Microtubule-associated proteins

Microtubule stabilization in pressure overload cardiac hypertrophy

Increased microtubule density, for which microtubule stabilization is one potential mechanism, causes contractile dysfunction in cardiac hypertrophy. After microtubule assembly, alpha-tubulin undergoes two, likely sequential, time-dependent posttranslational changes: reversible carboxy-terminal detyrosination (Tyr-tubulin left and right arrow Glu-tubulin) and then irreversible deglutamination (Glu-tubulin --> Delta2-tubulin), such that Glu- and Delta2-tubulin are markers for long-lived, stable microtubules. Therefore, we generated antibodies for Tyr-, Glu-, and Delta2-tubulin and used them for staining of right and left ventricular cardiocytes from control cats and cats with right ventricular hypertrophy. Tyr- tubulin microtubule staining was equal in right and left ventricular cardiocytes of control cats, but Glu-tubulin and Delta2-tubulin staining were insignificant, i.e., the microtubules were labile. However, Glu- and Delta2-tubulin were conspicuous in microtubules of right ventricular cardiocytes from pressure overloaded cats, i.e., the microtubules were stable. This finding was confirmed in terms of increased microtubule drug and cold stability in the hypertrophied cells. In further studies, we found an increase in a microtubule binding protein, microtubule-associated protein 4, on both mRNA and protein levels in pressure-hypertrophied myocardium. Thus, microtubule stabilization, likely facilitated by binding of a microtubule-associated protein, may be a mechanism for the increased microtubule density characteristic of pressure overload cardiac hypertrophy.

Calcium-stimulated phosphorylation of MAP-2 in pancreatic betaTC3-cells is mediated by Ca2+/calmodulin-dependent kinase II

An understanding of the role of CaM kinase II in the pancreatic beta-cell is dependent on the identification of its cellular targets. One of the best substrates of CaM kinase II in vitro that could function in secretory events is the microtubule-associated protein, MAP-2. By immunoblot analysis, a high molecular weight protein with electrophoretic properties characteristic of MAP-2, was identified in rat insulinoma betaTC3 cells and isolated rat islets. In immunoprecipitation experiments employing alpha-toxin-permeabilized betaTC3 cells, elevation of intracellular Ca2+ or addition of forskolin, an adenylate cyclase activator, induced significant phosphorylation of MAP-2 in situ. The effect of Ca2+ was rapid, concentration-dependent and closely correlated with activation of CaM kinase II under similar experimental conditions. H-89, a specific and potent inhibitor of cAMP-dependent protein kinase (PKA), prevented forskolin-induced MAP-2 phosphorylation but had little effect on MAP-2 phosphorylation stimulated by elevated Ca2+. Phosphopeptide mapping revealed that the phosphorylation pattern observed in situ upon incubation of the betaTC3 cells with increased free Ca2+, was strikingly similar to that generated in vitro by CaM kinase II, most notably with regard to the increased phosphate incorporated into one prominent site. These data provide evidence that MAP-2 is phosphorylated by CaM kinase II in the pancreatic beta-cell in situ, and that this event may provide an important link in the mediation of Ca2+-dependent insulin secretion.

Direct demonstration of the bifunctional property of Tetrahymena 14-nm filament protein/citrate synthase following expression of the gene in Escherichia coli

Tetrahymena 14-nm filament protein/citrate synthase (49K protein) is a bifunctional protein with roles in the cytoskeleton and as a citrate synthase. Though previous studies have shown that the 49K protein is derived from a single transcript of a single gene, direct demonstration of the 49K protein's bifunctional property remained to be elucidated. In this study, a recombinant 49K protein was expressed in Escherichia coli, purified and characterized. The citrate synthase activity of the recombinant 49K protein was comparable to that of the 49K protein purified from Tetrahymena. The recombinant 49K protein formed 14-nm filaments, but only of short length. The filaments were elongated in the presence of a soluble fraction of Tetrahymena. These results suggest that the 49K protein itself is bifunctional, but some co-factor(s) is necessary for elongation of filaments.

Age-dependent organotypic expression of microtubule-associated proteins (MAP1, MAP2, and MAP5) in rat brain

Age-dependent changes in the distribution of microtubule-associated proteins (MAPs) were analyzed in young (3-months, N = 3) and old (24-months, N = 3) rat brain. In the young rats, MAP1 and MAP5 exhibited prominent immunostaining in the perikarya and dendrites whereas MAP2 was selectively localized in the dendrites. In the cerebellum, MAP2 was preferentially localized in finer and distal branches of Purkinje cell dendrites and in punctate deposits surrounding glomeruli. In general, aging resulted in obvious declines in MAP2- >> MAP1- and MAP5-immunoreactivities in the hippocampus and parietal cortex but no change in cerebellum. The results indicate that: (1) hippocampus is the most affected and cerebellum is the least affected region with regard to declines in MAPs-immunoreactivities in the aged rat brain; (2) dendrite-specific MAP2 is almost completely depleted from most dendrites in the hippocampus and cortex. In summary, loss of MAP2-immunoreactivity in the affected brain areas may be associated with age-related impairment of synaptic plasticity, cognition and memory functions.

A 60-kDa plant microtubule-associated protein promotes the growth and stabilization of neurotubules in vitro

The search for microtubule-associated proteins (MAPs) in plants is relatively recent. In particular, the "classical MAPs," which stimulate the polymerization and stabilization of microtubules, have only been examined in heterogeneous fractions. As a first step in dissecting the role of individual MAPs, we have chromatographically purified a single 60-kDa protein from a carrot MAP fraction and analyzed its effects on tubulin assembly. MAP60 promoted the formation of long, morphologically regular brain microtubules in vitro, an effect inhibited by preincubation of the MAP with affinity-purified antibodies against this protein. MAP60 also increased the stability of microtubules to dilution and significantly enhanced cold stability to the normally cold-sensitive neurotubules. These in vitro properties are consistent with a role for MAP60 in regulating the turnover/assembly of dynamic plant microtubules in vivo.

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.

Involvement of the mutated M protein in altered budding polarity of a pantropic mutant, F1-R, of Sendai virus

Wild-type Sendai virus buds at the apical plasma membrane domain of polarized epithelial MDCK cells, whereas a pantropic mutant, F1-R, buds at both the apical and basolateral domains. In F1-R-infected cells, polarized protein transport and the microtubule network are impaired. It has been suggested that the mutated F and/or M proteins in F1-R are responsible for these changes (M. Tashiro, J. T. Seto, H.-D. Klenk, and R. Rott, J. Virol. 67:5902-5910, 1993). To clarify which gene or mutation(s) was responsible for the microtubule disruption which leads to altered budding of F1-R, MDCK cell lines containing the M gene of either the wild type or F1-R were established. When wild-type M protein was expressed at a level corresponding to that synthesized in virus-infected cells, cellular polarity and the integrity of the microtubules were affected to some extent. On the other hand, expression of the mutated F1-R M protein resulted in the formation of giant cells about 40 times larger than normal MDCK cells. Under these conditions, the effects on the microtubule network were enhanced. The microtubules were disrupted and polarized protein transport was impaired as indicated by the nonpolarized secretion of gp80, a host cell glycoprotein normally secreted from the apical domain, and bipolar budding of wild-type and F1-R Sendai viruses. The mutated F glycoprotein of F1-R was transported bipolarly in cells expressing the F1-R M protein, whereas it was transported predominantly to the apical domain when expressed alone or in cells coexpressing the wild-type M protein. These findings indicate that the M protein of F1-R is involved in the disruption of the microtubular network, leading to impairment of cellular polarity, bipolar transport of the F glycoprotein, and bipolar budding of the virus.

