The C-terminus of stathmin-like proteins governs the stability of their complexes with tubulin

Microtubule dynamics is modulated by many cellular factors including stathmin family proteins. Vertebrate stathmins sequester two αβ-tubulin heterodimers into a tight complex that cannot be incorporated in microtubules. Stathmins are regulated at the expression level during development and among tissues; they are also regulated by phosphorylation. Here, we study the dissociation kinetics of tubulin:stathmin assemblies in presence of different tubulin-binding proteins and identify a critical role of the C-terminus of the stathmin partner. Destabilizing this C-terminal region may represent an additional regulatory mechanism of the interaction with tubulin of stathmin proteins.

Ultrafast Force-Clamp Spectroscopy of Microtubule-Binding Proteins

Optical trapping has been instrumental for deciphering translocation mechanisms of the force-generating cytoskeletal proteins. However, studies of the dynamic interactions between microtubules (MTs) and MT-associated proteins (MAPs) with no motor activity are lagging. Investigating the motility of MAPs that can diffuse along MT walls is a particular challenge for optical-trapping assays because thermally driven motions rely on weak and highly transient interactions. Three-bead, ultrafast force-clamp (UFFC) spectroscopy has the potential to resolve static and diffusive translocations of different MAPs with sub-millisecond temporal resolution and sub-nanometer spatial precision. In this report, we present detailed procedures for implementing UFFC, including setup of the optical instrument and feedback control, immobilization and functionalization of pedestal beads, and preparation of MT dumbbells. Example results for strong static interactions were generated using the Kinesin-7 motor CENP-E in the presence of AMP-PNP. Time resolution for MAP-MT interactions in the UFFC assay is limited by the MT dumbbell relaxation time, which is significantly longer than reported for analogous experiments using actin filaments. UFFC, however, provides a unique opportunity for quantitative studies on MAPs that glide along MTs under a dragging force, as illustrated using the kinetochore-associated Ska complex.

The STARD9/Kif16a kinesin associates with mitotic microtubules and regulates spindle pole assembly

During cell division, cells form the microtubule-based mitotic spindle, a highly specialized and dynamic structure that mediates proper chromosome transmission to daughter cells. Cancer cells can show perturbed mitotic spindles and an approach in cancer treatment has been to trigger cell killing by targeting microtubule dynamics or spindle assembly. To identify and characterize proteins necessary for spindle assembly, and potential antimitotic targets, we performed a proteomic and genetic analysis of 592 mitotic microtubule copurifying proteins (MMCPs). Screening for regulators that affect both mitosis and apoptosis, we report the identification and characterization of STARD9, a kinesin-3 family member, which localizes to centrosomes and stabilizes the pericentriolar material (PCM). STARD9-depleted cells have fragmented PCM, form multipolar spindles, activate the spindle assembly checkpoint (SAC), arrest in mitosis, and undergo apoptosis. Interestingly, STARD9-depletion synergizes with the chemotherapeutic agent taxol to increase mitotic death, demonstrating that STARD9 is a mitotic kinesin and a potential antimitotic target.

Molecular encounters at microtubule ends in the plant cell cortex

The cortical arrays that accompany plant cell division and elongation are organized by a subtle interplay between intrinsic properties of microtubules, their self-organization capacity and a variety of cellular proteins that interact with them, modify their behaviour and drive organization of diverse, higher order arrays during the cell cycle, cell growth and differentiation. As a polar polymer, the microtubule has a minus and a plus end, which differ in structure and dynamic characteristics, and to which different sets of partners and activities associate. Recent advances in characterization of minus and plus end directed proteins provide insights into both plant microtubule properties and the way highly organized cortical arrays emerge from the orchestrated activity of individual microtubules.

Jupiter, a new Drosophila protein associated with microtubules

In this study we describe a novel Drosophila protein Jupiter, which shares properties with several structural microtubule-associated proteins (MAPs) including TAU, MAP2, MAP4. Jupiter is a soluble unfolded molecule with the high net positive charge, rich in Glycine. It possesses two degenerated repeats around the sequence PPGG, separated by a Serine-rich region. Jupiter associates with microtubules in vitro and, fused with the green fluorescent protein (GFP), is an excellent marker to follow microtubule dynamics in vivo. In a jupiter transgenic Drosophila strain generated by the "protein-trap" technique, Jupiter:GFP fusion protein localizes to the microtubule network through the cell cycle at the different stages of development. We found particularly high Jupiter:GFP concentrations in the young embryo, larval nervous system, precursors of eye photoreceptors and adult ovary. Moreover, from jupiter:gfp embryos we have established two permanent cell lines presenting strongly fluorescent microtubules during the whole cell cycle. In these cells, the distribution of the Jupiter:GFP fusion protein reproduces microtubule behavior upon treatment by the drugs colchicine and taxol. The jupiter cell lines and fly strain should be of wide interest for biologists interested in in vivo analysis of microtubule dynamics.

Tektin 4 is located on outer dense fibers, not associated with axonemal tubulins of flagella in rodent spermatozoa

Tektins, which are thought to be the constitutive proteins of microtubules in cilia, flagella, basal bodies, and centrioles, have been reported to be involved in the stability and structural complexity of axonemal microtubules. Four types of mammalian Tektins have been reported, and at least two types of Tektins, Tektin 2 and Tektin 4, have been verified to be present in sperm flagella. To elucidate the molecular localization of Tektin 4 in flagella of rodent spermatozoa, we performed immunocytochemistry, fractionation study followed by immunoblot analysis, and immunogold electron microscopy. Confocal laser scanning microscopy and immunogold electron microscopy indicated that Tektin 4 was associated with outer dense fibers (ODFs) in both the middle and principal piece of flagella in rat and mouse spermatozoa. Tektin 4 in rat spermatozoa is completely released by 6 M urea treatment, but not extracted by 1% Triton X-100 and 0.6 M potassium thiocyanate. Pre-embedding immunoelectron microscopy demonstrated that Tektin 4 located on the abaxial (convex) surface of ODFs in flagella, not associate with axonemal microtubules. Our data strongly suggested that Tektin 4 is not associated with axonemal tubulins but an ODFs-affiliated molecule in rodent spermatozoa.

