Antibodies to synthetic peptides from the tubulin regulatory domain interact with tubulin and microtubules

Microtubule Dynamics in Neuronal Development, Plasticity, and Neurodegeneration

Neurons are the basic information-processing units of the nervous system. In fulfilling their task, they establish a structural polarity with an axon that can be over a meter long and dendrites with a complex arbor, which can harbor ten-thousands of spines. Microtubules and their associated proteins play important roles during the development of neuronal morphology, the plasticity of neurons, and neurodegenerative processes. They are dynamic structures, which can quickly adapt to changes in the environment and establish a structural scaffold with high local variations in composition and stability. This review presents a comprehensive overview about the role of microtubules and their dynamic behavior during the formation and maturation of processes and spines in the healthy brain, during aging and under neurodegenerative conditions. The review ends with a discussion of microtubule-targeted therapies as a perspective for the supportive treatment of neurodegenerative disorders.

Human natural resistance-associated macrophage protein is a new type of microtubule-associated protein

Natural resistance-associated macrophage protein 1 (NRAMP1) is a putative membrane protein that dominates natural resistance to infection. An NRAMP1-glutathione S-transferase fusion protein was used to test the ability of the NRAMP1 NH2-terminal domain to bind to taxol-stabilized microtubules. Co-sedimentation analysis showed that the fusion protein binds to microtubules. Although the NH2-terminal domain of the NRAMP1 molecule has structural homology with the Pro-rich region of microtubule-associated protein 4 (MAP4), the presence of the MAP4 microtubule-binding domain fragment had little effect on the binding of the fusion protein to microtubules.

The interaction of Mip-90 with microtubules and actin filaments in human fibroblasts

The novel microtubule-interacting protein Mip-90 was originally isolated from HeLa cells by using affinity columns of agarose derivatized with peptides from the C-terminal regulatory domain on beta-tubulin. Biochemical and immunocytochemical data have suggested that the association of Mip-90 with the microtubule system contributes to its cellular organization. Here we report the interaction patterns of Mip-90 with microtubules and actin filaments in interphase human fibroblasts. A polyclonal monospecific antibody against Mip-90 was used for immunofluorescence microscopy analysis to compare the distribution patterns of this protein with tubulin and actin. A detailed observation of fibroblasts revealed the colocalization of Mip-90 with microtubules and actin filaments. These studies were complemented with experiments using cytoskeleton-disrupting drugs which showed that colocalization patterns of Mip-90 with microtubules and actin filaments requires the integrity of these cytoskeletal components. Interestingly, a colocalization of Mip-90 with actin at the leading edge of fibroblasts grown under subconfluency was observed, suggesting that Mip-90 could play a role in actin organization, particularly at this cellular domain. Mip-90 interaction with actin polymers was further supported in vitro by cosedimentation and immunoprecipitation experiments. The cosedimentation analysis indicated that Mip-90 bound to actin filaments with an association constant Ka = 1 x 10(6) M-1, while an stoichiometry Mip-90/actin of 1:12 mol/mol was calculated. Western blots of the immunoprecipitates revealed that Mip-90 associated to both actin and tubulin in fibroblasts extracts. These studies indicate that Mip-90, described as a microtubule-interacting protein, also bears the capacity to interact with the microfilament network, suggesting that it may play a role in modulating the interactions between these cytoskeletal filaments in nonneuronal cells.

Immunological characterization of epitopes on tau of Alzheimer's type and chemically modified tau

The microtubule-associated protein tau is the main structural component of paired helical filaments (PHFs), which in turn are one of the major aberrant polymers found in Alzheimer's disease. Immunological studies were carried out using site-directed monoclonal and polyclonal antibodies that recognize tubulin binding epitopes on tau, to further understand the mechanisms of tau self-association into PHFs. Tau protein was subjected to either carbamoylation with potassium cyanate (KCNO) or glycation with glucose, and the immunoreactivity of the chemically-modified protein with these antibodies was compared with tau derived from paired helical filaments and with normal brain tau. The data on the immunoblot patterns of tau isoforms and the ELISA titration curves revealed significant differences between the modified tau and normal controls. However, the Western blot patterns of immunoreactive tau from the chemically-modified protein and from Alzheimer brains were similar. The data on the differences in the electrophoretic profiles and Western blots of normal brain tau as compared with solubilized paired helical filaments, insoluble tangles and tau proteins of the Alzheimer's type, provide new clues to understand the anomalous interactions of tau in Alzheimer's disease.

