Distribution of MAP-4 in cells and in adult and developing mouse tissues

Microtubule-associated protein 4 promotes epithelial mesenchymal transition in hepatocellular cancer cells via regulating GSK3β/β-catenin pathway

Metastasis is a major obstacle in the treatment of hepatocellular carcinoma (HCC). Microtubule-associated protein 4 (MAP4) plays an important role as a coordinator between microtubules and microfilaments. However, the role of MAP4 in HCC migration and epithelial mesenchymal transition (EMT) is unclear. We compared the protein and mRNA levels of MAP4 in human HCC and adjacent normal tissues using western blotting, immunohistochemistry and RT-qPCR. The migration and invasion abilities and the levels of EMT markers (E-Cadherin, N-Cadherin, Vimentin, and Snail) were compared between MAP4-knockdown and MAP4-overexpressed HCC cells. Finally, we examined whether β-catenin and glycogen synthase kinase 3β (GSK3β) are involved in the stimulatory effects of MAP4 on HCC migration, invasion and EMT. The results revealed that MAP4 levels were higher in the HCC tissues than in the normal hepatic tissues. More importantly, MAP4 knockdown suppressed migration and invasion abilities and EMT processes in HCC cells, which were confirmed by the stimulatory effects of MAP4 overexpression on EMT processes in HCC cells. Further evidence demonstrated that the up-regulation of β-catenin activity induced by the interaction between MAP4 and GSK3β possibly accounted for the pro-migration and pro-EMT effects of MAP4 on HCC cells. Taken together, these results suggest that MAP4 promotes migration, invasion, and EMT in HCC cells by regulating the GSK3β/β-catenin pathway.

Phenotypic consequences of beta1-tubulin expression and MAP4 decoration of microtubules in adult cardiocytes

In pressure-overload cardiac hypertrophy, microtubule network densification is one cause of contractile dysfunction. Cardiac transcriptional upregulation of beta1-tubulin rather than the constitutive beta4-tubulin and of microtubule-associated protein (MAP)4 accompanies hypertrophy, with extensive microtubule decoration by MAP4. Because MAP4 stabilizes microtubules, and because the isoform-variable carboxy terminus of beta-tubulin binds to MAP4, we wished to determine whether one or both of these proteins has etiologic significance for cardiac microtubule network densification. Recombinant adenoviruses encoding beta1-tubulin, beta4-tubulin, and MAP4 were used to infect isolated cardiocytes. Overexpressed MAP4 caused a shift of tubulin dimers to the polymerized fraction and formation of a dense, stable microtubule network. Overexpressed beta1- or beta4-tubulin had neither any independent effect on these variables nor any effect additive to that of simultaneously overexpressed MAP4. Results from transgenic mice with cardiac overexpression of beta1-tubulin or MAP4 were confirmatory, but unlike the effects of brief adenovirus-mediated MAP4 overexpression in isolated cardiocytes, MAP4 transgenic hearts showed a marked increase in total alpha- and beta-tubulin. Thus MAP4 overexpression caused increased tubulin expression, formation of stable microtubules, and altered microtubule network properties, such that MAP4 upregulation may be one cause for the dense, stable microtubule network characteristic of pressure-overloaded, hypertrophied cardiocytes.

Casein kinase 2 as a potentially important enzyme in the nervous system

Protein kinase CK2 is a ubiquitous and pleiotropic seryl/threonyl protein kinase which is highly conserved in evolution indicating a vital cellular role for this kinase. The holoenzyme is generally composed of two catalytic (alpha and/or alpha') and two regulatory (beta) subunits, but the free alpha/alpha' subunits are catalytically active by themselves and can be present in cells under some circumstances. Special attention has been devoted to phosphorylation status and structure of these enzymic molecules, however, their regulation and roles remain intriguing. Until recently, CK2 was believed to represent a kinase especially required for cell cycle progression in non-neural cells. At present, with respect to recent findings, four essential features suggest potentially important roles for this enzyme in specific neural functions: (1) CK2 is much more abundant in brain than in any other tissue; (2) there appear to be a myriad of substrates for CK2 in both synaptic and nuclear compartments that have clear implications in development, neuritogenesis, synaptic transmission, synaptic plasticity, information storage and survival; (3) CK2 seems to be associated with mechanisms underlying long-term potentiation in hippocampus; and (4) neurotrophins stimulate activity of CK2 in hippocampus. In addition, some data are suggestive that CK2 might play a role in processes underlying progressive disorders due to Alzheimer's disease, ischemia, chronic alcohol exposure or immunodeficiency virus HIV. The present review focuses mainly on the latest data concerning the regulatory mechanisms and the possible neurophysiological functions of this enzyme.