Truncation of the projection domain of MAP4 (microtubule-associated protein 4) leads to attenuation of microtubule dynamic instability

MAP4, a ubiquitous heat-stable MAP, is composed of an asymmetric structure common to the heat-stable MAPs, consisting of an N-terminal projection (PJ) domain and a C-terminal microtubule (MT)-binding (MTB) domain. Although the MTB domain has been intensively studied, the role of the PJ domain, which protrudes from MT-wall and does not bind to MTs, remains unclear. We investigated the roles of the PJ domain on the dynamic instability of MTs by dark-field microscopy using various PJ domain deletion constructs of human MAP4 (PJ1, PJ2, Na-MTB and KDM-MTB). There was no obvious difference in the dynamic instability between the wtMAP4 and any fragments at 0.1 microM, the minimum concentration required to stabilize MTs. The individual MTs stochastically altered between polymerization and depolymerization phases with similar profiles of length change as had been observed in the presence of MAP2 or tau. We also examined the effects at the increased concentrations of 0.7 microM, and found that in some cases the dynamic instability was almost entirely attenuated. The length of both the polymerization and depolymerization phases decreased and "pause-phases" were occasionally observed, especially in the case of PJ1, PJ2 or Na-MTB. No obvious change was observed in the increased concentration of wtMAP4 and KDM-MTB. Additionally, the profiles of MT length change were quite different in 0.7 microM PJ2. Relatively rapid and long depolymerization phases were sometimes observed among quite slow length changes. Perhaps, this unusual profile could be due to the uneven distribution of PJ2 along the MT lattice. These results indicate that the PJ domain of MAP4 participates in the regulation of the dynamic instability.

Proteome analysis of the human mitotic spindle

The accurate distribution of sister chromatids during cell division is crucial for the generation of two cells with the same complement of genetic information. A highly dynamic microtubule-based structure, the mitotic spindle, carries out the physical separation of the chromosomes to opposite poles of the cells and, moreover, determines the cell division cleavage plane. In animal cells, the spindle comprises microtubules that radiate from the microtubule organizing centers, the centrosomes, and interact with kinetochores on the chromosomes. Malfunctioning of the spindle can lead to chromosome missegregation and hence result in aneuploidy, a hallmark of most human cancers. Despite major progress in deciphering the temporal and spatial regulation of the mitotic spindle, its composition and function are not fully understood. A more complete inventory of spindle components would therefore constitute an important advance. Here we describe the purification of human mitotic spindles and their analysis by MS/MS. We identified 151 proteins previously known to associate with the spindle apparatus, centrosomes, and/or kinetochores and 644 other proteins, including 154 uncharacterized components that did not show obvious homologies to known proteins and did not contain motifs indicative of a particular localization. Of these uncharacterized proteins, 17 were tagged and localized in transfected mitotic cells, resulting in the identification of six genuine spindle components (KIAA0008, CdcA8, KIAA1187, FLJ12649, FLJ90806, and C20Orf129). This study illustrates the strength of a proteomic approach for the analysis of isolated human spindles and identifies several novel spindle components for future functional studies.

Neuronal-associated microtubule proteins class III beta-tubulin and MAP2c in neuroblastoma: role in resistance to microtubule-targeted drugs

Advanced stage neuroblastoma has a poor clinical outcome and microtubule-destabilizing agents, such as the Vinca alkaloids, are an important component in the treatment of this childhood cancer. Vinca alkaloids bind to beta-tubulin on the alpha/beta-tubulin heterodimer and disrupt microtubule dynamics, leading to cell death. To date, studies examining the contribution of microtubules and associated proteins to the efficacy of microtubule-destabilizing agents in neuroblastoma have been limited. In this study, BE2-C neuroblastoma cells previously selected for resistance to either vincristine (BE/VCR10) or colchicine (BE/CHCb0.2) were found to display significant decreases in neuronal-specific class III beta-tubulin. Interestingly, vincristine-selected cells exhibited increased levels of polymerized tubulin that were not due to alpha-tubulin and class I, II, or III beta-tubulin mutations. Expression levels of the microtubule-depolymerizing protein stathmin were significantly increased in BE/VCR10 cells. In contrast, levels of MAP2a and MAP2b were relatively unaltered. A marked decrease in the neuronal protein, MAP2c, was identified in the vincristine-selected cells and, to a lesser extent, in the colchicine-selected cells. This is the first report describing specific microtubule alterations in neuroblastoma cells resistant to tubulin-targeted agents. The results indicate a need to identify the factors responsible for resistance to tubulin-targeted agents in neuroblastoma so that improved and novel treatment strategies can be developed for this drug refractory disease.

Differential Localization of Decidual Stathmin During Pregnancy in Rats

The present study was undertaken to determine the precise localization of stathmin, a protein associated with microtubule dynamics, during decidualization in rat uterus and to compare it with that of cyclin D3. Immunohistochemical analysis revealed that stathmin is exclusively localized in decidual cells, especially in the primary decidual zone surrounding the embryo, on days 7 and 9 of pregnancy. The intensity of staining was much higher on day 9 than day 7. On day 14, when the endometrial stromal cells had completely differentiated into decidual cells, the staining of decidual cells was faint. Cyclin D3 was expressed in decidual cells of the secondary but not the primary decidual zone on days 7 and 9. On day 14, cyclin D3 levels were low in decidua. Proliferating cell nuclear antigen (PCNA) was broadly detected in the uterus on days 7 and 9, and in the placenta and fetus on day 14. In an artificial decidualization model, cyclin D3 expression was stimulated as deciduoma was formed after an artificial stimulus. Stathmin mRNA levels also increased within 24 h and peaked at 48 h. The specific spatio-temporal uterine expression of stathmin and cyclin D3 suggest that they have a specific role in decidualization in rats.

Cloning and characterization of testis-specific tektin in Bombyx mori

A testis-specific cDNA library of Bombyx mori was constructed by an mRNA subtraction technique. Several clones were randomly selected and determined for their nucleotide sequences. One of them, designated as BmTST, contained a 3'-part of an open reading frame homologous to tektin, the protein known to form filamentous polymers in the walls of ciliary and flagellar microtubules. Also isolated was a genomic fragment, which contains the 5'-part of the coding sequence of BmTST and its promoter region. As a whole, the complete open reading frame was found to encode 508 amino acid residues, whose sequence had 28, 28 and 30% identities with the Strongylocentrotus purpuratus tektins A1, B1 and C1, respectively. Expression analysis by reverse transcription polymerase chain reaction with the cDNA and Western blotting with a polyclonal antibody indicated that the BmTST gene was expressed specifically in the testis during sperm maturation. The protein was immunologically detected exclusively in the fraction expected to contain the 9 + 2 flagellar axonemes of sperms. We infer that the BmTst protein is possibly involved in the spermatogenesis of B. mori.

Tektins as structural determinants in basal bodies

Tektins, present as three equimolar 47-55 kDa protein components, form highly insoluble protofilaments that are integral to the junctional region of outer doublet microtubules in cilia and flagella. To identify and quantify tektins in other compound microtubules such as centrioles or basal bodies, a rabbit antiserum was raised against tektin filaments isolated from Spisula solidissima (surf clam) sperm flagellar outer doublets and affinity-purified with nitrocellulose blot strips of tektins resolved by SDS- or SDS-urea-PAGE. These antibodies recognized analogous tektins in axonemes of organisms ranging from ctenophores to higher vertebrates. Quantitative immunoblotting established that outer doublet tektins occur in a 1:17 weight ratio to tubulin. Cilia and basal apparatuses were prepared from scallop gill epithelial cells; cilia and deciliated cells were prepared from rabbit trachea. Tektins were detected by immunoblotting in basal body-enriched preparations while tektins were localized to individual basal bodies by immunofluorescence. Supported by greater fluorescence in basal bodies than in adjacent axonemes in tracheal cells, analysis of basal apparatuses demonstrated both a proportionately greater ratio of tektin to tubulin (approximately 1:13) and two distinct solubility classes of tektins, consistent with tektins comprising the B-C junction of triplets in addition to the A-B junction as in doublets.