Expression of a developmentally regulated, phosphorylated isoform of microtubule-associated protein 1B in sprouting and regenerating axons in vitro

We have developed a novel culture system for studying axonal regeneration. Short lengths of spinal nerves with their attached dorsal root ganglia were removed from adult mice, explanted into Matrigel and maintained in serum-free medium for up to eight days. Profuse outgrowth of unfasciculated, naked axons occurred within 6 h from the cut ends of the peripheral nerve, dorsal roots and eventually from the ganglion itself, and continued to grow throughout the observation period. Some axons were entirely smooth, whilst others showed prominent varicosities. The former stained with antibody RT97, a marker for large-calibre, myelinated axons, whilst the latter stained with antibodies to calcitonin gene-related peptide, predominantly a marker for unmyelinated and small-diameter myelinated sensory axons. All axons stained with a monoclonal antibody (150) that recognizes a developmentally regulated phosphorylated isoform of the microtubule-associated protein 1B [Gordon-Weeks P. R. et al. (1993) Eur. J. Neurosci. 5, 1302-1311]. Monoclonal antibody 150 staining was observed along the entire length of all axons growing out of the explant; the proximal regions of these axons within the explant itself did not stain. The staining extended to the growth cones, which had elaborate morphologies. Other antibodies (e.g. to growth-associated protein 43) labelled axons within the nerve, as well as those growing in Matrigel. In preparations where the peripheral nerve had been crushed half-way along its length at the time of explantation, monoclonal antibody 150 staining was absent from axons in the nerve proximal to the crush, but present in axons which had regenerated within the nerve distal to the crush. The results indicate that re-expression during axonal regeneration of the phosphorylated isoform of microtubule-associated protein 1B recognized by monoclonal antibody 150 is restricted to the newly formed lengths of regenerated axons. The correlation between its expression and axonal growth during development and regeneration suggests that it may play a role in axonal extension. Our observations also demonstrate the usefulness of these explant cultures for studying axonal regeneration.

Blackjack, a novel protein associated with microtubules in embryonic neurons

Microtubule-associated proteins can influence the organization, stability and dynamics of microtubules. We characterize a novel protein that associates with microtubules as assessed by immunofluorescence, immunoelectron microscopy, and co-sedimentation. The protein is expressed heavily in embryonic neurons and, to a lesser extent, in epithelial and mesodermal cells. The cDNA sequence predicts a protein of 1,547 amino acids and approximately 170 kDa. Immunoblot of embryo lysate demonstrates bands of approximately 240 and 260 kDa. The predicted amino acid sequence contains 77 potential serine/threonine phosphorylation sites. A distinctive feature is a predicted alpha-helical central domain comprising 21 identical repeats of an 11 amino acid sequence (PLEELRKDAAE). The protein is thermostable and has two major charge-domains: the amino-terminal 80% has an estimated pI of 4.0 and the carboxy-terminal 20%, a pI of 12.2. The protein shares several general biochemical and molecular features of MAPs, but its sequence is not similar to that of any described MAP.

High molecular weight microtubule-associated proteins contain O-linked-N-acetylglucosamine

We have examined the post-translational modification of high molecular weight microtubule-associated proteins (MAPs) have shown that MAP1, MAP2, and MAP4 are glycosylated. The presence of carbohydrate residues on these proteins was indicated by labeling with biotin hydrazide following periodate oxidation, a specific and well established method for detecting saccharide moieties on proteins. Both MAP2 and MAP4 were also labeled in vitro by UDP-[3H]galactose in the presence of galactosyltransferase. Labeling by galactosyltransferase indicated that MAP2 and MAP4 contained terminal nonreducing GlcNAc residues, and they appeared to be O-linked to the proteins as shown by their sensitivity to beta-elimination. Chromatographic analysis showed that the GlcNAc residues were directly linked to the proteins as monosaccharides. Thus, we have added MAP2 and MAP4 to the list of intracellular O-GlcNAc-modified proteins, which includes other cytoskeletal proteins such as cytokeratins 8, 13, and 18 and neurofilament proteins NF-L and NF-M. We further characterized the O-GlcNAc modification of MAP2, and stoichiometric analysis indicated that nearly 10% of the MAP2 isolated from rat brain is modified by O-GlcNAc. However, this estimate is thought to reflect the minimal level of O-GlcNAc modification present on MAP2. We have also shown that both the O-GlcNAc and biotin hydrazide-reactive carbohydrate moieties are located on the projection domain of MAP2. Three O-GlcNAc-containing peaks were observed following fast protein liquid chromatography of a tryptic digest of MAP2, suggesting that multiple modification sites exist. The specific modification sites and functional significance of the O-GlcNAc glycosylation on the high Mr MAPs remain to be determined.

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.

Characterization of microtubule-associated protein 1-associated protein kinases from rat brain

The microtubule-associated protein (MAP) 1 preparation, MAP1A and 1B, obtained from rat brain microtubules was associated with protein kinases that were insensitive to cAMP, cGMP, calcium, calcium/calmodulin and calcium/phosphatidylserine. The fractionation of highly purified MAP1 by phosphocellulose chromatography revealed that protein kinase activity to phosphorylate phosvitin was separated into three major peaks (MAP1 kinases A, B and C). MAP1 was recovered in the MAP1 kinase A fraction and phosphorylated by the contained kinase. MAP1 kinase A is a novel protein kinase that is remarkably activated by poly-L-lysine and poly-L-arginine, but very insensitive to heparin among the kinases. Photoaffinity labeling using [alpha-32P]8-azido ATP indicated that the 65 kDa polypeptide is identified as an ATP-binding protein on sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the highly purified MAP1 and MAP1 kinase A fractions. MAP1 kinases B and C may be identified as casein kinase I- and II-like kinases. The present results show that MAP1 is associated with at least three kinases and provide an insight for understanding thoroughly the MAP1-mediated microtubule functions.

Molecular analysis of cellular loci disrupted by papillomavirus 16 integration in cervical cancer: frequent viral integration in topologically destabilized and transcriptionally active chromosomal regions

To discern the structural features of cellular loci that are disrupted by type 16 human papillomavirus (HPV-16) integration in cervical cancer, a polymerase chain reaction (PCR)-based strategy was employed for direct amplification and sequence analysis of four such cellular loci in cancer biopsy samples. One of the HPV-16-disrupted loci was found to be the microtubule-associated protein (MAP-2) gene and the other three loci were uncharacterized and were designated PID-1 to -3 (for papillomavirus integration-disrupted). The junctional sequences of the viral integration sites in the four loci analyzed are bracketed by long tracts of homogeneous purine or pyrimidine or alternating purine-pyrimidine which are known to destabilize the B-form conformation of the DNA structure. Using a panel of human/hamster hybrid cell DNAs and PCR analysis, the four loci were assigned to chromosomes 2 (MAP-2), 9 (PID-1), 1 (PID-2) and 8 (PID-3), respectively. These chromosomes carry numerous other previously determined viral integration and chromosomal fragile sites and the myc oncogenes. The PID-1 locus was further found in Southern analysis to be rearranged and amplified in another cervical cancer biopsy and a cervical carcinoma cell line (CaSki). On Northern analysis, the PID-1 and -3 probes detected a 3.0- and a 3.6-kb transcript, respectively, in normal cervical cells and in cervical cancer cell lines. The findings suggest that HPV-16 genome integrates frequently into topologically destabilized and transcriptionally active chromosomal sites. It remains to be elucidated whether the MAP-2 and the PID loci contribute to the pathogenesis of cervical cancer.