Mitotic phosphoepitopes precede paired helical filaments in Alzheimer's disease

We have shown previously that the TG-3 and MPM-2 antibodies recognize phosphoepitopes common to mitosis and degenerating neurons of Alzheimer's disease(AD) brain. Here, we have evaluated their occurrence in human brain biopsy tissue, and confirm that they are absent in mature neurons of adult brain, but reappear during neurodegeneration in AD. The TG-3 epitope appears ahead of the MPM-2 epitope and is distributed throughout the neuronal soma. Tau is the major TG-3 antigen in AD brain. The initial localization of MPM-2 immunoreactivity in primary dendrites, it's robust occurrence in granulovacuolar bodies, and the increased immunoreactivity with 300-350-kDa proteins, suggest MAPI B as a candidate MPM-2 antigen in AD. Production of mitotic phosphepitopes in more than one type of human neurodegenerative lesion implicates mitotic kinases as common mediators of neuronal death. Because mitotic phosphoepitopes appear before paired helical filaments, it is suggested that mitotic kinase activation triggers neurofibrillary tangle formation. Future studies will need to focus on factors influencing mitotic kinase activity, a point with potential for early diagnosis and disease abrogation.

Chicken microtubule-associated protein 4 (MAP4): a novel member of the MAP4 family

Chicken gizzard smooth muscle has often been used as a source of proteins of the contractile and cytoskeletal apparatus. In the present study, we isolated a hitherto unknown doublet of proteins, with apparent molecular weights of 200 kDa, from embryonic chicken gizzard and showed its association with the microtubules (MTs) and by immunofluorescence staining of cultured cells. Immunoblot analysis also revealed the ubiquitous expression of this protein in all embryonic chicken tissues examined. Molecular cloning techniques allowed its identification as the chicken homologue of the microtubule-associated protein 4 (MAP4), known from mammalian species, and revealed approximately 90% of its amino acid sequence. MAP4 is the major MAP of non-neuronal tissues and cross-species comparisons clearly demonstrated its highly conserved overall structure, consisting of a basic C-terminal MT-binding region and an acidic N-terminal projection domain of unknown function. Despite these conserved features, overall sequence homologies to its mammalian counterparts are rather low and focused to distinct regions of the molecule. Among these are a conserved 18-amino acid motif, which is known to mediate binding to MTs and a part of the MT-binding domain known as the proline-rich region, which is thought to be the regulatory domain of MAP4. The N-terminal 59 amino acids are a conserved and unique feature of the MAP4 sequence and might be an indication that MAP4 performs other functions besides the enhancement of MT assembly.

Microinjection of intact MAP-4 and fragments induces changes of the cytoskeleton in PtK2 cells

The molecular cloning and sequencing of microtubule-associated protein (MAP)-4 identified microtubule-binding repeats near the C-terminus and a projection domain near the N-terminus. Although it is well known that MAP-4 stimulates the assembly of and stabilizes microtubules (MT) in vitro, the function of MAP-4 in vivo is still unclear. In this study, we examined the function of MAP-4 in the cytoskeleton both in vitro and in vivo. Intact MAP-4 was prepared from bovine adrenal cortex, and the truncated fragments of the N- and the C-terminal halves (named NR and PA4 fragments, respectively) were expressed in Escherichia coli and isolated. In vitro studies demonstrated that in a solution containing a physiological concentration of NaCl, intact MAP-4 and the PA4 fragment were bound to MT, but not to F-actin. The NR fragment was not bound to MT or to F-actin. We also examined the MT changes in PtK2 cells after they had been microinjected with intact MAP-4 and the truncated fragments of PA4 and NR. The injection of intact MAP-4 or PA4 into the cells induced an increase in the number of cytoplasmic MT, as well as MT bundling. The NR fragment did not affect the MT array. Injected MAP-4 and PA4 were associated with the increased MT. In addition, injection with MAP-4 and PA4 stabilized MT in relation to treatment with the MT-disrupting drug, nocodazole. These results indicated that intact MAP-4 and the PA4 fragment promoted MT assembly and stabilized MT, by binding to MT, in vivo as well as in vitro. Further, the injection of the PA4 fragment induced an increase in stress fibers. However, these proteins did not show any association with the stress fibers. Our results suggest that there is an indirect effect of MAP-4 on stress fibers rather than a direct interaction between MAP-4 and stress fibers.

