Purification of microtubule proteins from Xenopus egg extracts: identification of a 230K MAP4-like protein

miR-103-3p Regulates the Differentiation and Autophagy of Myoblasts by Targeting MAP4

Skeletal muscle is the most abundant tissue in mammals, and myogenesis and differentiation require a series of regulatory factors such as microRNAs (miRNAs). In this study, we found that miR-103-3p was highly expressed in the skeletal muscle of mice, and the effects of miR-103-3p on skeletal muscle development were explored using myoblast C2C12 cells as a model. The results showed that miR-103-3p could significantly reduce myotube formation and restrain the differentiation of C2C12 cells. Additionally, miR-103-3p obviously prevented the production of autolysosomes and inhibited the autophagy of C2C12 cells. Moreover, bioinformatics prediction and dual-luciferase reporter assays confirmed that miR-103-3p could directly target the microtubule-associated protein 4 (MAP4) gene. The effects of MAP4 on the differentiation and autophagy of myoblasts were then elucidated. MAP4 promoted both the differentiation and autophagy of C2C12 cells, which was contrary to the role of miR-103-3p. Further research revealed that MAP4 colocalized with LC3 in C2C12 cell cytoplasm, and the immunoprecipitation assay showed that MAP4 interacted with autophagy marker LC3 to regulate the autophagy of C2C12 cells. Overall, these results indicated that miR-103-3p regulated the differentiation and autophagy of myoblasts by targeting MAP4. These findings enrich the understanding of the regulatory network of miRNAs involved in the myogenesis of skeletal muscle.

MAP4 as a New Candidate in Cardiovascular Disease

Microtubule and mitochondrial dysfunction have been implicated in the pathogenesis of cardiovascular diseases (CVDs), including cardiac hypertrophy, fibrosis, heart failure, and hypoxic/ischemic related heart dysfunction. Microtubule dynamics instability leads to disrupted cell homeostasis and cell shape, decreased cell survival, and aberrant cell division and cell cycle, while mitochondrial dysfunction contributes to abnormal metabolism and calcium flux, increased cell death, oxidative stress, and inflammation, both of which causing cell and tissue dysfunction followed by CVDs. A cytosolic skeleton protein, microtubule-associated protein 4 (MAP4), belonging to the family of microtubule-associated proteins (MAPs), is widely expressed in non-neural cells and possesses an important role in microtubule dynamics. Increased MAP4 phosphorylation results in microtubule instability. In addition, MAP4 also expresses in mitochondria and reveals a crucial role in maintaining mitochondrial homeostasis. Phosphorylated MAP4 promotes mitochondrial apoptosis, followed by cardiac injury. The aim of the present review is to highlight the novel role of MAP4 as a potential candidate in multiple cardiovascular pathologies.

The elegans of spindle assembly

The Caenorhabditis elegans one-cell embryo is a powerful system in which to study microtubule organization because this large cell assembles both meiotic and mitotic spindles within the same cytoplasm over the course of 1 h in a stereotypical manner. The fertilized oocyte assembles two consecutive acentrosomal meiotic spindles that function to reduce the replicated maternal diploid set of chromosomes to a single-copy haploid set. The resulting maternal DNA then unites with the paternal DNA to form a zygotic diploid complement, around which a centrosome-based mitotic spindle forms. The early C. elegans embryo is amenable to live-cell imaging and electron tomography, permitting a detailed structural comparison of the meiotic and mitotic modes of spindle assembly.

The p38/MAPK pathway regulates microtubule polymerization through phosphorylation of MAP4 and Op18 in hypoxic cells

In both cardiomyocytes and HeLa cells, hypoxia (1% O(2)) quickly leads to microtubule disruption, but little is known about how microtubule dynamics change during the early stages of hypoxia. We demonstrate that microtubule associated protein 4 (MAP4) phosphorylation increases while oncoprotein 18/stathmin (Op18) phosphorylation decreases after hypoxia, but their protein levels do not change. p38/MAPK activity increases quickly after hypoxia concomitant with MAP4 phosphorylation, and the activated p38/MAPK signaling leads to MAP4 phosphorylation and to Op18 dephosphorylation, both of which induce microtubule disruption. We confirmed the interaction between phospho-p38 and MAP4 using immunoprecipitation and found that SB203580, a p38/MAPK inhibitor, increases and MKK6(Glu) overexpression decreases hypoxic cell viability. Our results demonstrate that hypoxia induces microtubule depolymerization and decreased cell viability via the activation of the p38/MAPK signaling pathway and changes the phosphorylation levels of its downstream effectors, MAP4 and Op18.