Tubulin domains for the interaction of microtubule associated protein DMAP-85 from Drosophila melanogaster

The interaction of microtubule associated proteins (MAPs) with the microtubule system has been characterized in depth in neuronal cells from various mammalian species. These proteins interact with well-defined domains within the acidic tubulin carboxyl-terminal regulatory region. However, there is little information on the mechanisms of MAPs-tubulin interactions in nonmammalian systems. Recently, a novel tau-like protein designated as DMAP-85 has been identified in Drosophila melanogaster, and the regulation of its interactions with cytoskeletal elements was analyzed throughout different developmental stages of this organism. In this report, the topographic domains involved in the binding of DMAP-85 with tubulin heterodimer were investigated. Affinity chromatography of DMAP-85 in matrixes of taxol-stabilized microtubules showed the reversible interaction of DMAP-85 with domains on the microtubular surface. Co-sedimentation studies using the subtilisin-treated tubulin (S-tubulin) indicated the lack of association of DMAP-85 to this tubulin moiety. Moreover, studies on affinity chromatography of the purified 4 kDa C-terminal tubulin peptide bound to an affinity column, confirmed that DMAP-85 interacts directly with this regulatory domain on tubulin subunits. Further studies on sequential affinity chromatography using a calmodulin affinity column followed by the microtubule column confirmed the similarities in the interaction behaviour of DMAP-85 with that of tau. DMAP-85 associated to both calmodulin and the microtubular polymer. These studies support the idea that the carboxyl-terminal region on tubulin constitutes a common binding domain for most microtubule-interacting proteins.

Syk, activated by cross-linking the B-cell antigen receptor, localizes to the cytosol where it interacts with and phosphorylates alpha-tubulin on tyrosine

Syk (p72syk) is a 72-kDa, nonreceptor, protein-tyrosine kinase that becomes tyrosine-phosphorylated and activated in B lymphocytes following aggregation of the B-cell antigen receptor. To explore the subcellular location of activated Syk, anti-IgM-activated B-cells were fractionated into soluble and particulate fractions by ultracentrifugation. Activated and tyrosine-phosphorylated Syk was found predominantly in the soluble fraction and was not associated with components of the antigen receptor. Similarly, the activated forms of Syk and its homolog, ZAP-70, were found in soluble fractions prepared from pervanadate-treated Jurkat T-cells. A 54-kDa protein that co-immunoprecipitated with Syk from the soluble fraction of activated B-cells was identified by peptide mapping as alpha-tubulin. alpha-Tubulin was an excellent in vitro substrate for Syk and was phosphorylated on a single tyrosine present within an acidic stretch of amino acids located near the carboxyl terminus. alpha-Tubulin was phosphorylated on tyrosine in intact cells following aggregation of the B-cell antigen receptor in a reaction that was inhibited by the Syk-selective inhibitor, piceatannol. Thus, once activated, Syk releases from the aggregated antigen receptor complex and is free to associate with and phosphorylate soluble proteins including alpha-tubulin.

Subpopulations of tau interact with microtubules and actin filaments in various cell types

It has been demonstrated that microtubule-associated proteins (MAPs) interact with tubulin in vitro and in vivo. However, there is no clear evidence on the possible roles of the interactions of MAPs in vivo with other cytoskeletal components in maintaining the integrity of the cell architecture. To address this question we extracted the neuronal cytoskeleton from brain cells and studied the selective dissociation of specific molecular isospecies of tau protein under various experimental conditions. Tau, and in some cases MPA-2, were analysed by the use of anti-idiotypic antibodies that recognize epitopes on their tubulin binding sites. Fractions of microtubule-bound tau isoforms were extracted with 0.35 M NaCl or after the addition of nocodazole to allow microtubule depolymerization. Protein eluted with this inhibitor contained most of the assembled tubulin dimer pool and part of the remaining tau and MAP-2. When the remaining cytoskeletal pellet was treated with cytochalasin D to allow depolymerization of actin filaments, only tau isoforms were extracted. Immunoprecipitation studies along with immunolocalization experiments in cell lines containing tau-like components supported the findings on the roles of tau isospecies as linkers between tubulin in the microtubular structure with actin filaments. Interestingly, in certain types of cells, antibody-reactive tau isospecies were detected by immunofluorescence with a discrete distribution pattern along actin filaments, which was affected by cytochalasin disruption of the actin filament network. These results suggest the possible in vivo roles of subsets of tau protein in modulating the interactions between microtubules and actin filaments.