Microtubule-associated proteins in developing oligodendrocytes: transient expression of a MAP2c isoform in oligodendrocyte precursors

The morphological differentiation of oligodendrocytes is characterized by the formation of multiple, microtubule-rich processes which endow these cells with the ability to myelinate many axons simultaneously. Since microtubule-associated proteins (MAPs) strongly influence the structure and function of microtubules, we have investigated their expression in cultured differentiating oligodendrocytes in order to gain insights into MAP function during process formation and stabilization. MAP1B has been compared with two other structural MAPs: MAP4, which is an ubiquitously expressed protein, and MAP2, which hitherto was thought to be confined to neurons and reactive astrocytes. Immunofluorescence microscopy showed that the colocalization of MAP4 with microtubules in oligodendrocyte processes is not as extensive as found previously for MAP1B (Vouyiouklis and Brophy: J Neurosci Res 35:257-267, 1993). Nevertheless, like MAP1B, the expression of MAP4 increases during oligodendrocyte differentiation. In contrast, the expression of MAP2 is transiently elevated in preoligodendrocytes but declines precipitously at the onset of terminal differentiation. Cells of the oligodendrocyte lineage exclusively express a novel isoform of MAP2c which is primarily localized in the cell bodies of preoligodendrocytes. This suggests that MAP2c assists in the initiation of process extension rather than in the stabilization of microtubules in the cytoplasm-filled membranous extensions of mature cells. MAP-tau was not expressed at any developmental stage by oligodendrocytes. The distinct subcellular localizations and patterns of developmental expression of MAP1B, MAP4, and MAP2c suggest that these MAPs have different roles in the regulation of the microtubule network during the differentiation of myelin-forming oligodendrocytes.

Identification and molecular characterization of E-MAP-115, a novel microtubule-associated protein predominantly expressed in epithelial cells

A novel microtubule-associated protein (MAP) of M(r) 115,000 has been identified by screening of a HeLa cell cDNA expression library with an anti-serum raised against microtubule-binding proteins from HeLa cells. Monoclonal and affinity-purified polyclonal antibodies were generated for the further characterization of this MAP. It is different from the microtubule-binding proteins of similar molecular weights, characterized so far, by its nucleotide-insensitive binding to microtubules and different sedimentation behavior. Since it is predominantly expressed in cells of epithelial origin (Caco-2, HeLa, MDCK), and rare (human skin, A72) or not detectable (Vero) in fibroblastic cells, we name it E-MAP-115 (epithelial MAP of 115 kD). In HeLa cells, E-MAP-115 is preferentially associated with subdomains or subsets of perinuclear microtubules. In Caco-2 cells, labeling for E-MAP-115 increases when they polarize and form blisters. The molecular characterization of E-MAP-115 reveals that it is a novel protein with no significant homologies to other known proteins. The secondary structure predicted from its sequence indicates two domains connected by a putative hinge region rich in proline and alanine (PAPA region). E-MAP-115 has two highly charged regions with predicted alpha-helical structure, one basic with a pI of 10.9 in the NH2-terminal domain and one neutral with a pI of 7.6 immediately following the PAPA region in the acidic COOH-terminal half of the molecule. A novel microtubule-binding site has been localized to the basic alpha-helical region in the NH2-terminal domain using in vitro microtubule-binding assays and expression of mutant polypeptides in vivo. Overexpression of this domain of E-MAP-115 by transfection of fibroblasts lacking significant levels of this protein with its cDNA renders microtubules stable to nocodazole. We conclude that E-MAP-115 is a microtubule-stabilizing protein that may play an important role during reorganization of microtubules during polarization and differentiation of epithelial cells.

Mouse microtubule-associated protein 4 (MAP4) transcript diversity generated by alternative polyadenylation

Mouse microtubule-associated protein 4 (MAP4) is a protein that co-locates with microtubules in vivo. It is encoded by a single-copy gene that expresses multiple transcripts in most cell types [West et al., J. Biol. Chem. 266 (1991) 21886-21896]. This report describes the identification of two distinct 3'-untranslated regions (UTR) for MAP4 transcripts. The 3'-UTRs of the transcripts are identical up to the site of polyadenylation of the shorter mRNA. The longer transcript contains an additional 775 nucleotides after the first polyadenylation site. Both poly(A) tails follow the canonical polyadenylation site motif, AAUAAA. These data show that two different UTRs arise as a result of alternative polyadenylation site usage. Northern blots of RNA from different tissues probed with coding sequence show hybridization to the common 5.5- and 6.5-kb transcripts, whereas blots probed with sequence unique to the longer 3'-UTR show hybridization only to the 6.5-kb band. Both transcripts are found within the same cell type. In addition, muscle contains additional transcripts of 8 and 9 kb, of which only the 9-kb transcript hybridizes to the longer 3'-UTR probe.