Decreasing oncoprotein 18/stathmin levels reduces microtubule catastrophes and increases microtubule polymer in vivo

Oncoprotein 18/stathmin (Op18) has been identified recently as a protein which destabilizes microtubules. To characterize the function of Op18 in living cells, we used microinjection of anti-Op18 antibodies or antisense oligonucleotides to block either Op18 activity or expression in interphase newt lung cells. Anti-tubulin staining of cells microinjected with anti-Op18 and fixed 1–2 hours after injection showed an increase in total microtubule polymer. In contrast, microinjection of either non-immune IgG or anti-Op18 preincubated with bacterially-expressed Op18 had little effect on microtubule polymer level. Cells treated with Op18 antisense oligonucleotides for 4 days had (greater than or equal to)50% reduced levels of Op18 with no change in the soluble tubulin level. Measurement of MT polymer level in untreated, antisense or nonsense oligonucleotide treated cells demonstrated that reduced Op18 levels resulted in a 2.5-fold increase in microtubule polymer. Next, the assembly dynamics of individual microtubules at the peripheral regions of living cells were examined using video-enhanced contrast DIC microscopy. Microinjection of antibodies against oncoprotein 18 resulted in a 2.2-fold reduction in catastrophe frequency and a slight reduction in plus end elongation velocity compared to uninjected cells or cells microinjected with non-immune IgG. Preincubation of anti-Op18 antibody with recombinant Op18 greatly diminished the effects of the antibody. Similarly, treatment of cells with antisense oligonucleotides reduced catastrophes 2.5- to 3-fold compared to nonsense oligonucleotide treated or untreated cells. The other parameters of dynamic instability were unchanged after reducing Op18 with antisense oligonucleotides. These studies are consistent with Op18 functioning to regulate microtubule catastrophes during interphase in vivo.

Molecular cloning of haploid germ cell-specific tektin cDNA and analysis of the protein in mouse testis

Tektins are a class of proteins that form filamentous polymers in the walls of ciliary and flagellar microtubules. We report here the molecular cloning of a new member of the tektin family, tektin-t, identified from a mouse haploid germ cell-specific cDNA library. Tektin-t mRNA encodes a protein of 430 deduced amino acids possessing RSNVELCRD, the conserved sequence of tektin family proteins. Western blotting showed a single band having a molecular weight of 86 kDa in the mouse testis. Immunohistochemistry of the testis showed that tektin-t is localized in the flagella of elongating spermatids from developmental step 15 to maturity.

Drosophila centrosomes are unable to trigger parthenogenetic development of Xenopus eggs

Centrosomes are powerful and exclusive parthenogenetic agents in the Xenopus egg. We have previously shown that heterologous centrosomes from various vertebrate species were able to promote egg cleavage in Xenopus and that human centrosome activity was associated with an insoluble proteinacious structure that is not significantly simpler than the native centrosome. In this work, we have investigated the parthenogenetic capacity of more evolutionary distant centrosomes. We show that centrosomes devoid of centrioles, such as SPBs isolated from Saccharomyces cerevisiae, do not form asters of microtubules in cytoplasmic extracts from Xenopus eggs, and are inactive in the parthenogenetic test. We further show that Drosophila centrosomes which possess a typical centriole architecture, and are quite active to nucleate microtubules in Xenopus cytoplasmic extracts, are unable to trigger egg cleavage. This was observed both with centrosomes isolated from Drosophila syncytial embryos and nucleus-centrosome complexes from the Drosophila Kc23 cell line. We demonstrate that this inability could not be restored after pre-incubation of Drosophila centrosomes in the egg cytoplasm before injection. We conclude that the parthenogenetic activity of a centrosome is not directly linked to its capacity to nucleate microtubules from the egg tubulin, and that the evolutionary conserved nine-fold symmetrical structure of the centriole cannot be considered as sufficient for triggering procentriole assembly.

Morphological and immunocytochemical effects of dihydroartemisinin on differentiating NB2a neuroblastoma cells

The toxicity caused by the artemisinin derivative dihydroartemisinin in differentiated NB2a neuroblastoma cells was studied by transmission and scanning electron microscopy, Western blotting and immunocytochemistry. Western blotting with monoclonal antibodies failed to detect any specific changes in the cell cytoskeleton, nor were any changes detected by immunocytochemistry. This was consistent with electron microscopy of surviving cell neurites. Transmission electron microscopy revealed that dihydroartemisinin damaged NB2a cell mitochondrial cristae and endoplasmic reticulum. Scanning electron microscopy revealed that dihydroartemisinin depleted the filopodia-like processes projecting from the surface of the cell body and neurites. Some, or all, of these drug-induced changes in differentiating NB2a cells may have a role in the neurotoxicity of artemisinin derivatives.

Localization of motor-related proteins and associated complexes to active, but not inactive, centromeres

Multicentric chromosomes are often found in tumor cells and certain cell lines. How they are generated is not fully understood, though their stability suggests that they are non-functional during chromosome segregation. Growing evidence has implicated microtubule motor proteins in attachment of chromosomes to the mitotic spindle and in chromosome movement. To better understand the molecular basis for the inactivity of centromeres associated with secondary constrictions, we have tested these structures by immunofluorescence microscopy for the presence of motor complexes and associated proteins. We find strong immunoreactivity at the active, but not inactive, centromeres of prometaphase multicentric chromosomes using antibodies to the cytoplasmic dynein intermediate chains, three components of the dynactin complex (dynamitin, Arp1 and p150 Glued ), the kinesin-related proteins CENP-E and MCAK and the proposed structural and checkpoint proteins HZW10, CENP-F and Mad2p. These results offer new insight into the assembly and composition of both primary and secondary constrictions and provide a molecular basis for the apparent inactivity of the latter during chromosome segregation.

Nod factor internalization and microtubular cytoskeleton changes occur concomitantly during nodule differentiation in alfalfa

Reorganization of the plant cytoskeleton is thought to play an important role during nodule ontogeny. In situ immunolocalisation of tubulin reveals that important cytoskeletal changes, implying a transient disorganization followed by a newly patterned reorganization, occur in indeterminate and determinate nodules. In alfalfa nodules, cytoskeletal changes closely parallel the symbiotic differentiation features related to cell infection, bacterial release, endopolyploidization, cell enlargement, cell spatial organization and organelle ultrastructure and positioning. Moreover, the fact that microtubule disorganization can be correlated with Nod factor internalization in central infected cells suggests that Nod factors are possibly involved in the control of cytoskeletal changes which direct the differentiation of bacteria-containing cells.

