Mapping of distinct structural domains on microtubule-associated protein 2 by monoclonal antibodies

Chronic treatment with sabeluzole protects cultured rat brain neurons from the neurotoxic effects of excitatory amino acids

The neuroprotective properties of the cognitive enhancer sabeluzole were investigated in rat brain neuronal cultures derived from the hippocampal formation of 17-day-old rat embryos. Measurement of the neuronal cytoskeletal microtubule-associated protein, MAP2, was used to assess survival of neurons after exposure of neuronal cultures to glutamate. MAP2 was quantified in neuronal cell homogenates by means of an enzyme-linked immunosorbent assay (ELISA) using a mouse monoclonal MAP2 antibody, peroxidase-labeled goat anti-mouse Ig antiserum, and 2,2'-azido-di-[3-ethylbenz-thiazoline] sulphonate (ABTS) as substrate. Exposure of 7-day-old neuronal cultures to 1 mM glutamate for 16 hours led to a three-fold increase in released lactate dehydrogenase (LDH) and a 40% decrease in cellular MAP2 content. Acute treatment of neuronal cultures with 10 microM sabeluzole yielded a 40% drop in released LDH induced by glutamate. Cultures treated chronically with 0.1 microM sabeluzole on days 1 and 4 in culture showed, after 1 week in culture, a MAP2 content and total LDH activity that was not different from control cultures. A 16-hour exposure to 1 mM glutamate did not induce LDH release or changes in MAP2 levels in sabeluzole-treated cultures. A single treatment with 0.1 microM sabeluzole between day 1 to 5 induced a 70-80% drop in glutamate-induced released LDH in 7-day-old neuronal cultures. Full and partial neuronal protection after chronic sabeluzole treatment at 0.1 microM was also observed for neurotoxicity induced by 5 mM N-methyl-D-aspartate (NMDA) and 1 mM kainic acid or 30 microM veratridine, respectively. Within a series of compounds such as Ca++ and Na+ channel antagonists, glutamate receptor antagonists and various neurotransmitter receptor antagonists, sabeluzole, chronically given, were the most potent for inhibition of released LDH induced by 1 mM glutamate (IC50-value: 34 +/- 13 nM). In conclusion, chronic sabeluzole treatment protects cultured rat brain neurons from excitotoxic aggression.

Characterization and intracellular distribution of microtubule-associated protein 2 in differentiating human neuroblastoma cells

The use of a panel of monoclonal antibodies (mAbs) directed against different determinants of microtubule-associated protein 2 (MAP2) enabled us to identify two distinct high-molecular-mass MAP2 species (270 and 250 kDa) and a substantial amount of MAP2c (70 kDa) in human neuroblastoma cells. The 250-kDa MAP2 species appears to be confined to the human neuroblastoma cells and was not observed in microtubules (MTs) from bovine and rat brain, mouse neuroblastoma, or MTs from human cerebellum. A new overlay method was developed, which demonstrates binding of tubulin to human neuroblastoma high-molecular-mass MAP2 by exposing nitrocellulose-bound MT proteins under polymerization conditions to tubulin. Bound tubulin was detected with a mAb directed against beta-tubulin. The binding of tubulin to MAP2 could be abolished by a peptide homologous to positions 426-445 of the C-terminal region of beta-tubulin. Immunological cross-reactivity with several mAbs directed against bovine brain MAP2, taxol-promoted coassembly into MTs, and immunocytochemical visualization within cells were further criteria utilized to characterize these proteins as true MAPs. Indirect immunofluorescence with anti-MAP2 and anti-beta-tubulin mAbs demonstrated that there is a change in the spatial organization of MTs during induced cell differentiation, as indicated by the appearance of MT bundles and the redistribution of MAP2.

The RII subunit of cAMP-dependent protein kinase binds to a common amino-terminal domain in microtubule-associated proteins 2A, 2B, and 2C

Three products of the MAP2 gene are known: MAP2A and MAP2B (Mr approximately 200,000) and MAP2C (Mr 70,000). The structural relationship between these MAPs and the basis for their diversity in size are unknown. Previously, we found that a significant fraction of type II cAMP-dependent protein kinase was associated via its regulatory subunits with MAP2A and MAP2B. We now use an antibody prepared against the microtubule binding domain of MAP2A and MAP2B to identify MAP2C. All three forms of MAP2 bound to cAMP affinity columns and reacted with 32P-labeled RII in a blot overlay assay. By assaying proteolytic fragments of MAP2A and MAP2B as well as segments of MAP2 expressed in E. coli, the binding site for RII was localized to an 83 amino acid stretch at the distal (amino-terminal) end of the MAP2 arm domain. Therefore, the microtubule binding and RII binding domains are located at extreme opposite ends of MAP2A and MAP2B, and both are conserved in the much shorter MAP2C.