Association of mitogen-activated protein kinases with microtubules in mouse macrophages

Taxol, a microtubule-binding diterpene, mimics many effects of lipopolysaccharide (LPS) on mouse macrophages. The LPS-mimetic effects of taxol appear to be under the same genetic control as responses to LPS itself. Thus we have postulated a role for microtubule-associated proteins (MAP) in the response of macrophages to LPS. Stimulation of macrophages by LPS quickly induces the activation of mitogen-activated protein kinases (MAPK). MAPK are generally considered cytosolic enzymes. Herein we report that much of the LPS-activatable pool of MAPK in primary mouse peritoneal macrophages is microtubule associated. By immunofluorescence, MAPK were localized to colchicine- and nocodazole-disruptible filaments. From both mouse brain and RAW 264.7 macrophages, MAPK could be coisolated with polymerized tubulin. Fractionation of primary macrophages into cytosol-, microfilament-, microtubule-, and intermediated filament-rich extracts revealed that approximately 10% of MAPK but none of MAPK kinase (MEK1A and MEK2) was microtubule bound. Exposure of macrophages to LPS did not change the proportion of MAPK bound to microtubules, but preferentially activated the microtubule-associated pool. These findings confirm the prediction that LPS activates a kinase bound to microtubules. Together with LPS-mimetic actions of taxol and the shared genetic control of responses to LPS and taxol, these results support the hypothesis that a major LPS-signaling pathway in mouse macrophages may involve activation of one or more microtubule-associated kinases.

Development and activity-dependent expression of neuronal marker proteins in organotypic cultures of rat visual cortex

We are interested in activity-dependent mechanisms which govern the structural and functional maturation of neurons in the visual cortex. We have asked whether the expression of neuronal markers microtubule-associated proteins tau, MAP-2, synaptophysin (p38), and the growth-associated protein GAP-43 are dependent on cortical afferents or spontaneous activity. As a model system we have employed organotypic monocultures of rat visual cortex (OTCs, isolated from subcortical structures) in comparison with visual cortex in vivo (innervated by thalamic and other afferents) at different postnatal ages. We know from previous work that the OTCs, like the cortex in vivo, display a high rate of spontaneously generated action potentials. Therefore, as a third objective, we have analysed OTCs grown as monocultures under chronic blockade of spontaneous action potentials. Protein expression was detected by protein blots and/or immunohistochemistry. The proteins examined in this study are expressed in OTCs, even when grown under activity blockade. However, the pattern of expression differs from the cortex in vivo. Tau is expressed much weaker in OTCs than in cortex in vivo. The expression of the major band of about 50 kDa increases over time in vivo and in OTCs. Smaller isoforms of tau are dramatically downregulated, and larger (adult) isoforms do not appear within 35 days in vitro (DIV). Under activity blockade the expression of tau reaches a maximum by 21 DIV and decreases dramatically, so that the protein is hardly detectable by 47 DIV. MAP-2-immunoreactive proteins are localized in somata and dendrites, but also persist in axons. The expression in OTCs of p38 and GAP-43 correlates well with the expression observed in vivo. Synaptophysin (p38) occurs with a similar time course and amount in OTCs as in cortex in vivo. Synaptic boutons appear in all layers, and specialized terminal elements have been observed. Activity blockade slightly affects the p38 expression, although the late postnatal decline in p38 immunoreactivity observed on protein blots from cortex in vivo and in normal OTCs appears more accentuated in activity-blocked OTCs. The GAP-43 expression is prominent from birth onwards in vivo and in OTCs. However, in normal OTCs GAP-43 is not declining as it is in vivo, although it is downregulated in activity-blocked OTCs. As a major finding we report that neuronal markers which are normally expressed in immature neurons and axons during the period of differentiation and structural plasticity are continuously expressed in OTCs, suggesting that a monocultured cortex retains the ability for growth and structural changes longer than the cortex in vivo.

Basis for increased microtubules in pressure-hypertrophied cardiocytes

BACKGROUND

We have shown the levels of the sarcomere and the cardiocyte that a persistent increase in microtubule density accounts to a remarkable degree for the contractile dysfunction seen in pressure-overload right ventricular hypertrophy. In the present study, we have asked whether these linked phenotypic and contractile abnormalities are an immediate and direct effect of load input into the cardiocyte or instead a concomitant of hypertrophic growth in response to pressure overloading.

METHODS AND RESULTS

The feline right ventricle was pressure-overloaded by pulmonary artery banding. The quantity of microtubules was estimated from immunoblots and immunofluorescent micrographs, and their mechanical effects were assessed by measuring sarcomere motion during microtubule depolymerization. The biogenesis of microtubules was estimated from Northern and Western blot analyses of tubulin mRNAs and proteins. These measurements were made in control cats and in operated cats during and after the completion of right ventricular hypertrophy; the left ventricle from each heart served as a normally loaded same-animal control. We have shown that the alterations in microtubule density and sarcomere mechanics are not an immediate consequence of pressure overloading but instead appear in parallel with the load-induced increase in cardiac mass. Of potential mechanistic importance, both these changes and increases in tubulin poly A+ mRNA and protein coexist indefinitely after a new, higher steady state of right ventricular mass is reached.

CONCLUSIONS

Because we find persistent increases both in microtubules and in their biosynthetic precursors in pressure-hypertrophied myocardium, the mechanisms for this cytoskeletal abnormality must be sought through studies of the control both of microtubule stability and of tubulin synthesis.

NuMA assembles into an extensive filamentous structure when expressed in the cell cytoplasm

NuMA is a 236 kDa protein that participates in the organization of the mitotic spindle despite its strict localization in the nucleus during interphase. To test how cells progress through mitosis when NuMA is localized in the cytoplasm instead of the nucleus, we have deleted the nuclear localization sequence of NuMA using site-directed mutagenesis and transiently expressed this mutant protein (NuMA-DeltaNLS) in BHK-21 cells. During interphase, NuMA-DeltaNLS accumulates in the cytoplasm as a large mass approximately the same size as the cell nucleus. When cells enter mitosis, NuMA-DeltaNLS associates normally with the mitotic spindle without causing any apparent deleterious effects on the progression of mitosis. Examination of the cytoplasmic mass formed by NuMA-DeltaNLS using transmission electron microscopy (TEM) revealed an extensive network of approximately 5 nm filaments that are further organized by the presence of dynamic microtubules into a dense web of solid, approximately 23 nm cables. Using flow cytometry, we have isolated the intact filamentous mass formed by NuMA-DeltaNLS from lysates of transiently transfected cells. These isolated structures are constructed of networks of interconnected 5 nm filaments and are composed exclusively of NuMA. These data demonstrate that NuMA is capable of assembling into an extensive filamentous structure supporting the possibility that NuMA serves a structural function either in the nucleus during interphase or at the polar ends of the mitotic spindle.