Nucleation and capture of large cell surface-associated microtubule arrays that are not located near centrosomes in certain cochlear epithelial cells

This report deals with the as yet undetermined issue of whether cell-surface associated microtubules in certain cochlear epithelial cells are centrosomally nucleated and subsequently migrate to microtubule-capturing sites located at the surface regions in question. Alternatively, the cells may possess additional nucleating sites which are noncentrosomal and surface-associated. These alternative possibilities have been investigated for highly polarised epithelial cells called supporting cells in the mouse and guinea pig organ of Corti using antibodies to pericentrin and gamma-tubulin. There is substantial evidence that both proteins are essential components of microtubule-nucleating sites in cells generally. Each mature supporting cell possesses a large microtubule array that is remotely located with respect to its centrosome (more than 10 microns away). The antibodies bind to a cell's centrosome. No binding has been detected at 2 other microtubule-organising centres that are associated with the ends of the centrosomally-remote microtubule array while it is being constructed. Such arrays include thousands of microtubules in some of the cell types that have been examined. If all a cell's microtubules are nucleated by its centrosome then the findings reported above imply that microtubules escape from the centrosomal nucleating site and migrate to a new location. Furthermore capture of the plus and minus ends of the errant microtubules is taking place because both ends of a centrosomally-remote microtubule array are attached to sites that are precisely positioned at certain cell surface locations. Minus ends are locating targets with an exactitude comparable to that which has been demonstrated for plus ends in certain cell types. These cells apparently operate a single control centre strategy for microtubule nucleation that is complemented by precise positioning of plus and minus end-capturing sites at the cell surface.

The effect of sodium tetradecyl sulfate on mobility and antigen detectability of microtubule proteins in sodium dodecyl sulfate-polyacrylamide gel electrophoresis

Several factors been reported to influence the mobility of polypeptide in sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) including the brand of SDS. Using microtubule proteins from axonemes of Lytechinus pictus and Spisula solidissima sperm and meiotic spindles of Spisula solidissima we demonstrate that the change in mobility was caused by sodium tetradecyl sulfate (STS), a major contaminant of many commercial SDS brands. We also examined the use of sodium tetradecyl sulfate and different SDS brands as a tool in extracting more information from immunoblot studies. Commercial SDS containing contaminants other than sodium tetradecyl sulfate reduced or eliminated the immunosignal from certain polypeptides and the loss of antigenicity could not even be recovered by immunoblot under "renaturing" conditions. It can thus be concluded that STS can be useful in separating and identifying comigrating polypeptides and in detecting additional immunobands in immunoblots.

Endoplasmic reticulum to Golgi trafficking in multinucleated skeletal muscle fibers

The organization of membrane trafficking between endoplasmic reticulum and Golgi within multinucleated muscle fibers was analyzed. We found that markers for the compartment involved in endoplasmic reticulum to Golgi trafficking exhibited perinuclear as well as interfibrillar localization. Furthermore, these markers showed prominent colocalization with microtubules. To analyze membrane trafficking, we followed the temperature-controlled transport of the G protein of the mutant vesicular stomatitis virus, tsO45, in isolated myofibers. Perinuclear and cross-striated staining were seen at 39 degrees C, while at 15 degrees C a diffuse staining component appeared along a subset of interfibrillar microtubules. At 20 degrees C, bright Golgi spots were seen to be associated with microtubules that appeared as circumnuclear rings and longitudinal bundles. Beneath the motor end plate, however, the organization of the Golgi elements and microtubules was found to be distinctive. Retrograde trafficking induced by brefeldin A resulted in the disappearance of the Golgi spots throughout the myofibers and the appearance of staining along microtubules. Thus, interfibrillar membranes seem to be active in protein export, and trafficking between endoplasmic reticulum and Golgi elements occurred throughout the myofibers. The results suggest that microtubules served as tracks for the two-way trafficking between the endoplasmic reticulum and the Golgi compartment.

5-Azacytidine and BDNF enhance the maturation of neurons derived from EGF-generated neural stem cells

EGF-generated neural stem cells can form astrocytes, neurons, and oligodendrocytes upon differentiation; however, the proportion of cells that actually form neurons is very small. In the present study, we have studied the effect that 5-azacytidine (5AzaC), a demethylation agent, and brain-derived growth factor (BDNF) have on the differentiation and maturation of neurons originating from EGF-generated neural stem cells. Stem cells were maintained under a variety of culture conditions using combinations of 5AzaC and BDNF either alone or together. More neurons, as determined by the number of beta-tubulin III-immunoreactive somata, were present in cultures maintained in BDNF medium (a nearly fourfold increase compared to control cultures). 5AzaC did not significantly affect neuronal number, regardless of the presence of BDNF. In addition to neuronal number, the effect of 5AzaC and BDNF on the distribution of the microtubule proteins MAP2 and Tau was analyzed. In most cultures, MAP2 and Tau were colocalized throughout the neuron. In contrast, neurons cotreated with 5AzaC and BDNF contained neurons that began to exhibit cytoskeletal segregation of MAP2 into the somatodendritic compartments. Tau remained dispersed within the somata and the axon. This effect was not produced when 5AzaC or BDNF was used individually. These results demonstrate that 5AzaC and BDNF cooperate to produce more mature neurons from EGF-generated neural stem cells then either molecule can alone.

Centractin (ARP1) associates with spectrin revealing a potential mechanism to link dynactin to intracellular organelles

Centractin (Arp1), an actin-related protein, is a component of the dynactin complex. To investigate potential functions of the protein, we used transient transfections to overexpress centractin in mammalian cells. We observed that the overexpressed polypeptide formed filamentous structures that were significantly longer and more variable in length than those observed in the native dynactin complex. The centractin filaments were distinct from conventional actin in subunit composition and pharmacology as demonstrated by the absence of immunoreactivity of these filaments with an actin-specific antibody, by resistance to treatment with the drug cytochalasin D, and by the inability to bind phalloidin. We examined the transfected cells for evidence of specific associations of the novel centractin filaments with cellular organelles or cytoskeletal proteins. Using immunocytochemistry we observed the colocalization of Golgi marker proteins with the centractin polymers. Additional immunocytochemical analysis using antibodies to non-erythroid spectrin (fodrin) and Golgi-spectrin (beta I sigma *) revealed that spectrin colocalized with the centractin filaments in transfected cells. Biochemical assays demonstrated that spectrin was present in dynactin-enriched cellular fractions, was coimmunoprecipitated from rat brain cytosol using antibodies to dynactin subunits, and was coeluted with dynactin using affinity chromatography. Immunoprecipitations and affinity chromatography also revealed that actin is not a bona fide component of dynactin. Our results indicate that spectrin is associated with the dynactin complex. We suggest a model in which dynactin associates with the Golgi through an interaction between the centractin filament of the dynactin complex and a spectrin-linked cytoskeletal network.