Xenopus interphase and mitotic microtubule-associated proteins differentially suppress microtubule dynamics in vitro

Based on observations of microtubule dynamics in Xenopus extracts and in vivo, it has been assumed that the pool of interphase microtubule-associated proteins (MAPs) are more potent microtubule stabilizers than their mitotic counterparts. The aim of this study was to test that assumption, and two questions were addressed here. First, are there differences in the composition of interphase and mitotic MAPs? Second, do interphase MAPs more potently promote microtubule assembly than mitotic MAPs? Biochemical purification from Xenopus egg extracts shows that the composition of interphase and mitotic MAPs is similar. XMAP215, XMAP230, and XMAP310, which are the three characterized Xenopus MAPs, show decreased microtubule binding in mitotic extracts, and mitotic MAPs are slightly more phosphorylated than interphase MAPs. Bulk polymerization and time-lapse video microscopy show that microtubules polymerized two times faster in the presence of total interphase MAPs compared with total mitotic MAPs. Interphase but not mitotic MAPs strongly promoted microtubule nucleation in solution. Video microscopy showed that microtubules never underwent catastrophes in the presence of either MAP fraction. It is proposed that the increase in microtubule dynamics at the onset of mitosis results from phosphorylation dependent decreased microtubule stabilization by MAPs, allowing destabilizing factors to increase the catastrophe frequency and dismantle the interphase microtubule network.

Ser787 in the proline-rich region of human MAP4 is a critical phosphorylation site that reduces its activity to promote tubulin polymerization

p34cdc2 kinase-phosphorylation sites in the microtubule (MT)-binding region of MAP4 were determined by peptide sequence of phosphorylated MTB3, a fragment containing the carboxy-terminal half of human MAP4. In addition to two phosphopeptides containing Ser696 and Ser787 which were previously indicated to be in vivo phosphorylation sites, two novel phosphopeptides, containing Thr892 or Thr901 and Thr917 as possible phosphorylation sites, were isolated, though only in in vitro phosphorylation. The role of phosphorylation at Ser696 and Ser787, which were differently phosphorylated during the cell cycle (Ookata et al., (1997). Biochemistry, 36: 15873-15883), was investigated in MT-polymerization, using MAP4 Ser to Glu mutants, which mimic phosphorylation at each site. Mutation of Ser787 to Glu strikingly reduced the MAP4's MT-polymerization activity, while Glu-mutation at Ser696 did not. These results suggest that Ser787 could be the critical phosphorylation site causing MTs to be dynamic at mitosis.

XMAP230 is required for normal spindle assembly in vivo and in vitro

XMAP230 is a high molecular mass microtubule-associated protein isolated from Xenopus oocytes and eggs, and has been recently shown to be a homolog of mammalian MAP4. Confocal immunofluorescence microscopy revealed that XMAP230 is associated with microtubules throughout the cell cycle of early Xenopus embryos. During interphase XMAP230 is associated with the radial arrays of microtubules and midbodies remaining from the previous division. During mitosis, XMAP230 is associated with both astral microtubules and microtubules of the central spindle. Microinjection of affinity-purified anti-XMAP230 antibody into blastomeres severely disrupted the assembly of mitotic spindles during the rapid cleavage cycles of early development. Both monopolar half spindles and bipolar spindles were assembled from XMAP230-depleted extracts in vitro. However, spindles assembled in XMAP230-depleted extracts exhibited a reduction in spindle width, reduced microtubule density, chromosome loss, and reduced acetylation of spindle MTs. Similar defects were observed in the spindles assembled in XMAP230-depleted extracts that had been cycled through interphase. Depletion of XMAP230 had no effect on the pole-to-pole length of spindles, and depletion of XMAP230 from both interphase and M-phase extracts had no effect on the rate of microtubule elongation. From these results, we conclude that XMAP230 plays an important role in normal spindle assembly, primarily by acting to stabilize spindle microtubules, and that the observed defects in spindle assembly may result from enhanced microtubule dynamics in XMAP230-depleted extracts.