Neuron-like phenotypic changes in pancreatic β-cells induced by NGF, FGF, and dbcAMP

We studied the effects of nerve growth factor (NGF), fibroblast growth factor (FGF), and dibutyryl-cAMP (dbcAMP) on rat pancreatic β-cell morphology and of NGF and dbcAMP on insulin secretion. After 2 wk in culture, nearly 3% of β-cells extended neurite-like processes spontaneously; when cells were treated with NGF, almost 30% of them extended processes. In the presence of dbcAMP, almost all β-cells flattened, and the extension of neurite-like processes was more pronounced in fetal than in adult cells. The most prominent effect, regardless of age, was observed in cells treated with NGF and dbcAMP together, since the percentage of neurite-like bearing β-cells increased to 50%. β-cells cultured under these conditions maintained their immunoreactivity to insulin and nearly all β-cells and their neurite-like processes were also positive to GABA, tubulin, tau protein, and N-CAM. FGF increased the percentage of adult β-cells bearing neurite-like processes to 13%, and FGF and dbcAMP applied together to 40%. β-cells treated with NGF and dbcAMP for 5 to 7 d preserved their capability to secrete the hormone in response to different extracellular glucose concentrations. Insulin secretion of dbcAMP-treated β-cells was 2.5-fold higher than in control cells. NGF-treated cells were able to discriminate between different glucose concentrations, a property lost in control cells with time in culture.

Modulated induction of tau proteins in cultured human neuroblastoma cells

Previous studies indicated that a chemically-defined, differentiation medium (DM) induces neuroblastoma cells, especially IMR32K cells, to exhibit phenotypes of mature neurons (including neurite outgrowth and synthesis of neurofilament polypeptides) and develop certain attributes of the neurons which are affected by neurofibrillary degeneration in Alzheimer's disease, such as expression of tangle-associated epitopes and accumulation of paired helical filaments-(PHF-) like fibrils. Immunocytochemical staining suggested that this cytoskeletal abnormality most likely results from altered expression of tau proteins. In the current study, we addressed this issue by analyzing tau-enriched preparations of IMR32K cells that were previously exposed to different incubation media using a panel of antibodies specific to tau and related microtubule-associated proteins. These cultured cells exhibited three groups of tau immunoreactivities which differ in molecular weight. Among them the level of high molecular weight tau (MW 90-112 kDa) was selectively augmented after DM incubation. The tau proteins produced in these neuron-like cells shared phosphorylated sites with PHF-tau and fetal tau, but differed from PHF-tau in their lack of the N-terminal insert which characterizes adult isoforms.

Electrodynamics of microtubular motors: the building blocks of a new model

Microtubules are ubiquitous components of the cytoskeleton. They participate in many motility processes ranging from intracellular transport or chromosome movement during mitosis to ciliary and flagellar beating. The biophysical mechanism inherent in the generation and control of movement in all these motility phenomena has not yet been entirely elucidated. The authors propose a new model based on a charge transfer mechanism capable of shedding a new light on the molecular foundations of all motility processes. Electron transfer along the microtubular lattice is responsible for activation and control of all microtubule-associated ATPases (i.e. force generating enzymes). Microtubules are thus shown to be the basic motors of cell dynamics. The model is first applied to intracellular transport and ciliary and flagellar beating. Through two additional examples, the authors show the heuristic capabilities of the suggested hypothesis. The application of charge transfer control to the Protozoan Euglena gracilis leads to a plausible model capable of accounting for its phototactic response mechanism. Furthermore, the model allows a new interpretation of the electrophysiological response in vertebrate photoreceptors.

Stimulation of microtubule dynamic turnover in living cells treated with okadaic acid

We have examined the effects of okadaic acid, an inhibitor of protein phosphatases type 1 and 2A, on the dynamic instability behavior of individual microtubules in living cells. Addition of 1 microM okadaic acid to PtK1 epithelial cells induced ruffling of lamellar regions; after 50 min in okadaic acid, many cells were observed to round up. Confocal microscopy of okadaic acid-treated cells stained with an antibody to tubulin showed that microtubules were more densely packed near the periphery of the rounded cells, and in many cells, a reduction in the density of microtubules near the microtubule-organizing center was observed. The dynamic behavior of individual microtubules in cells previously injected with rhodamine-labeled tubulin was quantified by tracking individual microtubules from image sequences. Microtubule dynamic turnover was markedly stimulated in cells treated with 1 microM okadaic acid for 50-60 min: The average rates of both microtubule growing and shortening increased, and the average duration of pause, or attenuation, a phase in which neither growth nor shortening could be detected, was significantly decreased. Further, okadaic acid induced an approximately twofold increase in the frequency of catastrophe transitions and a threefold decrease in the frequency of rescue transitions. Dynamicity, a measure of the net gain and loss of polymer at microtubule plus ends, increased nearly threefold in okadaic acid-treated cells. These results demonstrate that microtubule turnover is stimulated in okadaic acid-treated cells and suggest that phosphorylation of molecules which interact with microtubules may result in increased microtubule dynamic turnover in vivo.

Multifunctional proteins in Tetrahymena: 14-nm filament protein/citrate synthase and translation elongation factor-1 alpha

One gene encoding a protein has been shown to have two entirely different functions. Such a phenomenon, which has been called "gene sharing," was first known in crystallins. We found two multifunctional proteins in the ciliated protozoan Tetrahymena: 14-nm filament protein and protein translation elongation factor 1-alpha (EF-1 alpha). The 14-nm filament protein has dual functions as a citrate synthase in mitochondria and as a cytoskeletal protein in cytoplasm. In cytoplasm, the 14-nm filament protein was involved in oral morphogenesis and in pronuclear behavior during conjugation. The observation that Tetrahymena intramitochondrial filamentous inclusions contain the 14-nm filament protein and that the citrate synthase activity of the 14-nm filament protein is decreased by polymerization and increased by depolymerization, suggests a possible modulating mechanism of citrate synthase activity by monomer-polymer conversion in mitochondria in situ. The EF-1 alpha functions as an F-actin-bundling protein and a 14-nm filament-associated protein as well as an elongation factor in protein synthesis. The F-actin-bundling activity of EF-1 alpha was regulated by Ca2+ and calmodulin. Here we review the properties and functions of two multifunctional proteins in Tetrahymena.

Interactions between the protein-tyrosine kinase ZAP-70, the proto-oncoprotein Vav, and tubulin in Jurkat T cells

Two molecules involved in signal transduction via the T cell antigen receptor, namely the protein-tyrosine kinase ZAP-70 and the proto-oncoprotein Vav, were found to be constitutively associated with tubulin in Jurkat T cells. Both were able to bind to tubulin independently of one another, as determined by transient transfection into COS-7 cells. The ZAP-70 associated with tubulin was preferentially tyrosine-phosphorylated after T cell antigen receptor stimulation of Jurkat T cells, suggesting that this interaction was functionally significant. Vav was also found to co-immunoprecipitate with ZAP-70 from cell extracts depleted of tubulin. This raised the possibility that Vav might be a substrate for ZAP-70 protein-tyrosine kinase activity. However, tyrosine phosphorylation of Vav preceded that of ZAP-70, indicating that Vav was unlikely to be a downstream target of ZAP-70. The association of ZAP-70 and Vav with tubulin implies that the microtubules may be involved in the signaling function of these two molecules, perhaps by targeting them to their appropriate intracellular location.