Functional analysis of dynactin and cytoplasmic dynein in slow axonal transport

The neuron moves protein and membrane from the cell body to the synapse and back via fast and slow axonal transport. Little is known about the mechanism of microtubule movement in slow axonal transport, although cytoplasmic dynein, the motor for retrograde fast axonal transport of membranous organelles, has been proposed to also slide microtubules down the axon. We previously showed that most of the cytoplasmic dynein moving in the anterograde direction in the axon is associated with the microfilaments and other proteins of the slow component b (SCb) transport complex. The dynactin complex binds dynein, and it has been suggested that dynactin also associates with microfilaments. We therefore examined the role of dynein and dynactin in slow axonal transport. We find that most of the dynactin is also transported in SCb, including dynactin, which contains the neuron-specific splice variant p135(Glued), which binds dynein but not microtubules. Furthermore, SCb dynein binds dynactin in vitro. SCb dynein, like dynein from brain, binds microtubules in an ATP-sensitive manner, whereas brain dynactin binds microtubules in a salt-dependent manner. Dynactin from SCb does not bind microtubules, indicating that the binding of dynactin to microtubules is regulated and suggesting that the role of SCb dynactin is to bind dynein, not microtubules. These data support a model in which dynactin links the cytoplasmic dynein to the SCb transport complex. Dynein then may interact transiently with microtubules to slide them down the axon at the slower rate of SCa.

Hydrostatic pressure has different effects on the assembly of tubulin, actin, myosin II, vinculin, talin, vimentin, and cytokeratin in mammalian tissue cells

Hydrostatic pressures in the range of hundreds of atmospheres are known to disrupt cytoskeletal organization in tissue culture cells, with profound changes in cell shape. The molecular mechanisms of these effects are poorly understood. To determine the effect of pressure on the cytoskeleton, and thus to provide better indicators of the molecular mechanisms, we used fluorescent antibody staining to compare the organizations of seven different cytoskeletal proteins in HeLa cells and rat osteosarcoma cells (ROS-17/2.8) subjected to different pressures up to 400 atm. Pressures of 300 atm or more caused cells of both lines to "round up" and to withdraw their lamellar extensions. However, this response varied within a population of cells, with some cells remaining spread at pressures that caused their neighbors to round up. The most resistant to rounding were those cells touching other cells, and the occasional giant cells. As expected, the rounded cells showed disruption of actin stress fibers and of vinculin and talin at focal contacts. The unrounded cells showed less disruption in the organization of these same proteins. Microtubules and myosin II filaments appeared resistant to 400 atm pressure in both cell types, whether rounded or unrounded. However, in HeLa cells, the intermediate filaments, vimentin and cytokeratin, depolymerized and formed small vesicles when pressures exceeded 200 atm, and this occurred in rounded as well as unrounded cells. In osteosarcoma cells, which do not have cytokeratin, vimentin did not depolymerize. We discuss different mechanisms that might explain these responses to pressure, including direct effects on the equilibria of protein polymerization and less direct effects on regulatory mechanisms, such as phosphorylation pathways, that control cytoskeletal organization. The later type of explanation seems more consistent with both the variability of response within cell populations and the difference in vimentin's response in one cell line compared with the other.

PF16 encodes a protein with armadillo repeats and localizes to a single microtubule of the central apparatus in Chlamydomonas flagella

Several studies have indicated that the central pair of microtubules and their associated structures play a significant role in regulating flagellar motility. To begin a molecular analysis of these components we have generated central apparatus-defective mutants in Chlamydomonas reinhardtii using insertional mutagenesis. One paralyzed mutant recovered in our screen, D2, is an allele of a previously identified mutant, pf16. Mutant cells have paralyzed flagella, and the C1 microtubule of the central apparatus is missing in isolated axonemes. We have cloned the wild-type PF16 gene and confirmed its identity by rescuing pf16 mutants upon transformation. The rescued pf16 cells were wild-type in motility and in axonemal ultrastructure. A full-length cDNA clone for PF16 was obtained and sequenced. Database searches using the predicted 566 amino acid sequence of PF16 indicate that the protein contains eight contiguous armadillo repeats. A number of proteins with diverse cellular functions also contain armadillo repeats including pendulin, Rch1, importin, SRP-1, and armadillo. An antibody was raised against a fusion protein expressed from the cloned cDNA. Immunofluorescence labeling of wild-type flagella indicates that the PF16 protein is localized along the length of the flagella while immunogold labeling further localizes the PF16 protein to a single microtubule of the central pair. Based on the localization results and the presence of the armadillo repeats in this protein, we suggest that the PF16 gene product is involved in protein-protein interactions important for C1 central microtubule stability and flagellar motility.

Identification of the chromosome localization domain of the Drosophila nod kinesin-like protein

The nod kinesin-like protein is localized along the arms of meiotic chromosomes and is required to maintain the position of achiasmate chromosomes on the developing meiotic spindle. Here we show that the localization of ectopically expressed nod protein on mitotic chromosomes precisely parallels that observed for wild-type nod protein on meiotic chromosomes. Moreover, the carboxyl-terminal half of the nod protein also binds to chromosomes when overexpressed in mitotic cells, whereas the overexpressed amino-terminal motor domain binds only to microtubules. Chromosome localization of the carboxyl-terminal domain of nod depends upon an 82-amino acid region comprised of three copies of a sequence homologous to the DNA-binding domain of HMG 14/17 proteins. These data map the two primary functional domains of the nod protein in vivo and provide a molecular explanation for the directing of the nod protein to a specific subcellular component, the chromosome.

Flagellar assembly in two hundred and fifty easy-to-follow steps

The eukaryotic flagellum is a complex biochemical machine that moves cells or moves materials over the surface of cells, such as in the mammalian esophagus, oviduct or in protozoa. It is composed of over 250 polypeptides that must be assembled into a number of different structures and each structure must be attached with an exact periodicity along the microtubules. Once the flagellum is assembled, each of the components must act in concert and in three dimensions to produce a complex waveform. This review provides an outline of the composition and function of the different structures found in the flagella of Chlamydomonas.

DNA binding and meiotic chromosomal localization of the Drosophila nod kinesin-like protein

The Drosophila no distributive disjunction (nod) gene encodes a kinesin-like protein that has been proposed to push chromosomes toward the metaphase plate during female meiosis. We report that the nonmotor domain of the nod protein can mediate direct binding to DNA. Using an antiserum prepared against bacterially expressed nod protein, we show that during prometaphase nod protein is localized on oocyte chromosomes and is not restricted to either specific chromosomal regions or to the kinetochore. Thus, motor-based chromosome-microtubule interactions are not limited to the centromere, but extend along the chromosome arms, providing a molecular explanation for the polar ejection force.