Actin-binding and microtubule-associated proteins in the organ of Corti

Actin-binding and microtubule-associated proteins regulate microfilament and microtubule number, length, organization and location in cells. In freeze-dried preparations of the guinea pig cochlea, both actin and tubulin are found in the sensory and supporting cells of the organ of Corti. Fodrin (brain spectrin) co-localized with actin in the cuticular plates of both inner and outer hair cells and along the lateral wall of the outer hair cells. Alpha-actinin co-localized with actin in the cuticular plates of the hair cells and in the head and foot plates of the supporting cells. It was also found in the junctional regions between hair cells and supporting cells. Profilin co-localized with actin in the cuticular plates of the sensory hair cells. Myosin was detected only in the cuticular plates of the outer hair cells and in the supporting cells in the region facing endolymph. Gelsolin was found in the region of the nerve fibers. Tubulin is found in microtubules in all cells of the organ of Corti. In supporting cells, microtubules are bundled together with actin microfilaments and tropomyosin, as well as being present as individual microtubules arranged in networks. An intensely stained network of microtubules is found in both outer and inner sensory hair cells. The microtubules in the outer hair cells appear to course throughout the entire length of the cells, and based on their staining with antibodies to the tyrosinated form of tubulin they appear to be more dynamic structures than the microtubules in the supporting cells. The microtubule-associated protein MAP-2 is present only in outer hair cells within the organ of Corti and co-localizes with tubulin in these cells. No other MAPs (1,3,4,5) are present. Tau is found in the nerve fibers below both inner and outer hair cells and in the osseous spiral lamina. It is clear that the actin-binding and microtubule-associated proteins present in the cochlea co-localize with actin and tubulin and that they modulate microfilament and microtubule structure and function in a manner similar to that seen in other cell types. The location of some of these proteins in outer hair cells suggests a role for microfilaments and microtubules in outer hair cell motility.

Cell cycle-dependent changes in the dynamics of MAP 2 and MAP 4 in cultured cells

To examine the behavior of microtubule-associated proteins (MAPs) in living cells, MAP 4 and MAP 2 have been derivatized with 6-iodoacetamido-fluorescein, and the distribution of microinjected MAP has been analyzed using a low light level video system and fluorescence redistribution after photobleaching. Within 1 min following microinjection of fluoresceinated MAP 4 or MAP 2, fluorescent microtubule arrays were visible in interphase or mitotic PtK1 cells. After cold treatment of fluorescent MAP 2-containing cells (3 h, 4 degrees C), microtubule fluorescence disappeared, and the only fluorescence above background was located at the centrosomes; microtubule patterns returned upon warming. Loss of microtubule immunofluorescence after nocodozole treatment was similar in MAP-injected and control cells, suggesting that injected fluorescein-labeled MAP 2 did not stabilize microtubules. The dynamics of the MAPs were examined further by FRAP. FRAP analysis of interphase cells demonstrated that MAP 2 redistributed with half-times slightly longer (60 +/- 25 s) than those for MAP 4 (44 +/- 20 s), but both types of MAPs bound to microtubules in vivo exchanged with soluble MAPs at rates exceeding the rate of tubulin turnover. These data imply that microtubules in interphase cells are assembled with constantly exchanging populations of MAP. Metaphase cells at 37 degrees C or 26 degrees C showed similar mean redistribution half-times for both MAP 2 and MAP 4; these were 3-4 fold faster than the interphase rates (MAP 2, t1/2 = 14 +/- 6 s; MAP 4, t1/2 = 17 +/- 5 s). The extent of recovery of spindle fluorescence in MAP-injected cells was to 84-94% at either 26 or 37 degrees C. Although most metaphase tubulin, like the MAPs, turns over rapidly and completely under physiologic conditions, published work shows either reduced rates or extents of turnover at 26 degrees C, suggesting that the fast mitotic MAP exchange is not simply because of fast tubulin turnover. Exchange of MAP 4 bound to telophase midbodies occurred with dynamics comparable to those seen in metaphase spindles (t1/2 = approximately 27 s) whereas midbody tubulin exchange was slow (greater than 300 s). These data demonstrate that the rate of MAP exchange on microtubules is a function of time in the cell cycle.