Antibodies directed against microtubule proteins from Drosophila melanogaster cross react with similar proteins in the rat brain

Monoclonal antibodies (Mabs) were used to delineate the localization of three proteins in rat cerebral cortex, hippocampus and cerebellum. The proteins were identified by Mabs directed against Drosophila melanogaster microtubule proteins (MTP). We have provisionally designated these proteins as Drosophila microtubule-associated proteins (DMAPs). The corresponding monoclonal antibodies are designated Mab DMAP-45, -55 and -66 indicating the molecular weights of each protein. All three Mabs cross-react with proteins of similar molecular weights in the rat brain. Correspondingly, these rat proteins are designated DMAPRs. DMAP-45 binds microtubules in an ATP-dependent manner. The molecular weight and subcellular localization of DMAP-45R differs significantly from previously described mammalian brain MAPs suggesting that it represents a novel MAP. Biochemical evidence suggests it may be an actin-related protein. DMAP-55R co-purifies stoichiometrically with rat brain microtubules and appears to be a previously undescribed isoform of tubulin. DMAP-66, which co-purifies stoichiometrically with Drosophila microtubules, does not do so in the rat brain. Immunohistochemistry performed with all three Mabs revealed a general pattern of staining of cell somata and dendrites in the cortex, hippocampus and cerebellum. Mab DMAP-55 also stained axons. In cerebral cortex all three Mabs preferentially, but not exclusively, stained layer V neuronal somata and dendrites. In hippocampus, Mabs DMAP-45 and -66 stained cell somata and dendrites in all hippocampal subfields, particularly the subiculum and CA3, whereas Mab DMAP-55 was most prevalent in mossy fibers. All three Mabs stain Purkinje cells in cerebellum with additional staining of cerebellar basket cells and Golgi cells observed with Mab DMAP-66.

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.

Biochemical characterization of related microtubule proteins in Drosophila melanogaster and adult rat brain

We describe the biochemical characteristics of three proteins isolated from Drosophila embryos and the rat brain. We refer to these proteins as DMAPs (Drosophila microtubule-associated proteins) since they were identified by monoclonal antibodies generated against microtubule protein (MTP) purified from Drosophila melanogaster embryos. DMAP-45 is a 45 kDa protein that binds microtubules in an ATP dependent manner. Preliminary biochemical evidence suggests that DMAP-45 may be an actin-related protein. DMAP-55 is a 55 kDa protein and based on its molecular weight and isoelectric point, may be a novel isoform of tubulin. DMAP-66 is a 66 kDa protein that binds strongly to microtubules in vitro and has multiple isoforms. Analyses of proteins in rat brain tissue extracts and purified rat brain MTP identified proteins of similar molecular weight and isoelectric points and are designated DMAP-45R, -55R and -66R. The presence of proteins with common biochemical properties in these widely divergent animal species suggests that they are related proteins.

SCG10, a neuron-specific growth-associated protein in Alzheimer's disease

Neuronal growth-associated proteins (nGAPs) are markers of neuronal process outgrowth and are associated with both degenerative and sprouting responses in Alzheimer's disease (AD) brain. To study possible involvement of SCG10, an nGAP, in AD, we cloned human SCG10 cDNA and analyzed SCG-10 at mRNA and protein levels in control and AD brains. The deduced amino acid sequence of human SCG10 was 69% identical to stathmin, another nGAP. By in situ hybridization, both SCG10 and stathmin mRNAs were detected in selected neuronal populations in aged human brains. Quantitative analysis by RNase protection revealed that levels of neither SCG10 nor stathmin mRNAs were significantly altered in AD. Using an SCG10-specific antibody, Western blot analysis did not reveal any quantitative changes of SCG10 in AD. However, when the concentration of SCG10 protein was plotted against the number of tangles, a positive correlation was found. SCG10 levels did not correlate with plaque numbers. Furthermore, immunohistochemical study revealed that neuronal SCG10 protein accumulated in the cell bodies in AD-affected regions. Thus, SCG10 compartmentalization and metabolism may be altered in AD possibly due to mechanisms related to tangle formation in this disease.

The regulation of acetylated microtubules during outgrowth from cultured neurons of the snail, Helisoma

Axonal stumps of cultured Helisoma trivolvis neurons express abundant acetylated microtubules, as a subset of total microtubules. Label completely disappears from the axonal remnants within approximately 1 day, and reappears in newly extended neurites over the course of the next 3-4 days, first in the proximal neurite as short, isolated segments. Acetylated microtubules occur in the neuritic shaft, but never in growth cones or membranous veils. Thus, acetylated microtubules are very labile to the signals generated by axotomy, and their proximodistal re-expression occurs at well separated sites within the neurite as it matures.

Myelin mutants: new models and new observations

The myelin mutants have been extensively used as tools to study the complex process of myelination in the central and peripheral nervous system. A multidisciplinary approach to the study of these models ultimately allows a correlation to be made between phenotype and genotype. This correlation may then lead to the formation of new hypotheses about the functions of the products of genes involved in myelination. This review presents a number of new myelin mutants which have recently been described. The species involved include mouse, rat, rabbit, hamster, and dog models. The genetic defect has not been elucidated in all of these animals, but most have been characterized clinically and pathologically, and, in some cases, biochemically. In addition, a better known myelin mutant, the trembler mouse, is discussed. Recent molecular findings have brought this fascinating mutant to the forefront of the field of peripheral nervous system research. The range of abnormalities in the mutants described in this review includes defects in specific myelin proteins, suspected abnormalities in membrane formation, and apparent defects of the oligodendrocyte cytoskeleton. These findings underscore the complexity of the myelination process and highlight the numerous ways in which it can be disrupted.

Production and utilization of detyrosinated tubulin in developing Artemia larvae: evidence for a tubulin-reactive carboxypeptidase

The reversible, enzymatically driven removal and readdition of its carboxy-terminal tyrosine are major posttranslational modifications of alpha-tubulin. To study these processes isoform-specific antibodies were produced and subsequently used to characterize tyrosinated and detyrosinated tubulin in the brine shrimp, Artemia. Tyrosinated tubulin existed in relatively constant amounts on western blots of cell-free protein extracts from Artemia at all developmental stages examined, whereas detyrosinated tubulin was present after 20-24 h of postgastrula growth. In agreement with the blots, the detyrosinated isoform was observed in immunofluorescently stained larvae after 24 h of incubation, appearing first in structures of a transient nature, namely spindles and midbodies. The elongated muscle cells encircling the gut and the epithelium bordering the gut lumen were stained extensively with antibody to detyrosinated tubulin. Detyrosination was accompanied by the appearance of a tubulin-reactive carboxypeptidase, which used both nonpolymerized and polymerized tubulin as substrate. The enzyme bound to microtubules very poorly, if at all, under conditions used in this work. Several inhibitors of carboxypeptidase A had no effect on the carboxypeptidase from Artemia and revealed similarities between this enzyme and others thought to be tubulin specific. The use of inhibitors also indicated that the carboxypeptidase from Artemia recognized aspects of tubulin structure in addition to the carboxy-terminal tyrosine. Our results support the idea that detyrosinated tubulin appears in microtubules of varying stability, and they demonstrate that Artemia possess a carboxypeptidase with the potential to detyrosinate tubulin during growth of larvae.

