible peripheral marker for phenylalanine neurotoxicity in PKU

In this work we tested human mononuclear cells as a peripheral marker to study neurotoxicity of phenylalanine (Phe). Slices of cerebral cortex of rats or human mononuclear cells were incubated with different concentrations of Phe and/or Ala in the presence of 32P-orthophosphate, the cytoskeletal fraction was extracted, and the radioactivity incorporated into intermediate filament proteins was measured. Our results show that 2 mM Phe as well as 1 mM Ala are effective in increasing the 32P in vitro incorporation into IFs in both tissues. When cerebral cortex slices or mononuclear cells were incubated with different concentrations of Phe and/or Ala, the effects on the 32P in vitro incorporation into IF proteins was compatible with an antagonistic mechanism of action of the two amino acids on the enzymes of the phosphorylating system. In addition, these blood cells may be a possible peripheral marker to study neurotoxicity of Phe in patients with PKU.

Katanin-mediated microtubule severing can be regulated by multiple mechanisms

Microtubules are essential for a wide range of cellular processes that vary between cell types. Katanin is a microtubule-severing protein that carries out an essential role in meiotic spindles in Caenorhabditis elegans and a non-essential role in mitotic spindles of vertebrates. In contrast to these M-phase associated roles, katanin is also essential for post-mitotic differentiation events in vertebrate neurons and in Arabidopsis. This diversity of function suggests that katanin's activity might be regulated by multiple mechanisms. Because katanin is active in M-phase Xenopus extracts but not in interphase extracts, we assayed for regulators of katanin's activity in these extracts. The microtubule-severing activity of purified katanin was inhibited by interphase Xenopus extracts. Fractionation revealed that this inhibition was due to at least 4 separable components, one of which contains the MAP4 homolog, XMAP230. Inhibition of katanin-mediated microtubule-disassembly activity by the XMAP230-containing fraction was reversible by cyclinB/cdk1, suggesting one possible mechanism for the increased severing activity observed in M-phase Xenopus extracts. In a previous study, spindle pole association by katanin was essential for its activity during mitosis suggesting that katanin's activity might also be regulated by co-localization with an activator. The polo-like kinase, Plx1, co-localized with katanin at spindle poles in vivo and purified Plx1 increased the microtubule-severing activity of katanin in vitro. These in vitro experiments illustrate the potential complexity of the regulation of katanin's activity in vivo and may explain how katanin can carry out widely different functions in different cell types.

The plant cytoskeleton: recent advances in the study of the plant microtubule-associated proteins MAP-65, MAP-190 and the Xenopus MAP215-like protein, MOR1

The microtubule cytoskeleton is a dynamic filamentous structure involved in many key processes in plant cell morphogenesis including nuclear and cell division, deposition of cell wall, cell expansion, organelle movement and secretion. The principal microtubule protein is tubulin, which associates to form the wall of the tubule. In addition, various associated proteins bind microtubules either to anchor, cross-link or regulate the microtubule network within cells. Biochemical, molecular biological and genetic approaches are being successfully used to identify these microtubule-associated proteins (MAPs) in plants, and we describe recent progress on three of these proteins.

Plk phosphorylation regulates the microtubule-stabilizing protein TCTP

The mitotic polo-like kinases have been implicated in the formation and function of bipolar spindles on the basis of their respective localizations and mutant phenotypes. To date, this putative regulation has been limited to a kinesin-like motor protein, a centrosomal structural protein, and two microtubule-associated proteins (MAPs). In this study, another spindle-regulating protein, the mammalian non-MAP microtubule-binding and -stabilizing protein, the translationally controlled tumor protein (TCTP), was identified as a putative Plk-interacting clone by a two-hybrid screen. Plk phosphorylates TCTP on two serine residues in vitro and cofractionates with the majority of kinase activity toward TCTP in mitotic cell lysates. In addition, these sites were demonstrated to be phosphorylated in vivo. Overexpression of a Plk phosphorylation site-deficient mutant of TCTP induced a dramatic increase in the number of multinucleate cells, rounded cells with condensed ball-like nuclei, and cells undergoing cell death, similar to both the reported anti-Plk antibody microinjection and the low-concentration taxol treatment phenotypes. These results suggest that phosphorylation decreases the microtubule-stabilizing activity of TCTP and promotes the increase in microtubule dynamics that occurs after metaphase.