Microtubule-associated protein 4 (MAP4) regulates assembly, protomer-polymer partitioning and synthesis of tubulin in cultured cells

We depleted MAP4, a ubiquitously expressed microtubule (MT)-associated protein previously shown to be capable of stabilizing MTs, from HeLa cells by stably expressing antisense RNA. These HeLa-AS cells, in which the MAP4 level was decreased to 33% of the wild-type level, displayed decreased content of total tubulin (65% of the wild-type level). The partitioning of cellular tubulin into protomer and polymer was altered in HeLa-AS cells: polymeric tubulin was decreased to 46% of the level in control cells, while protomeric tubulin was increased to 226% of the level in control cells. Tubulin protein synthesis was decreased, consistent with the tubulin autoregulation model, which proposes that tubulin protomer inhibits its own synthesis. Following release from drug-induced depolymerization, MTs in HeLa-AS cells reformed more slowly, and showed an increased focus on the centrosome, as compared to control cells. HeLa-AS cells also appeared to be less bipolar in shape and flatter than control cells. Our data suggest that MAP4 regulates assembly level of MTs and, perhaps through this mechanism, is involved in controlling spreading and shape of cells.

Confocal microscopy and 3-D reconstruction of the cytoskeleton of Xenopus oocytes

Xenopus oocytes contain a complex cytoskeleton composed of three filament systems: (1) microtubules, composed of tubulin and at least three different microtubule-associated proteins (XMAPs); (2) microfilaments composed of actin and associated proteins; and (3) intermediate filaments, composed of keratins. For the past several years, we have used confocal immunofluorescence microscopy to characterize the organization of the oocyte cytoskeleton throughout the course of oogenesis. Together with computer-assisted reconstruction of the oocyte in three dimensions, confocal microscopy gives an unprecedented view of the assembly and reorganization of the cytoskeleton during oocyte growth and differentiation. Results of these studies, combined with the effects of cytoskeletal inhibitors, suggest that organization of the cytoskeleton in Xenopus oocytes is dependent upon a hierarchy of interactions between microtubules, microfilaments, and keratin filaments. This article presents a gallery of confocal images and 3-D reconstructions depicting the assembly and organization of the oocyte cytoskeleton during stages 0-VI of oogenesis, a discussion of the mechanisms that might regulate cytoskeletal organization during oogenesis, and speculates on the potential roles of the oocyte cytoskeleton during oogenesis and axis formation.

XMAP230 is required for the organization of cortical microtubules and patterning of the dorsoventral axis in fertilized Xenopus eggs

The dorsoventral axis of Xenopus embryos is specified by a rotation of the egg cortex relative to the underlying yolky cytoplasm. This cortical rotation, which occurs during the first cell cycle after fertilization, is dependent upon an array of parallel microtubules in the subcortical cytoplasm. We have used confocal immunofluorescent microscopy and microinjection of affinity-purified anti-XMAP230 antibody to address the role of XMAP230, one of three high-molecular-weight microtubule-associated proteins (MAPs) in Xenopus eggs, in the assembly and organization of the cortical microtubule array and specification of the dorsoventral axis. Confocal immunofluorescence microscopy revealed that XMAP230 was associated with cortical microtubules shortly after their appearance in the subcortical cytoplasm. XMAP230 staining became more prominent as microtubules were aligned and bundled during the cortical rotation. Loss of XMAP230 appeared to precede disassembly of cortical microtubules at the end of the first cell cycle. Deeper within the cytoplasm, XMAP230 was associated with microtubules early in the assembly of the sperm aster. However, later in the first cell cycle, XMAP230 was associated with microtubules (MTs) of the first mitotic spindle, spindle asters, and the cortical MTs, but not with microtubule remnants of the sperm aster. Microinjection of anti-XMAP230 antibody locally disrupted the assembly and organization of microtubules in the cortex of activated or fertilized eggs and resulted in defects in the dorsoventral patterning of embryos. These results indicate that the assembly and/or organization of cortical microtubules in fertilized Xenopus eggs and subsequent specification of the dorsoventral axis are dependent upon XMAP230.