Distribution of Big tau in the central nervous system of the adult and developing rat

The diversity of neuronal morphology and function is correlated with specific expression of various microtubule associated proteins (MAPs). One of the major neuronal MAPs, tau, has multiple isoforms formed as a result of alternative splicing and phosphorylation that are differentially expressed during development. Big tau is a high molecular weight isoform that contains an additional large exon (4a) and is expressed primarily by neurons in the peripheral nervous system (PNS). We cloned the complete 4a exon in an expression vector, isolated the recombinant protein and produced antibodies specific to Big tau that were used to localize Big tau in the developing spinal cord and in the adult central nervous system (CNS). In developing spinal cord, Big tau is first expressed in the central projections of the dorsal root ganglia neurons and in motor neurons at embryonic day 18 and postnatal day 2, respectively. In the adult rat CNS, almost all neurons that extend processes into the PNS express Big tau, including all cranial nerve motor nuclei and central processes of most sensory ganglia; of these ganglia, only the bipolar neurons of the olfactory, vestibular and spiral ganglia did not express Big tau. Retinal ganglion cells are the only CNS neurons, whose processes remain entirely within the CNS, that express high levels of Big tau. The limited and specific distribution of Big tau is consistent with a role in stabilizing microtubules in axons that are subjected to great shear forces.

Microtubule-severing activity in M phase

The stable cytoplasmic microtubules that emanate from centrosomes in eukaryotic cells disappear at the onset of M phase and are replaced by the dynamic microtubules of the mitotic spindle. Microtubule-severing activity increases significantly under the control of maturation-promoting factor at the transition between G2 phase and M phase, and is thought to be involved in the microtubule reorganization. This review highlights three microtubule-severing factors that may be responsible for microtubule-severing activity in M phase.

The expression and distribution of tau proteins and messenger RNA in rat dorsal root ganglion neurons during development and regeneration

Microtubule-associated proteins contribute to the balance between stability and plasticity of the neuronal cytoskeleton by modulating assembly and disassembly of microtubules. The tau microtubule-associated proteins exist in several isoforms which are developmentally regulated and differentially distributed. Our objective was to characterize the distribution of tau isoforms in developing and mature dorsal root ganglia neurons and during axonal regeneration following sciatic nerve axotomy. Immunocytochemical analysis was carried out using antibodies that recognize all tau isoforms and a novel antibody that specifically recognizes the high molecular weight isoform. The expression of tau is highly regulated during development. At E14, all dorsal root ganglion neurons express only the low molecular weight tau isoforms. These isoforms are still present in all dorsal root ganglion neurons in neonates, whereas high molecular weight tau isoforms are expressed in a subset of dorsal root ganglion neurons. The switch from low to exclusively high molecular weight tau expression begins at E18 and is completed during the first postnatal week. In the adult, high molecular weight tau is restricted to small- and medium-sized dorsal root ganglion neurons; its distribution largely coincides with the population of substance P and calcitonin gene related peptide peptidergic neurons. This differential distribution was observed in the cell body, dorsal roots and sciatic nerve axons. In contrast to the protein, however, the distribution of high molecular weight tau messenger RNA is not restricted; all dorsal root ganglion neurons express similar tau messenger RNA levels. The discrepancy between the distribution of protein and messenger RNA suggests control at the post-transcriptional or translational levels. Sciatic nerve axotomy which is followed by axonal regeneration did not alter the differential distribution of high molecular weight tau immunostaining. We conclude that the distribution and expression of tau isoforms during axonal regeneration in adult does not recapitulate the developmental pattern.

Cytoskeletal reorganization of human platelets induced by the protein phosphatase 1/2 A inhibitors okadaic acid and calyculin A

Okadaic acid (OA) and calyculin A (CLA), which are potent and specific inhibitors of serine/threonine protein phosphatases type 1 and 2A, have been shown to induce drastic changes in platelet morphology. The aim of this study was to analyse the molecular mechanisms of OA- or CLA-induced cytoskeletal reorganization, with a specific focus on microtubules and actin filaments. Confocal fluorescence microscopy revealed that OA or CLA altered the distribution of microtubules from marginal band arrangements to homogeneous patterns, consistent with the transmission-electron-microscopic finding that microtubules were fragmented and redistributed into pseudopod-like processes. In thrombin-activated platelets, OA or CLA induced extremely long pseudopods containing an array of microtubules and actin filaments, and a condensed mass of actin filaments in the centre of platelets. OA or CLA induced the constriction of actin filaments without an increase in filamentous (F)-actin, and also rather significantly inhibited actin polymerization in thrombin-activated platelets. Furthermore, neither OA or CLA enhanced phosphorylation of myosin light chain (MLC). By immunoprecipitation of platelet lysate with anti-alpha-tubulin antibody, a 90 kDa protein was co-precipitated with tubulin and was predominantly phosphorylated in the presence of OA. As the time-dependent phosphorylation of 90 kDa protein correlated well with the reorganization of microtubules, these data suggest that phosphorylation and dephosphorylation of this protein might play a role in the regulation of microtubule organization. These findings indicate that OA or CLA induces reorganization of microtubules and actin filaments via the phosphorylation of a microtubule-associated 90 kDa protein and an MLC-phosphorylation-independent mechanism. mechanism.

Identification of an M(r) 60,000 polypeptide unique to the meiotic spindle of the mouse oocyte

The mouse oocyte expresses an M(r) 60,000 (p60) polypeptide that is associated with the first and second meiotic spindles. Immunoreactive p60 was not detectable in the meiotic spindles of male germ cells or in mitotic spindles. P60 was identified with a polyclonal antibody whose predominant activity is directed against ankyrin. However, immunoadsorption experiments demonstrated that p60 is not an ankyrin isoform and represents a secondary activity of the polyclonal antibody. Circumstantial evidence suggest that p60 may be a microtubule-associated protein. Since the most obvious difference between the female meiotic spindle and other spindles is the long half-life of the former, we hypothesize that p60 may function in the maintenance of the long-lived female meiotic apparatus.

Modulation of microtubule dynamic instability in vivo by brain microtubule associated proteins

Heat-stable brain microtubule associated proteins (MAPs) and purified microtubule associated protein 2 (MAP-2) were microinjected into cultured BSC-1 cells which had been previously injected with rhodamine-labeled tubulin. The dynamic instability behavior of individual microtubules was then examined using low-light-level fluorescence microscopy and quantitative microtubule tracking methods. Both MAP preparations suppressed microtubule dynamics in vivo, by reducing the average rate and extent of both growing and shortening events. The average duration of growing events was not affected. When measured as events/unit time, heat-stable MAPs and MAP-2 did not significantly alter the frequency of rescue; the frequency of catastrophe was decreased approximately two-fold by heat-stable MAPs and MAP-2. When transition frequencies were calculated as events/unit distance, both MAP preparations increased the frequency of rescue, without altering the frequency of catastrophe. The percentage of total time spent in the phases of growth, shrink and pause was determined. Both MAP-2 and heat-stable MAPs decreased the percentage of time spent shortening, increased the percentage of time spent paused, and had no effect on percentage of time spent growing. Heat-stable MAPs increased the average pause duration, decreased the average number of events per minute per microtubule and increased the probability that a paused microtubule would switch to growing rather than shortening. The results demonstrate that addition of MAPs to living cells reduces the dynamic behavior of individual microtubules primarily by suppressing the magnitude of dynamic events and increasing the time spent in pause, where no change in the microtubule length can be detected. The results further suggest that the expression of MAPs directly contributes to cell type-specific microtubule dynamic behavior.