The projection domain of MAP4 suppresses the microtubule-bundling activity of the microtubule-binding domain

Microtubule-associated protein 4 (MAP4), a major MAP expressed in proliferating non-neuronal cells, consists of an N-terminal projection (PJ) domain and a C-terminal microtubule-binding (MTB) domain. The PJ domain of MAP4 is divided into three regions; the N-terminal acidic region (the Na-region), the multiple KDM-repeated sequence region (the KDM-region), and the b-region followed by the MTB domain. To investigate roles of the PJ domain, we prepared three truncated forms of human MAP4 with different PJ domain lengths; PJ1, PJ2 and MTB with deletion of about one-third, two-third and all of the PJ domain, respectively, and examined their effects on bundle formation of microtubules (MTs). MTs polymerized by full length MAP4 were singly distributed as observed by both negative staining electron microscopy and dark field microscopy. MTs with PJ1 were also separated in solution but became pairs when pelleted by centrifugation. PJ2 formed planar two-dimensional bundles consisting of several MTs (the 2D-bundle). MTB induced large bundles of many MTs, tightly packed without space in between (termed the 3D-bundle). To study how the PJ domain decreases the bundle-forming activity of the MTB domain of MAP4, we made three additional deletion-mutants of MAP4, called Na-MTB, KDM-MTB and Na-PJ2. Na-MTB and KDM-MTB, in which the KDM/b-region and both of Na- and b-regions were deleted respectively, were prepared by fusing the Na-region or KDM-region to MTB. Both of Na-MTB and KDM-MTB suppressed the 3D-bundle formation as effectively as PJ2. MTs polymerized with Na-PJ2, the KDM-deletion mutant made by adding the Na-region to PJ2, were singular and did not become bundles. These results indicated that the PJ domain kept individual MTs separated by suppressing the bundle-forming ability of the MTB domain. The suppressive activity of the PJ domain was correlated with the length, but not the amino acid sequence, of the PJ.

MAP4 counteracts microtubule catastrophe promotion but not tubulin-sequestering activity in intact cells

Microtubules are polar polymers that continually switch between phases of elongation and shortening, a property referred to as dynamic instability. The ubiquitous microtubule associated protein 4 (MAP4) shows rescue-promoting activity during in vitro assembly of microtubules (i.e., promotes transitions from shortening to elongation), but its regulatory role in intact cells is poorly defined. Here, we demonstrate that ectopic MAP4 promotes outgrowth of extended MTs during beta1-integrin-induced cell spreading. An inducible cotransfection protocol was employed to further analyze the regulatory role of MAP4 in human leukemia cells with microtubules partially destabilized by either ectopic tubulin-sequestering proteins or proteins that promote catastrophes (i.e., transitions from elongation to shortening). Coexpression of proteins that sequester free tubulin heterodimers with different efficiencies was found to abolish microtubule stabilization by MAP4. In contrast, however, the microtubule-stabilizing activity of MAP4 was found to suppress the activities of two distinct and specific catastrophe promoters, namely, XKCM1 and a nonsequestering truncation derivative of Op18/stathmin. These observations reveal specificity in the microtubule-stabilizing activity of MAP4 that differentiates between two mechanistically distinct types of MT destabilization.

Shigella deliver an effector protein to trigger host microtubule destabilization, which promotes Rac1 activity and efficient bacterial internalization

Shigella deliver a subset of effectors into the host cell via the type III secretion system, that stimulate host cell signal pathways to modulate the actin dynamics required for invasion of epithelial cells. Here we show that one of the Shigella effectors, called VirA, can interact with tubulin to promote microtubule (MT) destabilization, and elicit protrusions of membrane ruffling. Under in vitro conditions, VirA inhibited polymerization of tubulin and stimulated MT destabilization. Upon microinjection of VirA into HeLa cells, a localized membrane ruffling was induced rapidly. Overexpression of VirA in host cells caused MT destruction and protruding membrane ruffles which were absent when VirA was co-expressed with a dominant-negative Rac1 mutant. Indeed, Shigella but not the virA mutant stimulated Rac1, including the formation of membrane ruffles in infected cells. Importantly, the MT structure beneath the protruding ruffling was destroyed. Furthermore, drug-induced MT growth in HeLa cells greatly enhanced the Shigella entry. These results indicate that VirA is a novel type of bacterial effector capable of inducing membrane ruffling through the stimulation of MT destabilization.

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.

Toward reconstitution of in vivo microtubule dynamics in vitro

The transition from interphase to mitosis is marked by a dramatic change in microtubule dynamics resulting in the reorganization of the microtubule network that culminates in mitotic spindle formation. While the molecular basis for this change in microtubule organization remains obscure, it is currently thought that a balance in the activity of microtubule stabilizing and destabilizing factors regulates how dynamic cellular microtubules are. By mixing the microtubule stabilizer XMAP215 and the microtubule destabilizer XKCM1, reconstitution of in vivo-like microtubule dynamics has now been achieved in vitro.