ncd and kinesin motor domains interact with both alpha- and beta-tubulin

Motor domains of the Drosophila minus-end-directed microtubule (MT) motor protein ncd, were found to saturate microtubule binding sites at a stoichiometry of approximately one motor domain per tubulin dimer. To determine the tubulin subunit(s) involved in binding to ncd, mixtures of ncd motor domain and MTs were treated with the zero-length cross-linker 1-ethyl-3-(3-dimethylaminopropyl-carbodiimide) (EDC). EDC treatment generated covalently cross-linked products of ncd and alpha-tubulin and of ncd and beta-tubulin, indicating that the ncd motor domain interacts with both alpha- and beta-tubulin. When the Drosophila kinesin motor domain protein was substituted for the ncd motor domain, cross-linked products of kinesin and alpha-tubulin and of kinesin and beta-tubulin were produced. EDC treatment of mixtures of ncd motor domain and unassembled tubulin dimers or of kinesin motor domain and unassembled tubulin dimers produced the same motor-tubulin products generated in the presence of MTs. These results indicate that kinesin family motors of opposite polarity interact with both tubulin monomers and support a model in which some portion of each protein's motor domain overlaps adjacent alpha- and beta-tubulin subunits.

A 205 kDa protein from non-neuronal cells in culture contains tubulin binding epitopes

Microtubule-associated proteins (MAPs) interact with tubulin in vitro and in vivo. Despite that there is a large amount of information on the roles of these proteins in neurons, the data on non-neuronal MAPs or MAPs-related proteins is scarce. There is an increasing number of microtubule-interacting proteins that have been identified in different cultured cell lines, and some of them share common functional epitopes with the most well-known MAPs, MAP-2 and tau. In a search for tubulin-interacting proteins in non-neuronal cells we identified a 205 kDa protein in the monkey kidney Vero cells in culture, on the basis of immunological studies and affinity chromatography. This protein interacts with the C-terminal moiety of beta-tubulin and cosediments with taxol assembled microtubules, but it was not recovered after successive cycles of assembly and disassembly. The presence of neuronal MAPs such as MAP-1, MAP-2 and tau was not detected in these cells. Interestingly, the studies showed that the 205 kDa protein contained a tubulin binding motif which was recognized by site-directed antibodies that also tag tubulin binding epitopes on MAP-2 and tau. This characteristic led us to designate this protein as MBD-205, a component that shares binding domains with these MAPs, rather than as a marker of the MAPs family. On the other hand, immunofluorescence experiments using site-specific antibodies, i.e. MAP-reacting monoclonal anti-idiotypic reagent MTB6.22 and a polyclonal antibody to the second tau repeat, revealed a MBD-205 co-localization with membrane structures and microtubule-organizing centers in Vero cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Carboxyl terminal sequences of beta-tubulin involved in the interaction of HMW-MAPs. Studies using site-specific antibodies

After the finding of the involvement of the C-terminal moieties of tubulin subunits in the interaction of MAPs, different studies have focused on the substructure of the binding domains for the different MAPs. Current biochemical evidence point to the role of a low-homology sequence between alpha and beta-subunits within the conserved region of the C-terminal domain of tubulin, in the binding of MAP-2 and tau. Another line of studies indicates that a site for interaction of the high molecular weight MAPs is located in the variable region defined by the glutamic-rich C-terminus of beta-tubulin. Here, we report the usefulness of idiotypic site-directed antibodies, produced by immunization with peptides from different beta-tubulin isoforms, to study both MAP-1 and MAP-2 binding sites on tubulin. On the basis of these results with site-specific antibodies along with previous structural information (Cross et al., 1991, Biochemistry 30: 4362-4366), we propose the role of consensus sequences, from the invariant beta-tubulin C-terminal domain in the binding of MAP-2 and from the variable domain in the interactions of MAP-1 and MAP-2.