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

We previously demonstrated (Ookata et al., 1992, 1993) that the p34cdc2/cyclin B complex associates with microtubules in the mitotic spindle and premeiotic aster in starfish oocytes, and that microtubule-associated proteins (MAPs) might be responsible for this interaction. In this study, we have investigated the mechanism by which p34cdc2 kinase associates with the microtubule cytoskeleton in primate tissue culture cells whose major MAP is known to be MAP4. Double staining of primate cells with anti-cyclin B and anti-MAP4 antibodies demonstrated these two antigens were colocalized on microtubules and copartitioned following two treatments that altered MAP4 distribution. Detergent extraction before fixation removed cyclin B as well as MAP4 from the microtubules. Depolymerization of some of the cellular microtubules with nocodazole preferentially retained the microtubule localization of both cyclin B and MAP4. The association of p34cdc2/cyclin B kinase with microtubules was also shown biochemically to be mediated by MAP4. Cosedimentation of purified p34cdc2/cyclin B with purified microtubule proteins containing MAP4, but not with MAP-free microtubules, as well as binding of MAP4 to GST-cyclin B fusion proteins, demonstrated an interaction between cyclin B and MAP4. Using recombinant MAP4 fragments, we demonstrated that the Pro-rich C-terminal region of MAP4 is sufficient to mediate the cyclin B-MAP4 interaction. Since p34cdc2/cyclin B physically associated with MAP4, we examined the ability of the kinase complex to phosphorylate MAP4. Incubation of a ternary complex of p34cdc2, cyclin B, and the COOH-terminal domain of MAP4, PA4, with ATP resulted in intracomplex phosphorylation of PA4. Finally, we tested the effects of MAP4 phosphorylation on microtubule dynamics. Phosphorylation of MAP4 by p34cdc2 kinase did not prevent its binding to microtubules, but abolished its microtubule stabilizing activity. Thus, the cyclin B/MAP4 interaction we have described may be important in targeting the mitotic kinase to appropriate cytoskeletal substrates, for the regulation of spindle assembly and dynamics.

Expression and distribution of phosphorylated MAP1B in growing axons of cultured hippocampal neurons

Microtubule associated proteins (MAPs) interact with tubulin to modulate neurite stability and growth during development. The phosphorylated form of one of these MAPs, MAP1B (MAP1B-P) is hypothesized to be of particular importance for the regulation of neurite outgrowth. To investigate the mechanisms by which MAP1B and MAP1B-P contribute to this regulation, we used a new antibody against an isoform of MAP1B-P to determine its pattern of expression during neuronal development in vitro. We examined cultured hippocampal neurons because these provide a well-established system to evaluate the development of axons and dendrites. MAP1B, MAP1B-P and MAP2 colocalized to the cell bodies and minor processes during the first 24 hours of culture, but MAP1B-P also extended well into the growth cones. As neurite outgrowth and differentiation proceeded, MAP1B and MAP1B-P became localized to the cell bodies and axons, and MAP2 to the cell bodies and dendrites. After 3 days, MAP1B-P declined in the cell body and was segregated to the distal axon; MAP1B remained in the cell body, but was also concentrated in the distal axon. Over 5-9 days in culture, MAP1B-P levels decreased and became undetectable; MAP1B levels decreased later (19-23 days). MAP2 levels, however, remained high through the entire culture period in cell bodies and dendrites. These results are consistent with the hypothesis that MAP1B-P plays an important role in the initiation and elongation of axons by regulating the dynamics of microtubules near the growth cone: MAP1B-P expression is greatest during the period of active neurite extension, is particularly prominent in growth cones where axon outgrowth is most active, and decreases along with the decline in active axon extension.

Identification of a novel Ca(2+)-regulated protein that is associated with the marginal band and centrosomes of chicken erythrocytes

We have identified a novel Ca(2+)-regulated protein, p23, that is expressed specifically in avian erythrocyte and thrombocyte lineages. Sequence analysis of this 23 kDa protein reveals that it bears no homology to any known sequence. In mature definitive erythrocytes p23 exists in equilibrium between a soluble and a cytoskeletal bound pool. The cytoskeletal fraction is associated with the marginal band of microtubules, centrosomes and nuclear membrane under conditions of low free [Ca2+]. An increase in free [Ca2+] to 10(−6) M is sufficient to induce dissociation of > 95% of bound p23 from its target cytoskeletal binding sites, yet this [Ca2+] has little effect on calmodulin-mediated MB depolymerization. Analysis of p23 expression and localization during erythropoiesis together with results from heterologous p23 expression in tissue cultured cells demonstrated that this protein does not behave as a bone fide microtubule-associated protein. In addition, the developmental analysis revealed that although p23 is expressed early in definitive erythropoeisis, its association with the MB, centrosome and nuclear membrane occurs only in the final stages of differentiation. This cytoskeletal association correlates with marked p23 stabilization and accumulation at a time p23 expression is being markedly downregulated. We hypothesize that the mechanism of p23 association to the MB and centrosomes may be induced in part by a decrease in intracellular [Ca2+] during the terminal stages of definitive erythropoiesis.

Sorting mechanisms of tau and MAP2 in neurons: suppressed axonal transit of MAP2 and locally regulated microtubule binding

Tau is abundant in the axon, whereas MAP2 is found in the cell body and dendrites. To understand their differential localization, we performed transfection studies on primary cultured neurons using tagged tau, MAP2, MAP2C, and their chimeric/deletion mutants. We found that MAP2 was prevented from entering the axon by its N-terminal projection domain and that microtubule binding of tau was stronger in the axon than in the cell body and dendrites, whereas that of MAP2/MAP2C was tighter in the cell body and dendrites than in the axon. These binding properties were determined by their microtubule-binding domains and were suggested to be regulated by phosphorylation, at least in the case of tau. Thus, the suppressed axonal transit of MAP2 and locally regulated microtubule binding may play important roles for their sorting in neurons.