Drosophila Aurora A kinase is required to localize D-TACC to centrosomes and to regulate astral microtubules

Disruption of the function of the A-type Aurora kinase of Drosophila by mutation or RNAi leads to a reduction in the length of astral microtubules in syncytial embryos, larval neuroblasts, and cultured S2 cells. In neuroblasts, it can also lead to loss of an organized centrosome and its associated aster from one of the spindle poles, whereas the centrosome at the other pole has multiple centrioles. When centrosomes are present at the poles of aurA mutants or aurA RNAi spindles, they retain many antigens but are missing the Drosophila counterpart of mammalian transforming acidic coiled coil (TACC) proteins, D-TACC. We show that a subpopulation of the total Aurora A is present in a complex with D-TACC, which is a substrate for the kinase. We propose that one of the functions of Aurora A kinase is to direct centrosomal organization such that D-TACC complexed to the MSPS/XMAP215 microtubule-associated protein may be recruited, and thus modulate the behavior of astral microtubules.

Accompanying protein alterations in malignant cells with a microtubule-polymerizing drug-resistance phenotype and a primary resistance mechanism

Microtubules (MTs) are cytoskeletal components whose structural integrity is mandatory for the execution of many basic cell functions. Utilizing parental and drug-resistant ovarian carcinoma cell lines that have acquired point mutations in beta-tubulin and p53, we studied the level of expression and modification of proteins involved in apoptosis and MT integrity. Extending previous results, we demonstrated phosphorylation of pro-survival Bcl-x(L) in an epothilone-A resistant cell line, correlating it with drug sensitivity to tubulin-active compounds. Furthermore, Mcl-1 protein turned over more rapidly following exposure to tubulin-modifying agents, the stability of Mcl-1 protein paralleling the drug sensitivity profile of the paclitaxel or epothilone-A resistant cell lines. The observed decreases in Mcl-1 were not a consequence of G(2)M arrest, as determined by flow cytometry analysis, which showed prominent levels of Mcl-1 in the absence of any drug treatment in populations enriched in mitotic cells. We also observed that a paclitaxel-resistant cell line expressed Bax at a much lower level than the sensitive parental line [A2780(1A9)], consistent with its mutant p53 status. MT-associated protein-4 (MAP4), whose phosphorylation during specific phases of the cell cycle reduces its MT-polymerizing and -stabilizing capabilities, was phosphorylated in response to drug challenge without a change in expression. Phosphorylation of MAP4 correlated with sensitivity to tubulin-binding drugs and with a dissociation from MTs. We propose that the tubulin mutations, which result in a compromised paclitaxel:tubulin or epothilone:tubulin interaction and paclitaxel or epothilone resistance, indirectly inhibit downstream events that lead to cell death, and this, in turn, may contribute to the drug-resistance phenotype

Sub-cellular localisation of GFP-tagged tobacco mitotic cyclins during the cell cycle and after spindle checkpoint activation

We have previously shown that the tobacco cyclin B1;1 protein accumulates during the G2 phase of the cell cycle and is subsequently destroyed during mitosis. Here, we investigated the sub-cellular localisation of two different B1-types and one A3-type cyclin during the cell cycle by using confocal imaging and differential interference contrast (DIC) microscopy. The cyclins were visualised as GFP-tagged fusion proteins in living tobacco cells. Both B1-type cyclins were found in the cytoplasm and in the nucleus during G2 but when cells entered into prophase, both cyclins became associated with condensing chromatin and remained on chromosomes until metaphase. As cells exited metaphase, the B1-type cyclins became degraded, as shown by time-lapse images. A stable variant of cyclin B1;1-GFP fusion protein, in which the destruction box had been mutated, maintained its association with the nuclear material at later phases of mitosis such as anaphase and telophase. Furthermore, we demonstrated that cyclin B1;1 protein is stabilised in metaphase-arrested cells after microtubule destabilising drug treatments. In contrast to the B1-type cyclins, the cyclin A3;1 was found exclusively in the nucleus in interphase cells and disappeared earlier than the cyclin B1 proteins during mitosis.