Polyribosome targeting to microtubules: enrichment of specific mRNAs in a reconstituted microtubule preparation from sea urchin embryos

A subset of mRNAs, polyribosomes, and poly(A)-binding proteins copurify with microtubules from sea urchin embryos. Several lines of evidence indicate that the interaction of microtubules with ribosomes is specific: a distinct stalk-like structure appears to mediate their association; ribosomes bind to microtubules with a constant stoichiometry through several purification cycles; and the presence of ribosomes in these preparations depends on the presence of intact microtubules. Five specific mRNAs are enriched with the microtubule-bound ribosomes, indicating that translation of specific proteins may occur on the microtubule scaffolding in vivo.

Dephosphorylation of sperm midpiece antigens initiates aster formation in rabbit oocytes

During fertilization in most mammals, the penetrating sperm organizes an aster of microtubules. We have investigated the mechanisms underlying this function of the sperm by a series of experiments based on microinjection of isolated sperm midpieces into unfertilized oocytes. These midpieces contain antigens recognized by the MPM-2 antibody. These antigens, which are absent from the rest of the tail fraction, correspond to three phosphorylated polypeptides of 77, 81, and 85 kDa. Dephosphorylation with alkaline phosphatase abolishes antigenicity on blots and in whole sperm. Reactivity to the antibody disappears between 1 and 3 hr after calcium stimulation of oocytes, following the decline in H1 kinase activity and coincident with aster formation. In unactivated oocytes, no aster forms and the antigen remains unchanged. MPM-2 treatment of midpieces prior to injection blocks their ability to form asters in oocytes activated by calcium stimulation. The epitope also disappears in 6-methyl-aminopurine-treated oocytes, implying that maintenance of the phosphorylated state requires kinase activity. A result that confirms this view is that sperm midpieces dephosphorylated by alkaline phosphatase can be rephosphorylated after injection into oocytes or by exposure in vitro to a Xenopus oocyte cytoplasmic fraction high in H1 kinase activity. We suggest that the microtubule nucleation activity of sperm midpieces after fertilization is triggered by the calcium-induced decrease in maturation promoting factor, which results in dephosphorylation of specific sperm centrosomal proteins.

The APC gene product associates with microtubules in vivo and promotes their assembly in vitro

Defects in the APC gene occur frequently in patients with familial adenomatous polyposis coli and are associated with the progression of sporadic tumors of the colon and stomach. We examined the subcellular location of adenomatous polyposis coli (APC) protein resulting from transient expression of full length and partial APC complementary DNAs in epithelial cells. Immunofluorescent detection revealed an association of APC with cytoplasmic microtubules. Expression of partial complementary DNA constructs indicated that the carboxy-terminal region of the APC protein, typically deleted in cancers, is essential for this association. The same APC polypeptides that associated with microtubules in vivo also dramatically promoted their assembly in vitro. These results suggest that wild-type APC protein binds to and affects the assembly of microtubules, whereas the mutants identified in tumors have lost this activity.

Wild-type but not mutant APC associates with the microtubule cytoskeleton

The adenomatous polyposis coli protein (APC) is mutated in familial adenomatous polyposis patients as well as in sporadic colorectal tumors. In an attempt to further understand the function of APC, the subcellular localization of APC was examined. Wild-type and mutant forms of APC were expressed in mammalian cells and protein detected by immunofluorescence using monoclonal and polyclonal antibodies. Staining of wildtype APC protein revealed a filamentous network which extended throughout the cytoplasm and colocalized with microtubules. In striking contrast, mutant APC protein gave a diffuse cytoplasmic staining pattern. Treatment with a microtubule depolymerizing agent, nocodazole, caused APC as well as tubulin to become diffusely cytoplasmic. In addition, immunoperoxidase staining of transfected APC protein followed by transmission electron microscopy revealed staining of microtubules. These results suggest that wild-type but not mutant APC protein may be associated with the microtubule cytoskeleton.

Quantitative determination of the proportion of microtubule polymer present during the mitosis-interphase transition

We have developed a new method for determining levels of tubulin polymer, based on quantitative fluorescence detection of x-rhodamine tubulin microinjected into living cells and we have applied this method to analysis of the mitosis-interphase transition. LLC-PK cells in interphase and mitosis were microinjected, then cooled and rewarmed to drive tubulin incorporation. Total tubulin fluorescence in individual, living cells was quantified using a cooled, scientific grade CCD image sensor. Cells were then washed and lysed into a microtubule-stabilizing buffer to extract the soluble pool. Total tubulin polymer fluorescence was determined for the extracted cells in the same way as for living cells. Fluorescence images were corrected by flat-fielding and background subtraction. The ratio of extracted cell fluorescence/living cell fluorescence for individual cells, was taken as the proportion of tubulin as polymer. Cells in M-phase, G1 and random interphase were analyzed. G1 cells had almost the same proportion as random interphase cells. Mitotic cells gave a value of 90 +/- 5% of G1 cells at 37 degrees C. Within M-phase, levels of tubulin as polymer in metaphase and early anaphase were not significantly different. In contrast to the general expectation of microtubule depolymerization at anaphase onset, these results indicate that as cells exit mitosis, the overall proportion of tubulin as polymer does not change dramatically even though the mitotic spindle disassembles. We conclude that the mitosis-interphase transition is accompanied by a redistribution of tubulin at an essentially constant polymer level. Therefore, a global shift to depolymerization conditions is not the driving force for anaphase chromosome movement.

Isolation and analysis of microtubule motor proteins

Isolation of microtubule motor proteins is needed both for the discovery of new motors and for characterization of the products of motor-related genes. The sequences of motor-related genes cannot yet be used to predict the mechanochemical properties of the gene products. This was illustrated by the first kinesin-related gene product to be characterized. Protein expressed from the ncd gene moved toward the minus ends of microtubules (Walker et al., 1990; McDonald et al., 1990), while kinesin itself moves toward the plus ends. Until the relationship between mechanochemical function and amino acid sequence is more thoroughly understood, biochemical isolation and characterization of microtubule motor proteins will remain essential. Two approaches for getting useful quantities of microtubule motor proteins have been used: isolation from cytosol as described under Section II above and isolation from bacteria carrying cloned motor protein genes in expression vectors. Bacterial expression of functional microtubule motors has been successful to date in only a few cases (Yang et al., 1990; Walker et al., 1990, McDonald et al., 1990). Additional progress is expected with the expression of cloned genes from viral vectors in cultured eukaryotic cells, but broad success has not yet been reported. Biochemical isolation of motors from their natural cytosol has some distinct advantages. One can have confidence that a given motor will be folded properly and have normal post-translational modifications. In addition, if it exists in vivo as a heteromultimer, a microtubule motor isolated from its native cytosol will carry with it a normal complement of associated proteins. Studies of such associated proteins will be important in learning how motors accomplish their tasks in vivo. Drosophila cytosol should be a rich source of microtubule motors. Drosophila carry at least 11 and perhaps as many as 30 genes that are related to kinesin (Stewart et al., 1991; Endow and Hatsumi, 1991). The work of Tom Hays' lab indicates that Drosophila carry more than nine dynein related genes (Rasmussen et al., 1994). Relatively little effort to isolate the products of these genes from cytosol has been made. The only work that I am aware of has produced a kinesin-like microtubule motor (D.G. Cole, K.B. Sheehan, W.M. Saxton, and J.M. Scholey, in progress) that may be the Drosophila homolog of Xenopus eg5 (Sawin et al., 1992). This isolation was straightforward, and efforts to identify additional motors are almost assured of success.