XMAP230 is required for the assembly and organization of acetylated microtubules and spindles in Xenopus oocytes and eggs

We used affinity-purified polyclonal antibodies to characterize the distribution and function of XMAP230, a heat-stable microtubule-associated protein isolated from Xenopus eggs, during oogenesis. Immunoblots revealed that XMAP230 was present throughout oogenesis and early development, but was most abundant in late stage oocytes, eggs, and early embryos. Immunofluorescence microscopy revealed that XMAP230 was associated with microtubules in oogonia, post-mitotic stage 0 oocytes, early stage I oocytes, and during stage IV-VI of oogenesis. However, staining of microtubules by anti-XMAP230 was not detectable during late stage I through stage III. In stage VI oocytes, anti-XMAP230 stained a large subset of microtubules that were also stained with monoclonal antibodies specific for acetylated (α)-tubulin. During oocyte maturation, XMAP230 was associated with the transient microtubule array that serves as the precursor of the first meiotic spindle, as well as both first and second meiotic spindles. The extensive array of cytoplasmic microtubules present throughout maturation was not detectably stained by anti-XMAP230. Microinjection of anti-XMAP230 locally disrupted the organization and acetylation of microtubules in stage VI oocytes, and reduced the re-acetylation of microtubules during recovery from cold-induced microtubule disassembly. Subsequent maturation of oocytes injected with anti-XMAP230 resulted in defects in the assembly of the transient microtubules array and first meiotic spindle. These observations suggest that XMAP230 is required for the stabilization and organization of cytoplasmic and spindle microtubules in Xenopus oocytes and eggs.

A propagated wave of MPF activation accompanies surface contraction waves at first mitosis in Xenopus

During the period of mitosis, two surface contraction waves (SCWs) progress from the animal to vegetal poles of the Xenopus egg. It has been shown that these SCWs occur in parallel with the activation of MPF and with its subsequent inactivation in the animal and vegetal hemispheres, suggesting that they are responses to propagated waves of MPF activity across the egg. We have analysed the mechanism of MPF regulation in different regions of the egg in detail in relation to SCW progression. The distributions of histone HI kinase activity and of Cdc2 and cyclin B (the catalytic and regulatory subunits of MPF) were followed by dissection of intact eggs following freezing and in cultured fragments separated by ligation. Cdc2 was found to be distributed evenly throughout the egg cytoplasm. Loss of phosphorylated (inactive) forms of Cdc2 coincided spatially with the wave of MPF activation, while cyclin B2 accumulation occurred in parallel in animal and vegetal regions. In ligated vegetal pole fragments no MPF activation or Cdc2 dephosphorylation were detectable. A wave of cyclin B destruction that occurred in concert with the second SCW was also blocked. Taken together these results indicate that the triggering mechanism for MPF activation requires components specific to the animal cytoplasm, acting via Cdc2 dephosphorylation, and that MPF activation subsequently propagates autocatalytically across the egg. SCW progression in the vegetal hemisphere was followed directly by time-lapse videomicroscopy of subcortical mitochondrial islands. The first SCW traversed the vegetal pole at the time of MPF activation in this region. Like MPF activation and inactivation, SCWs were blocked in the vegetal region by ligation. These observations reinforce the hypothesis that the first SCW is a direct consequence of the MPF activation wave. It may reflect depolymerisation of the subcortical microtubule network since it coincided exactly with the arrest of the microtubule-dependent movement of ‘cortical rotation’ and was related in direction in most eggs. The cyclin B destruction wave and associated cortical contraction of the second SCW may be localised downstream consequences of the MPF activation wave, or they may propagate independently from the animal cytoplasm.