P34(cdc2) kinase is associated with cortical microtubules from higher plant protoplasts

The cell cycle regulatory enzyme p34(cdc2) kinase is known to be localized to the preprophase band, the spindle and the phragmoplast, but not to interphase cortical microtubules. This was investigated further by mechanically cleaving substrate-attached protoplasts to leave plasma membrane disks bearing microtubules freed of nuclear and cytosolic signal. Antibodies to PSTAIRE and to specific C-terminal peptides of cdc2a, were used in immunofluorescence, protein blotting and immunogold electron microscopy to demonstrate that antigen is located on the cortical microtubules of carrot, tobacco BY-2 and Arabidopsis cells.

alpha-, beta-, and gamma-Tubulin polymerization in response to DNA damage

Microtubules provide structural support for a cell and play key roles in cell motility, mitosis, and meiosis. They are also the targets of several anticancer agents, indicating their importance in maintaining cell viability. We have investigated the possibility that alterations in microtubule structure and tubulin polymerization may be part of the cellular response to DNA damage. In this report, we find that gamma-radiation stimulates the production and polymerization of alpha-, beta-, and gamma- tubulin in hematopoeitic cell lines (Ramos, DP16), leading to visible changes in microtubule structures. We have found that this microtubule reorganization can be prevented by caffeine, a drug that concomitantly inhibits DNA damage-induced cell cycle arrest and apoptosis. Our results support the idea that microtubule polymerization is an important facet of the mammalian response to DNA damage.

Resistance to Taxol in lung cancer cells associated with increased microtubule dynamics

Microtubule dynamics are crucial for mitotic spindle assembly and chromosome movement. Suppression of dynamics by Taxol appears responsible for the drug's potent ability to inhibit mitosis and cell proliferation. Although Taxol is an important chemotherapeutic agent, development of resistance limits its efficacy. To examine the role of microtubule dynamics in Taxol resistance, we measured the dynamic instability of individual rhodamine-labeled microtubules in Taxol-sensitive and -resistant living human cancer cells. Taxol-resistant A549-T12 and -T24 cell lines were selected from a human lung carcinoma cell line, A549. They are, respectively, 9- and 17-fold resistant to Taxol and require low concentrations of Taxol for proliferation. We found that microtubule dynamic instability was significantly increased in the Taxol-resistant cells. For example, with A549-T12 cells in the absence of added Taxol, microtubule dynamicity increased 57% as compared with A549 cells. The length and rate of shortening excursions increased 75 and 59%, respectively. These parameters were further increased in A549-T24 cells, with overall dynamicity increasing by 167% compared with parental cells. Thus, the decreased Taxol-sensitivity of these cells can be explained by their increased microtubule dynamics. When grown without Taxol, A549-T12 cells were blocked at the metaphase/anaphase transition and displayed abnormal mitotic spindles with uncongressed chromosomes. In the presence of 2-12 nM Taxol, the cells grew normally, suggesting that mitotic block resulted from excessive microtubule dynamics. These results indicate that microtubule dynamics play an important role in Taxol resistance, and that both excessively rapid dynamics and suppressed dynamics impair mitotic spindle function and inhibit proliferation.

Phosphorylation of MAP4 affects microtubule properties and cell cycle progression

In human cells, MAP4, a microtubule-associated protein ubiquitously expressed in proliferating cells, has been shown to undergo in vivo phosphorylation. Two phosphorylation sites, serines 696 and 787, lie within the proline-rich region of its microtubule-binding domain. To test the hypothesis that phosphorylation at these sites influences microtubule properties or cell cycle progression, we prepared stable cell lines that inducibly express versions of MAP4 in which phosphorylation of these two serines was prevented by their replacement with alanine, lysine, or glutamate residues (AA-, KK-, or EE-MAP4). All non-phosphorylatable mutant forms of MAP4 expressed in mouse Ltk- cells were localized to MT arrays that were unremarkable in appearance. Expression of non-phosphorylatable mutants of MAP4 did not affect cell doubling time; however, expression of some mutants altered progression into or through cell division. Interactions of mutant MAP4 with MTs were examined in vitro. KK mutant MAP4 bound MTs more avidly than its wild-type counterpart, WT-MAP4. In vivo MT polymer also differed among the mutants: MTs in cells expressing the KK- and AA-MAP4 forms were more resistant to nocodazole depolymerization than those in cells expressing EE- or WT-MAP4 forms. Our results demonstrate that phosphorylation alters MAP4 properties and suggest a raison d'être for phosphorylation of the MAP4 microtubule-binding domain during cell cycle progression.