Purification and characterization of the high molecule weight microtubule associated proteins from neonatal rat brain

The changes in the levels of microtubule-associated proteins (MAPs) during advanced embryonic stages, neonatal and adult organisms reflect the importance of these cytoskeletal proteins in relation to the morphogenesis of the central nervous system. MAP-1B is found in prenatal brains and it appears to have the highest levels in neonatal rat brains, being a developmentally-regulated protein. In this research, a fast procedure to isolate MAP-1B, as well as MAP-2 and MAP-3 from neonatal rat brains was designed, based on the differential capacity of poly L-aspartic acid to release MAPs during temperature-dependent cycles of microtubule assembly in the absence of taxol. The high molecular weight MAP-1B was recovered in the warm supernatants after microtubular protein polymerization in the presence of low concentrations of polyaspartic acid. Instead, MAP-2 and a 180 kDa protein with characteristics of MAP-3 remained associated to the polymer after the assembly. Further purification of MAP-1B was attained after phosphocellulose chromatography. Isolation of MAP-2 isoforms together with MAP-3 was achieved on the basis of their selective interactions with calmodulin-agarose affinity columns. In addition, MAP-2 and MAP-3 were also purified on the basis of their capacities to interact with the tubulin peptide beta-II (422-434) derivatized on an Affigel matrix. However, MAP-1B did not interact with the beta-II tubulin fragment, but it showed interaction with the Affigel-conjugated beta-I (431-444) tubulin peptide. The different MAPs components were characterized by western blots using specific monoclonal antibodies.(ABSTRACT TRUNCATED AT 250 WORDS)

A tau fragment containing a repetitive sequence induces bundling of actin filaments

Much indirect evidence suggests that the interconnections of actin microfilaments with the microtubule system are mediated by microtubule-associated proteins (MAPs). In this study we provide new data to support the interaction of a specific tubulin-binding domain on tau with actin in vitro. In actin polymerization assays, the synthetic peptide VRSKIGSTENLKHQPGGG, corresponding to the first repetitive sequence of tau protein, increased turbidity at 320 nm in a dose-dependent fashion. A salient feature of the tau peptide-induced assembly process is the formation of a large amount of actin filament bundles, as revealed by electron microscopic analysis. An increase in the tau peptide concentration resulted in a proportional increase in the bundling of actin filaments. It is interesting that a gradual decrease of pH within the range 7.6-4.7 resulted in a higher effect of tau peptide in promoting bundles of actin filaments. A similar pH-dependent effect was observed for tau protein-induced bundling. An analysis of the mechanisms that operate in the peptide induction of actin filament bundles suggests the involvement of electrostatic forces, because the neutralization of epsilon-aminolysyl residues by selective carbamoylation resulted in a complete loss of the peptide induction of actin bundles. The data suggest that a tau repetitive sequence (also found in MAP-2 and MAP-4) containing a common tubulin binding motif may constitute a functional domain on tau for the dynamics of the interconnections between actin filaments and microtubules.

A tau-like protein interacts with stress fibers and microtubules in human and rodent cultured cell lines

The cytoskeletal integrity of human and rodent cell lines was analyzed using site-directed monoclonal antibodies prepared from hybridomas. Secreting hybridomas were produced by immunizing mice with synthetic peptides from the C-terminal domain of the beta II-tubulin isotype, beta II(422–434), YQQYQDATADEQG, and the first imperfect repeat from brain tau, Tau-I(187–204), VRSKIGSTENLKHQPGGG. Two hybridomas were selected for this work: MTB6.22, an anti-idiotypic monoclonal antibody, which was obtained from a mouse immunized with the beta II-peptide and recognizes specific tubulin-binding domains on MAP-2 and tau; and Tau-I/1, which recognizes the repetitive binding sequences on tau and MAP-2. Immunoblots of cytoskeletal protein preparations from the five different tumor cell lines studied, showed the interaction of the site-directed antibodies MTB6.22 and Tau-I/1 with a group of proteins that co-migrate with brain tau. Immunoreactive tau components were also identified using an anti-tau monoclonal antibody (clone Tau-2), and several polyclonal anti-tau antibodies that interact with tau epitopes, other than those of the tubulin-binding domains. These tau-like proteins bound to a calmodulin-Sepharose affinity column and were eluted using 2 mM EGTA. Interestingly, washing the extracted cytoskeleton pellet with 5 × 10(−3) M Ca2+ for short periods of time selectively released the tau-like protein components, whilst most of the other cytoskeletal proteins remained in the pellet. On the other hand, immunofluorescence microscopy of detergent-extracted cells showed immunostaining of MAP components that appear to be co-localized in a discrete dot-like distribution along the stress fibers, which were revealed using rhodamine-phallacidin. Further support for the specificity of tau interaction with sites on tubulin and actin polymers was obtained with double-immunofluorescence, using the MAP-reactive monoclonal antibody MTB6.22 and a polyclonal antibody to a tubulin peptide containing part of the tau-binding domain on tubulin. Considering the anti-idiotypic nature of the MTB6.22 monoclonal antibody, our studies indicate that, in all the cell lines analyzed, a tau-like protein component is involved in mediating the interaction of both actin and tubulin polymers.