Localization of tau and other proteins of isolated marginal bands

To determine which proteins were associated with and intrinsic to the marginal band (MB) of microtubules (MTs), we studied protein components of MBs isolated from nucleated erythrocytes by differential detergent solubilization of the membrane skeleton (MS). MBs isolated from dogfish erythrocytes contained major proteins in the tubulin M(r) range. A high molecular weight protein of approximately 290 kD that bound antibody to syncolin and to heat-stable brain MAPs was present in the whole cytoskeleton. However, most of it was solubilized by the MB isolation medium, together with the MS. Dogfish erythrocyte cytoskeletons and isolated MBs were examined with polyclonal and monoclonal antibodies against mammalian brain tau and chicken erythrocyte tau. As shown by immunofluorescence and immunoblotting, these antibodies bound to proteins in the 50 to 67 kD range, located along the length of isolated MBs. Two-dimensional SDS-PAGE revealed isolated MB proteins of pI approximately 6.8 in the same molecular weight range, as well as alpha- and beta-tubulin with pI approximately 5.4. Subtilisin or high-salt treatment of isolated MBs resulted in unbundling of MTs, indicating involvement of MAPs. MBs isolated from chicken erythrocyte cytoskeletons also contained tau as shown by anti-mammalian brain tau immunofluorescence. Both chicken and dogfish isolated MBs also bound phalloidin, but the binding was usually discontinuous and, for any given MB, matched the pattern of anti-syncolin binding. Both syncolin and F-actin were part of the MS remnant remaining after MT disassembly, supporting their assignment to a specialized MS region at the MB/MS interface. In contrast, tau protein appears to be intrinsic to the MB, where it may have an MT stabilizing and bundling function.

Centrosome repositioning immediately following karyokinesis and prior to cytokinesis

The behaviour of the centrosome immediately following cell division in tissue culture cells has been investigated. We find that following karyokinesis, but preceding cytokinesis, sister centrosomes relocate from the spindle poles to a position adjacent to the intercellular bridge. This repositioning is accompanied by the appearance of a microtubule bundle that extends from the poleward region of the cell to the centrosome and increases in length as the centrosome approaches the intercellular bridge. Disruption of this bundle with colcemid interrupts centrosome repositioning. In contrast, centrosome repositioning persists in late mitotic cells grown in the presence of cytochalasin D. However, the position of the microtubule-centrosome complex within the cell is randomized suggesting that the path, but not the process, of centrosome repositioning is dependent on an intact actin filament network. This study points out, for the first time, that the complex migration of the centrosome preceding mitosis is paralleled by an equally complex set of events following cell division. We suggest that post-mitotic centrosome repositioning may play a role in ensuring that daughter cells have equal but opposite polarity and may reflect an interrelationship between the establishment of the interphase cytoskeleton and the completion of cytokinesis.

Ca(2+)-calmodulin regulated effectors of microtubule stability in neuronal tissues

In general, microtubules are labile structures which depolymerize at low temperature and are sensitive to Ca2+. However, in brain tissue, axonal microtubules are disassembly-resistant and can exist without attachment to a microtubule organizing center. Stable microtubules cannot be purified by usual recycling procedures and this has made the elucidation of the molecular mechanisms involved in their stabilization difficult. This paper summarizes previous work in our laboratories, aimed at the identification of brain microtubule stabilizing proteins. We present assay methods which allow the detection of microtubule stability effectors in complex extracts and in chromatographic column fractions. Applied to brain crude extracts, they result in the isolation of Ca(2+)-calmodulin binding and Ca(2+)-calmodulin regulated proteins. One, called STOP, appears to account for microtubule stabilization in neurons. A second protein with similar activity is myelin basic protein. Non-neuronal tissues also contain Ca(2+)-calmodulin-regulated effectors which appear to differ in structure from their neuronal counterparts. Thus, in all tissues examined, microtubule stability seems to be accounted for by unique Ca(2+)-calmodulin regulated proteins, showing tissue specificity.

Suppression of MAP2 in cultured cerebellar macroneurons inhibits minor neurite formation

We show here that antisense MAP2 oligonucleotides inhibit neurite outgrowth in cultured cerebellar macroneurons. Unlike control neurons, which first extend a lamellipodial veil followed by a consolidation phase during which the cells extend minor neurites, MAP2-suppressed cells persist with lamellipodia and later become rounded. The induction of microtubules containing tyrosinated tubulin, which parallels neurite outgrowth in control neurons, was blocked under antisense conditions. The small but significant increase in acetylated microtubules was not affected. In contrast, the suppression of tau, which selectively blocks axonal elongation, completely prevented the increase of acetylated microtubules, but did not modify the induction of labile microtubules. These results suggest that MAP2 and tau have different functions: the initial establishment of neurites depends upon MAP2, whereas further neurite elongation depends upon tau and microtubule stabilization.

Localization of actin filaments and microtubules in the cells of the Limulus lateral and ventral eyes

The ommatidia of the lateral eye of the horseshoe crab, Limulus polyphemus, undergo rhythmic changes in structure that are driven by diurnal lighting and efferent neural activity from a circadian clock in the brain. This study uses cytochemical probes to investigate the cytoskeletal elements mediating these responses and to develop models for their control. Antibodies to actin and phallodin, a specific F-actin probe, label the rhabdom of lateral eye ommatidia, the cone cells of the ommatidial aperture, the ommatidial sheath, and the peripheral regions of the photoreceptor (retinular cell) cytoplasm. These probes also label the rhabdomere of ventral photoreceptors. Antibodies to tubulin label the eccentric cell dendrite and soma in each lateral eye ommatidium, the cone cells of the aperture, and the peripheral retinular cell cytoplasm. Models are proposed for the cytoskeletal mechanisms involved in controlling aperture and rhabdom shape, pigment movement, and shedding of rhabdomeral membrane.