Partial sequence of MAP2 in the region of a shared epitope with Alzheimer neurofibrillary tangles

A 3.3-kilobase DNA complementary to human microtubule-associated protein 2 (MAP2) was sequenced by the dideoxy method. The 3' end terminates at an internal EcoRI site before the polyA tail. Due to the arrangement of the cDNA insert in the lambda gt11 vector, the MAP2 fragment is not fused to beta-galactosidase when expressed. The Chou Fasman algorithm for the initial 58 amino acids from the first in-frame methionine predicts an alpha helix. Beyond this point, a series of turns is predicted until amino acid 160. The frequent presence of basic residues in proximity to serines or threonines is consistent with multiple phosphorylation sites. The minimum specificity determinant for Ca2+/calmodulin-dependent kinase is repeated 13 times. The sequence of a region containing a MAP2 epitope that is shared with the Alzheimer neurofibrillary tangle was determined by DNase treatment of the cDNA and antibody selecting the small resultant clones in a lambda gt11 sublibrary. Likewise, a MAP2 epitope that is not shared with the neurofibrillary tangle also has been located. Both epitopes are in the projection portion of the molecule. A bovine MAP2 cyanogen bromide fragment, which contains the epitope shared with the neurofibrillary tangle, is partially insoluble under aqueous conditions, probably due to the aggregation of oppositely charged residues. Thus, rapid cleavage of MAP2 to small peptides is probably necessary in vivo to prevent the aggregation of larger cleavage fragments.

Anti-idiotypic antibodies that react with microtubule-associated proteins are present in the sera of rabbits immunized with synthetic peptides from tubulin's regulatory domain

A fundamental question in microtubule research is how the interactions of tubulin subunits with microtubule-associated proteins (MAPs) are controlled. The answer should provide insight into the regulation of the cellular processes in which microtubules are implicated. Previous work demonstrated the interaction of MAPs with a 4-kDa C-terminal domain of tubulin alpha and beta subunits. Synthetic peptides from the variable region of the 4-kDa C-terminal moiety of tubulin subunits, alpha-(430-441) and beta-(422-434), bind to MAP-2 and to the MAP tau, and a preferential interaction of the beta peptide is observed. To define the regulatory significance of the substructure of the C-terminal tubulin domain, we produced rabbit antisera against these MAP-interacting peptides. We found that these antisera contained not only antibodies to the original synthetic peptides but also antibodies to MAPs. Here, we report that these antibodies, which react with MAP-1, MAP-2, and tau, appear to be a population of anti-idiotypic antibodies directed to the anti-peptide antibodies. They can inhibit MAP-induced tubulin assembly into microtubules in vitro, and the addition of MAPs overcomes the inhibition. The recognition by these anti-idiotypic antibodies of the tubulin-binding domain on MAPs provides unequivocal evidence that the tubulin region defined by the synthetic peptides is directly involved in the interaction with MAPs.

Heterogeneity of microtubule-associated protein (MAP2) in vertebrate brains

We have utilized monoclonal antibodies to investigate the antigenic diversity of MAP2-immunoreactive proteins in the nervous system of vertebrates. We found that domains defined by the monoclonal antibodies differed in their conservation across vertebrate evolution, ranging from wide cross-reactivity with almost all vertebrates (mammals, birds, reptiles and amphibians) to a very limited cross-reactivity with only few mammalian species. However, we did not find MAP2-immunoreactive proteins in fish species with either of the monoclonal or polyclonal antibodies. There was also a significant divergence in the apparent molecular weight of MAP2, even in closely related species. For example, different species of wild mice and strains of laboratory mice showed variations of up to 30 kDa in their apparent molecular mass. Using alkaline phosphatase, under conditions that dephosphorylate neurofilaments, we showed that the observed heterogeneity was not the result of variations in the phosphate content. The heterogeneity in molecular weight of MAP2 may, therefore, be the result of changes in primary structure, transcriptional variations or different post-translational modifications. The heterogeneity of MAP2, as well as its specific distribution and implicated interactions with other molecules, underscore the complexity of MAP2 and its potential for structural and functional diversity. The phylogenic analysis of such a complex molecule also provides a method to establish the uniqueness of monoclonal antibodies and the degree of their conservation for their corresponding epitopes.