The antineoplastic agent estramustine and the derivative estramustine-phosphate inhibit secretion of interleukin-3 in leukemic cells. Possible roles of MAPs

The antineoplastic drug estramustine is an adduct of estradiol and nor-nitrogen mustard. It has been shown that this drug interferes with microtubule assembly, an effect mediated by estramustine interaction with microtubule-associated proteins (MAPs). In the present report we demonstrate that estramustine and the phosphorylated derivative of the drug, estramustine-phosphate, inhibit the secretion of interleukin-3 by WEHI-3B cells. These studies also show that the estramustine derivative specifically interacts with a MAPs component found in these cells, which exhibited characteristics ressembling those of tau protein isoforms. Western blots using a unique monoclonal antibody MTB6.22 that recognizes microtubule-binding domains on MAPs, indicated that this WEHI protein factor contained the antigenic determinant that are functionally significant for microtubule assembly. ELISA assays using this antibody, also showed a decrease in the levels of the immunoreactive protein in WEHI cells after treatment with EMP. Interestingly, it has been recently described that the action of estramustine-phosphate is mediated by a direct interaction with MAP-binding sites on the microtubule surface, which are recognized by the site-specific monoclonal antibody. These findings together with immuno-precipitation experiments using anti-interleukin-3 antibodies and the inhibitory effect of the estramustine derivative on WEHI secretion process suggest that this anti-mitotic agent may block IL-3 secretion by a mechanism involving its interaction with a 'tau-like' MAPs component present in these cells.

Specific macromolecular interactions between tau and the microtubule system

The microtubule-associated protein Tau, a major component of brain microtubules, shares common repeated C-terminal sequences with the high molecular-weight protein MAP-2. It has been shown that tau peptides V187-G204 and V218-G235, representing two main repeats, induced brain tubulin assembly in a concentration-dependent fashion. The specific roles of these repeats in the interaction of tau with microtubules, and its antigenic nature were investigated using synthetic tau peptides and site-directed monoclonal antibodies. Tau peptides appeared to compete with MAP-2 incorporation into assembled microtubules. The interactions of the tau fragments with beta-tubulin peptides bearing the tau binding domain on tubulin were analyzed by fluorescence spectroscopy. The specificity of the binding was further demonstrated by the reactivity of tau and the tau peptides with a monoclonal anti-idiotypic antibody produced after immunization with the beta-II(422-434) tubulin peptide, as assessed by enzyme-linked immunoassay. Western blots confirmed the interaction of tau with the monoclonal antibody. In addition, immunoassays revealed a competition between the MAP-reacting monoclonal antibody and the tubulin peptide beta-II(422-434) for their interaction with the tau molecule.