Abundant expression of the microtubule-associated protein, ensconsin (E-MAP-115), alters the cellular response to Taxol

Correlation between expression level of a microtubule-associated protein called ensconsin (E-MAP-115) and degree of Taxol sensitivity in several cultured cell lines prompted us to investigate potential cause-and-effect relationships between ensconsin level and Taxol action. We used human MCF-7 or HeLa cells, which are sensitive to low Taxol concentrations (LD(50) of 30-35 and 3.5 nM, respectively) to prepare stably transfected populations of cells expressing heterogeneous levels of ensconsin chimeras, either green fluorescent protein (GFP) conjugated to full-length ensconsin (GFP-Ensc) or to ensconsin's microtubule-binding domain (GFP-EMTB). Both a subjective microscopic assay, i.e., scoring fluorescence of GFP-ensconsin chimeras following Taxol treatment, and a quantitative immunobiochemical assay, i.e., measuring level of GFP-ensconsin chimera in cells surviving treatment with Taxol, showed that cells expressing higher levels of GFP-ensconsin chimera were killed more readily by Taxol concentrations approaching the LD(50). In contrast, in TC-7 cells, which are relatively insensitive to Taxol (LD(50) > 600 nM), high-level expression of GFP-EMTB conferred no significant susceptibility to killing by Taxol. However, heightening the Taxol sensitivity of GFP-EMTB-TC-7 cells by pre-incubating cells with the p-glycoprotein inhibitor, verapamil, did result in selective killing of cells highly expressing GFP-EMTB. Taken together, results obtained in MCF-7, HeLa, and TC-7 cells suggest that elevated ensconsin level bestowed a selective disadvantage upon Taxol-sensitive cells. To probe potential mechanisms by which ensconsin could alter the Taxol response, we isolated microtubules from HeLa cells that were or were not pretreated with Taxol. In vivo Taxol treatment significantly tightened microtubule-binding of ensconsin, suggesting that Taxol alters ensconsin's microtubule-binding properties and may, in turn, alter the Taxol response of the microtubules. Our data support the hypothesis that Taxol works synergistically or in concert with microtubule-binding proteins in bringing about deleterious effects on the microtubule cytoskeleton.

The non-catalytic domain of the Xenopus laevis auroraA kinase localises the protein to the centrosome

Aurora kinases are involved in mitotic events that control chromosome segregation. All members of this kinase subfamily possess two distinct domains, a highly conserved catalytic domain and an N-terminal non-catalytic extension that varies in size and sequence. To investigate the role of this variable non-catalytic region we overexpressed and purified Xenopus laevis auroraA (pEg2) histidine-tagged N-terminal peptide from bacterial cells. The peptide has no effect on the in vitro auroraA kinase activity, but it inhibits both bipolar spindle assembly and stability in Xenopus egg extracts. Unlike the full-length protein, the N-terminal domain shows only low affinity for paclitaxel-stabilised microtubules in vitro, but localises to the centrosomes in a microtubule-dependent manner. When expressed in Xenopus XL2 cells, it is able to target the green fluorescent protein to centrosomes. Surprisingly, this is also true of the pEg2 catalytic domain, although to a lesser extent. The centrosome localisation of the N-terminal peptide was disrupted by nocodazole whereas localisation of the catalytic domain was not, suggesting that in order to efficiently localise to the centrosome, pEg2 kinase required the non-catalytic N-terminal domain and the presence of microtubules.

Type I protein kinase a is localized to interphase microtubules and strongly associated with the mitotic spindle

We show here that type I protein kinase A is localized to microtubules during the entire cell cycle in epithelial (hepatoma, cervical carcinoma) and nonepithelial (myoblast) cell lines. The association of the type Ialpha regulatory subunit is very strong in all phases of mitosis, from prophase to cytokinesis. In interphase, the association appears weaker, reflecting perhaps a more dynamic molecular interaction. This regulatory subunit appears to recruit catalytic subunits as the latter are also associated with microtubules. BW1J hepatoma cells, stably transfected with either wild-type or mutant Ialpha regulatory subunit, are enriched in aberrant mitoses with multipolar spindles and in mono- or multinucleated giant cells. This suggests that type I protein kinase A could have a role in centrosome duplication and/or segregation, sister chromatid separation, or cytokinesis.

Checkpoint genes in cancer

The mammalian cell cycle is exquisitely controlled by a 'machinery' composed of cyclin-dependent kinases and their binding partners, the cyclins. These kinases regulate transitions into DNA synthesis and mitosis, and their inactivity contributes to cellular quiescence, differentiation and senescence. Cell cycle transitions are, in turn, controlled by checkpoints that monitor ribonucleotide pools, oxygen tension, the extracellular environment, growth signalling programmes, the status of DNA replication, and the mitotic spindle apparatus. Genes positively controlling cell cycle checkpoints can be targets for oncogenic activation in cancer, whereas negative regulators, such as tumour suppressor genes, are targeted for inactivation. Understanding the molecular details of cell cycle regulation and checkpoint abnormalities in cancer offers insight into potential therapeutic strategies.