Common and distinct tubulin binding sites for microtubule-associated proteins

A specific binding assay was developed that monitors the interaction of 125I-labeled microtubule-associated proteins (MAPs) with tubulin or its fragments bound to nitrocellulose membrane. To identify the tubulin-binding domains for MAPs we have examined the binding of rat brain 125I-labeled MAP2 or 125I-labeled tau factors to 60 peptides derived from porcine alpha- and beta-tubulin. MAP2 and tau factors specifically interacted with two peptides derived from the carboxyl-terminal region of beta-tubulin, which are located between positions 392-445 and 416-445. In addition, there is a distinct tau-binding site at the amino-terminal region of alpha-tubulin. tau factors but not MAP2 displayed strong interaction with a peptide derived from the amino-terminal domain of alpha-tubulin between positions 1 and 75. To narrow down the location of the beta-tubulin binding site that is common to MAP2 and tau factors, we have synthesized five peptides that are homologous to the corresponding sequence from the porcine or rat carboxyl-terminal region. Binding studies with the synthetic peptides suggest that amino acid residues 434-440 of beta-tubulin are crucial for the interaction of MAP2 and tau factors.

Analysis of the microtubule-binding domain of MAP-2

We examined the microtubule-binding domain of the microtubule-associated protein (MAP), MAP-2, using rabbit antibodies that specifically bind to the microtubule-binding region ("stub") and the projection portion ("arm") of MAP-2. We found that (a) microtubules decorated with arm antibody look similar to those labeled with whole unfractionated MAP antibody, though microtubules are not labeled with stub antibody; (b) incubation of depolymerized microtubule protein with stub antibody prior to assembly partially inhibits the rate of microtubule elongation, presumably because MAPs that are complexed with antibody cannot bind to microtubules and stabilize elongating polymers; (c) the rate of appearance and amounts of 36- and 40-kD microtubule-binding peptides produced by digestion with chymotrypsin are distinct for MAPs associated with microtubules vs. MAPs free in solution. The enhanced stability of the 40-kD peptide when associated with microtubules suggests that this domain of the protein is closely associated with, or partially buried in, the microtubule surface; (d) MAP-2 is a slender, elongate molecule as determined by unidirectional platinum shadowing (90 +/- 30 nm), which is in approximate agreement with previous observations. Stub antibody labels MAP-2 in the terminal one-quarter of the extended protein, indicating an intrinsic asymmetry in the molecule.

MAP3: characterization of a novel microtubule-associated protein

Using monoclonal antibodies we have characterized a brain protein that copurifies with microtubules. We identify it as a microtubule-associated protein (MAP) by the following criteria: it copolymerizes with tubulin through repeated cycles of microtubule assembly in vitro; it is not associated with any brain subcellular fraction other than microtubules; in double-label immunofluorescence experiments antibodies against this protein stain the same fibrous elements in cultured cells as are stained by antitubulin; and this fibrous staining pattern is dispersed when cytoplasmic microtubules are disrupted by colchicine. Because it is distinct from previously described MAPs we designate this novel species MAP3. The MAP3 protein consists of a closely spaced pair of polypeptides on SDS gels, Mr 180,000, which are present in both glial (glioma C6) and neuronal (neuroblastoma B104) cell lines. In brain the MAP3 antigen is present in both neurons and glia. In nerve cells its distribution is strikingly restricted: anti-MAP3 staining is detectable only in neurofilament-rich axons. It is not, however, a component of isolated brain intermediate filaments.

Dynamic interactions of fluorescently labeled microtubule-associated proteins in living cells

Microtubule-associated proteins (MAPs) from calf brain were fluorescently labeled with 6-iodoacetamido fluorescein (I-AF). The modified MAPs (especially enriched for MAP2) were fully active in promoting tubulin polymerization in vitro and readily associated with cytoplasmic filaments when microinjected into living cultured cells. Double-labeling experiments indicated that the microinjected AF-MAPs were incorporated predominantly, if not exclusively, into cytoplasmic microtubules in untreated cells or paracrystals induced within vinblastine-treated cells. Similar results were obtained with different cell types (neuronal, epithelial, and fibroblastic) of diverse origin (man, mouse, chicken, and rat kangaroo). Mobility measurements of the microinjected AF-MAPs using the method of fluorescence-photobleaching recovery (FPR) revealed two populations of AF-MAPs with distinct dynamic properties: One fraction represents the soluble pool of MAPs and is mobile with a diffusion coefficient of D = 3 X 10(-9) cm2/s. The other fraction of MAPs is associated with the microtubules and is essentially immobile on the time scale of FPR experiments. However, it showed slow fluorescence recovery with an apparent half time of approximately 5 min. The slow recovery of fluorescence on defined photobleached microtubules occurred most probably by the incorporation of AF-MAPs from the soluble cytoplasmic pool into the bleached area. The bleached spot on defined microtubules remained essentially immobile during the slow recovery phase. These results suggest that MAPs can associate in vivo with microtubules of diverse cell types and that treadmilling of MAP2-containing microtubules in vivo, if it exists, is slower than 4 micron/h.