Microtubule assembly in clarified Xenopus egg extracts

Crude cytoplasmic extracts made from Xenopus eggs have proven to be uniquely useful in the studies of the mechanism of spindle microtubule assembly dynamics and chromosome movement during progression through the cell cycle. We examined microtubule dynamic instability in the Xenopus system using video-enhanced differential interference contrast microscopy (VE-DIC), which required high-speed centrifugation in order to clarify crude Xenopus extracts of refractile particles. Surprisingly, the resultant clarified, undiluted extracts exhibited virtually no microtubule catastrophe, even in the presence of high MPF (cyclin B/p34cdc2 kinase) activity and mitogen-activated protein (MAP) kinase activity, a down-stream kinase also implicated in regulating microtubule dynamics. Microtubule elongation occurred at plus ends, and interphase microtubules grew at 17-30 microns/min while metaphase [meiotic, myelin basic protein kinase activity which is diagnostic for cytostatic factor (CSF)-arrested] microtubules grew at about 10 microns/min. Plus-end shortening rates for both interphase and metaphase extracts were > 50 microns/min. Addition of okadaic acid, a protein phosphatase inhibitor known to activate MAP kinase activity and cause an increase in microtubule turnover in extracts made from sea urchin eggs, had no effect on microtubule catastrophe in either interphase or metaphase Xenopus extracts. In addition, the microtubules assembled in interphase extracts were less sensitive to dilution than those in metaphase. This study is the first to describe the dynamic instability of microtubules in Xenopus extracts without the addition of exogenous tubulins or other buffer contaminants.

Cell cycle-dependent phosphorylation of the 77 kDa echinoderm microtubule-associated protein (EMAP) in vivo and association with the p34cdc2 kinase

The most abundant microtubule-associated protein in sea urchin eggs and embryos is the 77 kDa echinoderm microtubule-associated protein (EMAP). EMAP localizes to the mitotic spindle as well as the interphase microtubule array and is a likely target for a cell cycle-activated kinase. To determine if EMAP is phosphorylated in vivo, sea urchin eggs and embryos were metabolically labeled with 32PO4 and a monospecific antiserum was used to immunoprecipitate EMAP from 32P-labeled eggs and embryos. In this study, we demonstrate that the 77 kDa EMAP is phosphorylated in vivo by two distinct mechanisms. In the unfertilized egg, EMAP is constitutively phosphorylated on at least five serine residues. During the first cleavage division following fertilization, EMAP is phosphorylated with a cell cycle-dependent time course. As the embryo enters mitosis, EMAP phosphorylation increases, and as the embryo exits mitosis, phosphorylation decreases. During mitosis, EMAP is phosphorylated on 10 serine residues and two-dimensional phosphopeptide mapping reveals a mitosis-specific site of phosphorylation. At all stages of the cell cycle, a 33 kDa polypeptide copurifies with the 77 kDa EMAP, regardless of phosphorylation state. Antibodies against the cdc2 kinase were used to demonstrate that the 33 kDa polypeptide is the p34cdc2 kinase. The p34cdc2 kinase copurifies with the mitotic apparatus and immunostaining indicates that the p34cdc2 kinase is concentrated at the spindle poles. Models for the interaction of the p34cdc2 kinase and the 77 kDa EMAP are presented.