Non-cooperative binding of the MAP-2 microtubule-binding region to microtubules

Microtubule-associated protein (MAP)-2 is a multi-domain cytoskeletal protein that copurifies with brain microtubules (MTs) through repeated cycles of warm polymerization and cold disassembly. Recent equilibrium binding studies of high molecular weight MAP-2ab to taxol-stabilized MTs suggest that the interactions are highly cooperative, as indicated by sigmoidal binding curves, non-linear Scatchard plots, and an apparent all-or-none response in MAP binding in titration experiments (Wallis, K. T., Azhar, S., Rho, M. B., Lewis, S. A., Cowan, N. J., and Murphy, D. B. (1993) J. Biol. Chem. 268, 15158-15167). To learn more about the mechanism of MAP-2 binding to MTs, we investigated the binding properties of bacterially expressed MT-binding region (MTBR) of bovine brain MAP-2. Scatchard plots of the binding data showed no evidence of cooperativity, as reflected by the linear plots of v/[MTBR]free versus v. The stoichiometry was 1-1.1 mol of MTBR/mol of tubulin dimer, and the dissociation constant for the MTBR was 1.1 microM. Bovine brain tau protein competitively inhibited MAP-2 binding, as evidenced by an increased Kd value for MTBR binding to MTs. Although the second repeat peptide m2 (VTSK-CGSLKNIRHRPGGG) is thought to play a dominant role in MAP-2 binding to MTs, a MTBR mutant (with m2 replaced by the third octadecapeptide repeat m3) displays an Kd of 2.8 +/- 0.1 microM and stoichiometry of 0.9 +/- 0.05 mol of MTBR/mol of tubulin dimer. Another mutant with additional copies of the second repeat, designated by us as MTBR[m12m2m32], displayed noncooperative binding with a Kd of 0.53 +/- 0.05 microM and a stoichiometry of 2.2 +/- 0.2 mol of mutant MTBR/tubulin dimer. Equilibrium sedimentation experiments demonstrated that the wild-type MTBR is monomeric, whereas MTBR[m12m2m32] self-associates to a stable dimer over the concentration range used in our MT binding studies. This finding indicates that only one of the two MT-binding sites on the dimer is probably linked to a microtubule at any given time.

Stabilization and bundling of subtilisin-treated microtubules induced by microtubule associated proteins

The acidic carboxy-terminal regions of alpha- and beta-tubulin subunits are currently thought to be centrally involved in microtubule stability and in microtubule association with a variety of proteins (MAPs) such as MAP2 and tau proteins. Here, pure tubulin microtubules were exposed to subtilisin to produce polymers composed of cleaved tubulin subunits lacking carboxy termini. Polymer exposure to subtilisin was achieved in buffer conditions compatible with further tests of microtubule stability. Microtubules composed of normal alpha-tubulin and cleaved beta-tubulin were indistinguishable from control microtubules with regard to resistance to dilution-induced disassembly, to cold temperature-induced disassembly and to Ca(2+)-induced disassembly. Microtubules composed of cleaved alpha- and beta-tubulins showed normal sensitivity to dilution-induced disassembly and to low temperature-induced disassembly, but marked resistance to Ca(2+)-induced disassembly. Polymers composed of normal alpha-tubulin and cleaved beta-tubulin or of cleaved alpha- and beta-tubulins were stabilized in the presence of added MAP2, myelin basic protein and histone H1. Cleavage of tubulin carboxy termini greatly potentiated microtubule stabilization by tau proteins. We show that this potentiation of polymer stabilization can be ascribed to tau-induced microtubule bundling. In our working conditions, such bundling upon association with tau proteins occurred only in the case of microtubules composed of cleaved alpha- and beta-tubulins and triggered apparent microtubule cross-stabilization among the bundled polymers. These results, as well as immunofluorescence analysis, which directly showed interactions between subtilisin-treated microtubules and MAPs, suggest that the carboxy termini of alpha- and beta-tubulins are not primarily involved in the binding of MAPs onto microtubules. However, interactions between tubulin carboxy termini and MAPs remain possible and might be involved in the regulation of MAP-induced microtubule bundling.

Human cerebral cortical cell lines from patients with unilateral megalencephaly and Rasmussen's encephalitis

Continuous cerebral cortical cell lines have been developed from two patients, an 11-month-old with unilateral megalencephaly and a seven-year-old with Rasmussen's encephalitis, designated HCN-1 and HCN-2, respectively. The two cell lines stain for neuronal markers such as neurofilament and neuron-specific enolase but not for non-neuronal markers such as glial fibrillary acidic protein and S-100 protein. In the presence of appropriate growth factors, the cells extend long, branched processes resembling neurons. Differentiation of HCN-1 cells can be induced with nerve growth factor, dibutyryl cyclic AMP and isobutylmethylxanthine, while for HCN-2 cells nerve growth factor, isobutylmethylxanthine and the phorbol ester 12-O-tetradecaoylphorbol-13-acetate are most effective. Immunohistochemical staining of both differentiated cell lines reveals intense staining for GABA, glutamate, somatostatin, cholecystokinin-8 and methionine enkephalin. Two human cortical neuronal cell lines have been developed which represent neuronal precursors. These cell lines propagate in culture and are capable of differentiating upon the addition of a variety of growth factors and chemical agents. These cell lines should prove to be useful models for the study of in vitro neuronal processes.

Calpain inhibitors block Ca(2+)-induced suppression of neurite outgrowth in isolated hippocampal pyramidal neurons

Ca2+ is an important regulator of neurite elongation and growth cone movements but the mechanism(s) mediating these Ca(2+)-dependent effects is unclear. Since cytoskeletal proteins are rapidly degraded by Ca(2+)-dependent proteinases (calpains) in vitro and in vivo, we investigated whether Ca(2+)-induced pruning or regression of neuronal processes is mediated by calpains. Isolated hippocampal pyramidal-like neurons were cultured and the ability of the membrane-permeable calpain inhibitors ethyl(+)-(2S,3S)-3-[(S)-methyl-1-(3-methylbutylcarbamoyl)-butyl carbamoyl]-2 - oxiranecarboxylate (EST) and carbobenzoxyl-valyl-phenylalanyl-H (MDL 28170) to block the Ca2+ ionophore A23187-induced suppression in neurite outgrowth was investigated. Addition of 100 nM A23187 to the culture medium resulted in a retraction of dendrites without altering axonal elongation. The addition of 300 nM A23187 to the culture medium resulted in a significant decrease in the rate of axonal elongation as well as a retraction of dendritic processes. Administration of EST (5 or 20 microM) to the culture medium completely blocked the pruning effect of 100 nM A23187 on dendrites and of 300 nM A23187 on axons, while EST alone did not significantly affect neurite outgrowth rate. MDL 28170 (20 microM) showed the same effect as EST in preventing ionophore-induced pruning of dendrites and axons at 100 and 300 nM concentrations, respectively, of A23187. EST (20 microM) did not block the A23187-induced rise of [Ca2+]i as measured with fura-2. These results suggest that calpains play a role in Ca(2+)-induced pruning of neurites in isolated hippocampal pyramidal neurons.

Spatial and temporal changes in signal transduction pathways during LTP

Following LTP induction in freely moving rats, in situ hybridization revealed discrete changes in the expression of one isoform in each of four families of serine/threonine kinases constitutively expressed in the dentate gyrus of the hippocampus. Expression of the alpha isoform of CaMKII showed a transient increase over the soma and a more persistent increase over the dendritic field of dentate granule cells. Of the PKC isoforms, only gamma PKC was up-regulated substantially 2 hr after LTP induction, declining to control levels 48 hr later. An increase in the expression of mRNA for ERK2 and raf-B was seen at 24 hr only. These results show that, during the maintenance phase of LTP in the hippocampus, there are selective increases in the expression of serine/threonine kinases and that these increases have specific and characteristic temporal and spatial profiles.