Microtubule assembly protects the region 28-38 of the beta-tubulin subunit

Polyclonal antibodies have been raised against the peptide 28-38 of the beta-subunit of the tubulin heterodimer in order to study the accessibility of this region in the tubulin heterodimer and in various tubulin assemblies. These antibodies were specific for all beta-tubulin subunits, except for beta'-tubulin isotypes, and did not recognize the alpha-tubulin subunit. The 28-38 region does not play a role in the interaction between the alpha- and beta-subunits since it was accessible to the antibodies on the native heterodimer. The accessibility of the antibodies was not modified by several microtubular poisons. In contrast, in all tubulin assemblies obtained in the presence of microtubular associated proteins, the region 28-38 was not available to the antibodies. These antibodies did not react with microtubules or tubulin spirals assembled either from microtubule proteins or from pure tubulin when these tubulin assemblies were probed in the absence of free tubulin after centrifugation on glass coverslips. In addition, antibodies failed to interact with the microtubule cytoskeleton in cultured Ptk2 cells indicating that the 28-38 region of beta-tubulin is also protected in cellular structures. These observations suggest that the 28-38 region of the beta-tubulin subunit is either located in a zone of interaction between two successive tubulin dimers within a protofilament or hidden by an allosteric conformational change which occurs during tubulin assembly.

The solution conformation of tubulin-beta(422-434)-NH2 and its Nac-DATADEQG-NH2 fragment based on NMR

The solution conformation of tubulin-beta(422-434)-NH2 (YQQYQDATADEQG-NH2) and its Nac-DATADEQG-NH2 fragment has been studied by two-dimensional 1H-nmr spectroscopy in CD3OH/H2O (90/10 v/v) at neutral and low pH. The 13 amino acid peptide is a segment of the C-terminal region of tubulin, and is directly involved in the selective binding site with microtubule-associated proteins MAP-2 and the tau protein. Based on correlated spectroscopy, total correlation spectroscopy, and rotating frame nuclear Overhauser effect spectroscopy experiments, a complete assignment of all proton resonances was achieved, and the conformation of the backbone could be deduced from coupling constants, NH temperature coefficients, and nuclear Overhauser effects. The spectroscopic evidence indicates that the T8-Q12 section of both molecules forms one complete alpha-helical turn, stabilized by a NH (Q12)-C = O (T8) hydrogen bond. Furthermore, strong pH-dependent backfolding of the E11 side chain to its own NH proton was found. In addition, close proximity between the aromatic side chains of Y1, Y4, and the alpha-helical part, resulting in some substantial chemical shift changes when comparing the entire 13-mer with the octamer, could be explained in terms of a nonclassical kink in the DATA section. The conformational space is dominated by extended structures and the nonextended conformers are only a minor, yet spectroscopically clearly discernible entity. The presence of the alpha-helical region at the C-terminus of the 13-mer is important because binding studies of this peptide with MAP-2 indicate that the D10-E11-Q12-G13 fragment is critical for the binding interaction.

Multiple epitope specificity of monoclonal antibodies to a single synthetic peptide: use in the characterization of the GP IIb-IIIa binding domain of von Willebrand factor

Monoclonal antibodies have been induced against the synthetic peptide with sequence Tyr-Glu-Val-Val-Thr-Gly-Ser-Pro-Arg-Gly-Asp-Ser-Gln-Ser-Ser. This peptide represents residues Glu1737-Ser1750 of the mature von Willebrand factor (vWF) subunit and contains the sequence Arg-Gly-Asp, thought to be important in mediating binding to the platelet receptor glycoprotein (GP) IIb-IIIa complex. Twelve antibodies were obtained, eight of which bound to native vWF as well as to the peptide immunogen insolubilized onto agarose beads. These antibodies defined at least three distinct epitopes, as demonstrated by antibody interaction with peptides having a single phenylalanine substitution at each position in the sequence. In particular, two antibodies bound to epitopes on vWF that included one or more of the three residues (arginine, glycine, aspartic acid) thought to be involved in binding to GP IIb-IIIa, whereas one antibody bound to an epitope that did not include any of those residues. Nevertheless, the three antibodies cross-reacted with each other, a finding explained by the fact that the corresponding epitopes had at least two residues in common, namely Gly1741 and Ser1742. In spite of the cross-reactivity for binding to vWF, only the two antibodies whose epitopes included residues in the Arg-Gly-Asp sequence inhibited vWF interaction with GP IIb-IIIa. The third antibody had no inhibitory effect even though it was bound to an epitope located at a distance of only few residues on the amino terminal side of Arg-Gly-Asp. These results demonstrate that monoclonal antibodies raised against a single synthetic peptide with sequence limited to fifteen residues may exhibit distinct epitope specificity and may be used to define functional domains in macromolecules with a high degree of resolution.