Phosphorylation of microtubule-associated proteins MAP2 and MAP4 by the protein kinase p110mark. Phosphorylation sites and regulation of microtubule dynamics

The phosphorylation of microtubule-associated proteins (MAPs) is thought to be a key factor in the regulation of microtubule stability. We have shown recently that a novel protein kinase, termed p110 microtubule-affinity regulating kinase ("MARK"), phosphorylates microtubule-associated protein tau at the KXGS motifs in the region of internal repeats and causes the detachment of tau from microtubules (Drewes, G., Trinczek, B., Illenberger, S., Biernat, J., Schmitt-Ulms, G., Meyer, H.E., Mandelkow, E.-M., and Mandelkow, E. (1995) J. Biol. Chem. 270, 7679-7688). Here we show that p110mark phosphorylates analogous KXGS sites in the microtubule binding domains of the neuronal MAP2 and the ubiquitous MAP4. Phosphorylation in vitro leads to the dissociation of MAP2 and MAP4 from microtubules and to a pronounced increase in dynamic instability. Thus, the phosphorylation of the repeated motifs in the microtubule binding domains of MAPs by p110mark might provide a mechanism for the regulation of microtubule dynamics in cells.

Microtubules and microtubule-associated proteins

Microtubule research is becoming increasingly diverse, reflecting the many isoforms and modifications of tubulin and the many proteins with which microtubules interact. Recent advances are particularly visible in four areas: microtubule motor proteins (their structures, stepping modes, and forces); microtubule nucleation (the roles of centrosomes and gamma-tubulin); tubulin folding (mediated by cytoplasmic chaperones); and the expanding list of microtubule-associated proteins, knowledge of their phosphorylation states, and information on their effects on microtubule dynamics.

Effect on microtubule dynamics of XMAP230, a microtubule-associated protein present in Xenopus laevis eggs and dividing cells

The reorganization from a radial [corrected] interphase microtubule (MT) network into a bipolar spindle at the onset of mitosis involves a dramatic change in MT dynamics. Microtubule-associated proteins (MAPs) and other factors are thought to regulate MT dynamics both in interphase and in mitosis. In this study we report the purification and functional in vitro characterization of a 230-KD MAP from Xenopus egg extract (XMAP230). This protein is present in eggs, oocytes, testis and a Xenopus tissue culture cell line. It is apparently absent from non-dividing cells in which an immunologically related 200-kD protein is found. XMAP230 is composed of two isoforms with slightly different molecular masses and pIs. It is localized to interphase MTs, dissociates from MTs at the onset of prophase and specifically binds to spindle MTs during metaphase and anaphase. The dissociation constant of XMAP230 is 500 nM, the stoichiometry of binding to MTs is between 1:8 and 1:4, and the in vivo concentration is approximately 200 nM. Both isoforms are phosphorylated and have reduced affinity for microtubules in mitotic extracts. Analysis of the effect of XMAP230 on MT dynamics by video microscopy shows that it increases the growth rate, decreases the shrinking rate of MTs and strongly suppresses catastrophes. These results suggest that in vivo, XMAP230 participates in the control of the MT elongation rate, stabilizes MTs and locally modulates MT dynamics during mitosis.

XMAP from Xenopus eggs promotes rapid plus end assembly of microtubules and rapid microtubule polymer turnover

We have used video-enhanced DIC microscopy to examine the effects of XMAP, a Mr 215,000 microtubule-associated protein from Xenopus eggs (Gard, D.L., and M. W. Kirschner. 1987. J. Cell Biol. 105:2203-2215), on the dynamic instability of microtubules nucleated from axoneme fragments in vitro. Our results indicate that XMAP substantially alters the parameters of microtubule assembly at plus ends. Specifically, addition of 0.2 microM XMAP resulted in (a) 7-10-fold increase in elongation velocity, (b) approximately threefold increase in shortening velocity, and (c) near elimination of rescue (the switch from rapid shortening to elongation). Thus, addition of XMAP resulted in the assembly of longer, but more dynamic, microtubules from the plus ends of axonemes which upon catastrophe disassembled back to the axoneme nucleation site. In agreement with previous observations (Gard, D.L., and M. W. Kirschner. 1987. J. Cell Biol. 105:2203-2215), the effects of XMAP on the minus end were much less dramatic, with only a 1.5-3-fold increase in elongation velocity. These results indicate that XMAP, unlike brain MAPs, promotes both polymer assembly and turnover, and suggests that the interaction of XMAP with tubulin and the function of XMAP in vivo may differ from previously characterized MAPs.