Evidence for a non-tubulin spindle matrix and for spindle components immunologically related to tektin filaments

Tektins were originally described as a set of three filamentous proteins (tektin A, B and C) associated with the walls of axonemal microtubules of sea urchin sperm. Using affinity-purified polyclonal antibodies raised against tektins of two sea urchin species, Lytechinus pictus and Strongylocentrotus purpuratus, we looked for tektin-like components in microtubule systems other than axonemes. By immunofluorescence microscopy we observed labeling of meiotic spindles in eggs of the surf clam Spisula solidissima and in several mammalian cell lines. In Spisula eggs the tektin-like antigens were still associated with the spindles after about 95% of the tubulin had been removed via a calcium/cold treatment. In pig kidney epithelial cells the tektin-like antigen appeared to be associated with bundles of calcium-stable spindle microtubules. By SDS-PAGE immunoblot the affinity-purified anti-tektins recognized several polypeptides in tubulin-depleted spindle remnants of Spisula eggs: A approximately 52 kDa, 1 M KCl-resistant component was identified by the antibody raised against tektin C from S. purpuratus, a approximately 48 kDa component was recognized by the antibody specific for tektin A from L. pictus, and three polypeptide bands (approximately 64 kDa, approximately 100 kDa and greater than 200 kDa) were detected by the antibody specific for tektin C from L. pictus. Only the latter antibody, however, stained Spisula spindles by immunofluorescence microscopy. We further report that the sensitivity of antibody recognition of proteins on immunoblots is dependent on the purity of sodium dodecyl sulfate.

A monoclonal antibody, raised against mammalian centrosomes and screened by recognition of plant microtubule organizing centers, identifies a pericentriolar component in different cell types

We have used monoclonal antibodies raised against isolated native calf thymus centrosomes to probe the structure and composition of the pericentriolar material. To distinguish prospective antibodies as specific to conserved elements of this material, we screened clones by their identification of microtubule organizing centers (MTOCs) in different animal and plant cells. Among the clonal antibodies that reacted with MTOCs in both plant and mammalian cells, we describe one (mAb 6C6) that was found to immunostain centrosomes in a variety of bovine and human cells. In cycling cells this signal persisted through the entire cell cycle. Microscopy showed that the mAb 6C6 antigen was a component of the pericentriolar material and this was confirmed by biochemical analysis of centrosomes. Using immunoblot analysis of protein fractions derived from purified components of centrosomes, we have characterized the mAb 6C6 antigen as a 180 kDa polypeptide. We conclude that we have identified a protein component permanently associated with the pericentriolar material. Surprisingly, monoclonal antibody 6C6 also stained other mitotic organelles in mammalian cells, in a cell-cycle-dependent manner. During prometaphase and metaphase the antibody stained both centrosomes and kinetochores. At the onset of anaphase the kinetochore-specific staining dissociated from chromosomes and was subsequently redistributed onto a newly characterized organelle, the telophase disc while the centrosomal stain remained intact. It is not known if the 180 kDa centrosomal protein itself redistributes during mitosis, or if the pattern observed represents other antigens with shared epitopes. The pericentriolar material is thought to be composed of conserved elements, which appeared very early during the evolution of eukaryotes. Our results strongly suggest that mAb 6C6 identifies one of these elements.

Identification of a tektin-like protein associated with neurofilaments in the developing chick nervous system

A 160-kD polypeptide, which is recognized by an affinity-purified polyclonal antibody to the 55-kD tektin-A polypeptide from sea urchin sperm flagellar microtubules, is associated with neurofilaments in embryonic chick nerve cells. Antibodies to tektin-A and monoclonal antibodies to the neurofilament triplet proteins colocalize to filaments in cultured nerve cells and to filaments in extracts of chick spinal cord, using indirect immunofluorescence microscopy and immunogold electron microscopy. The antigen reacting with anti-tektin-A in chick brain and spinal cord extracts has been identified as a 160-kD polypeptide by SDS-PAGE and has been shown to be distinct from the known neurofilament-triplet proteins by two-dimensional immunoblot analysis. These data suggest that a unique protein with limited sequence homology to tektin-A is a component of the neuronal cytoskeleton and is incorporated into or associated with neurofilaments.

Cytoskeletal specializations at the rod photoreceptor distal tip

We have examined microtubules and microtubule-like elements within the toad rod photoreceptor outer segment in order to define regional specializations of the photoreceptor cytoskeleton. "Ciliary" microtubules were localized within the rod outer segment (ROS) by using thin section electron microscopy, immunofluorescence, and rapid-freeze deep-etch microscopy. All three methods showed that ciliary microtubules stop short of the extreme ROS distal tip, although abundant microtubule-like structures distinct from the ciliary microtubules were found within the distal 10-15 microns of the ROS tip. These heretofore undescribed "distal ROS tubules" are clustered at the clefts or incisures of the disk membrane stack and resemble microtubules in overall size and shape, although they are not closely related antigenically to tubulin. The distal ROS tubules are more abundant in green rods than red rods and vary in number during the daily light/dark cycle. Quantitation of these tubules at two time points during the light/dark cycle suggests that there are three- to fourfold more tubules in the ROS tip one hour after light onset than one hour before light onset. Retinas prevented from normal disk membrane shedding by separation of the retina from the adjacent pigment epithelium, failed to develop increased numbers of tubules after light onset. This suggests that the newly described distal ROS tubules may modulate or be modulated by light-induced interactions between the photoreceptors and pigment epithelium, such as those that occur during the disk shedding phase of membrane turnover.

Differential in vitro action of S-12363, a new vinblastine derivative, and of its epimer on microtubule proteins

The action of two epimers of a new vinblastine derivative that differ in their in vivo antitumor activity and their cytotoxicity was studied in vitro in brain microtubule proteins. These two compounds, called S-12363 and S-12362, could not be distinguished from one another or from other active vinca alkaloids by their ability to prevent microtubule assembly. However, they differed strongly both from one another and from vincristine and vinblastine in their ability to induce the formation of tubulin paracrystals and in the stability of the paracrystals following temperature shifts from 0 degree to 37 degrees C and vice versa. The most potent drug, S-12363, induced considerable tubulin aggregation, which was even more pronounced than that observed in the presence of vincristine. Previous results have shown that S-12363, in contrast to vincristine, induces no neurotoxic effects. This observation is in disagreement with a direct relationship between tubulin aggregation and neurotoxicity.

Ultrastructural evidence that insoluble microtubules are components of the neurofibrillary tangle

The ultrastructure of Alzheimer's neurofibrillary tangles is heterogeneous and includes abnormal paired helical filaments (PHF) and various other insoluble structures. Insoluble non-PHF components isolated from neurofibrillary tangles were examined by electron microscopy. Comparison of these fractions with normal assembled neurofilaments and normal brain microtubules revealed scattered profiles which were morphologically (not chemically) identical to structures present in the microtubule, but not in the neurofilament preparations. These results support the notion that insoluble microtubules contribute to the make up of the neurofibrillary tangle. Based on these findings, preliminary experiments were conducted which suggest that non-enzymatic glycosylation may be a pathway leading to insolubility of the microtubules.