Immunocytochemical localization of microtubule-associated proteins 1A and 2 in the rat retina

We have studied the immunocytochemical localization of microtubule-associated proteins (MAPs) in rat retinal cells. Using biochemical and immunochemical methods we have identified microtubule-associated protein 1 (MAP1) and microtubule-associated protein 2 (MAP2) as major MAPs in the rat retina. With indirect immunofluorescence microscopy, the inner plexiform layer and the ganglion cell layer were stained with both anti-MAP1A antibody and anti-MAP2 antibody. Cells at the inner margin of the inner nuclear layer were prominently stained with anti-MAP2, but not with anti-MAP1A. Thin section-immunoelectron microscopy using colloidal gold-labeled secondary antibodies revealed MAP1A and MAP2 staining in the neuronal processes of the inner plexiform layer. A filamentous network between the microtubules in the neurites was stained with both antibodies. The developmental course of expression of MAP1A and MAP2 in rat retina was studied by indirect immunofluorescence microscopy. Although MAP2 was already present at 1 day postnatal, MAP1A was not detected until 7 days postnatal. These results indicate that: (1) retinal neurons are heterogeneous in their expression of MAPs; (2) retinal ganglion cells show the same intracellular distribution of MAP1A and MAP2 as typical nerve cells such as motor neurons; and (3) MAP1A and MAP2 are differentially expressed in developing rat retina.

Distribution of phosphorylated spindle-associated proteins in the diatom Stephanopyxis turris

Mitotic spindles isolated from the diatom Stephanopyxis turris become thiophosphorylated in the presence of ATP gamma S at specific locations within the mitotic apparatus, resulting in a stimulation of ATP-dependent spindle elongation in vitro. Here, using indirect immunofluorescence, we compare the staining pattern of an antibody against thiophosphorylated proteins to that of MPM-2, an antibody against mitosis-specific phosphoproteins, in isolated spindles. Both antibodies label spindle poles, kinetochores, and the midzone. Neither antibody exhibits reduced labeling in salt-extracted spindles, although prior salt extraction inhibits thiophosphorylation in ATP gamma S. Furthermore, both antibodies recognize a 205 kd band on immunoblots of spindle extracts. Microtubule-organizing centers and mitotic spindles label brightly with the MPM-2 antibody in intact cells. These results show that functional mitotic spindles isolated from S. turris are phosphorylated both in vivo and in vitro. We discuss the possible role of phosphorylated cytoskeletal proteins in the control of mitotic spindle function.

Estramustine binds a MAP-1-like protein to inhibit microtubule assembly in vitro and disrupt microtubule organization in DU 145 cells

The twofold purpose of the study was (a) to determine if a MAP-1-like protein was expressed in human prostatic DU 145 cells and (b) to demonstrate whether a novel antimicrotubule drug, estramustine, binds the MAP-1-like protein to disrupt microtubules. SDS-PAGE and Western blots showed that a 330-kD protein was associated with microtubules isolated in an assembly buffer containing 10 microM taxol and 10 mM adenylylimidodiphosphate. After purification to homogeneity on an A5m agarose column, the 330-kD protein was found to promote 6 S tubulin assembly. Turbidimetric (A350), SDS-PAGE, and electron microscopic studies revealed that micromolar estramustine inhibited assembly promoted by the 330-kD protein. Similarly, estramustine inhibited binding of the 330-kD protein to 6-S microtubules independently stimulated to assemble with taxol. Immunofluorescent studies with beta-tubulin antibody (27B) and MAP-1 antibody (MI-AI) revealed that 60 microM estramustine (a) caused disassembly of MAP-1 microtubules in DU 145 cells and (b) removed MAP-1 from the surfaces of microtubules stabilized with 0.1 microM taxol. Taken together the data suggested that estramustine binds to a 330-kD MAP-1-like protein to disrupt microtubules in tumor cells.

Reactivation of spindle elongation in vitro is correlated with the phosphorylation of a 205 kd spindle-associated protein

Mitotic spindles isolated from the diatom Stephanopyxis turris consist of two half-spindles of closely interdigitating microtubules that slide relative to one another in the presence of ATP, reinitiating spindle elongation (anaphase B) in vitro. Purified spindles that have been exposed to ATP-gamma-S undergo ATP-dependent reactivation more readily than do control spindles. Thiophosphorylated proteins in such spindles are located in the spindle midzone, kinetochores, and a portion of the pole complex. One major thiophosphorylated peptide of 205 kd is detected in extracts prepared from spindles labeled with [35S]ATP-gamma-S, and is also localized in the spindle midzone by using an antibody that recognizes thiophosphorylated proteins. It is likely that this 205 kd peptide is either a positive regulator or mechanochemical transducer of microtubule sliding when it is in a phosphorylated state.