Le spermatozoide de Ophidion sp. (Poisson,téléostéen): Particularités ultrastructurales du flagelle

The spermatozoon of Ophidion sp. possesses an elongated nucleus 8 μm long, a short midpiece (0,6 μm), and a long flagellum (100 μm). The flagellar membrane extends in the form of two diametrically opposed sidefins. Evolving spermatids and spermatozoa are found in the lumen of the seminiferous tubes. The sections of flagella show filamentary and tubular elements disposed parallel to the axoneme microtubules. We have divided the flagella into three types. In type 1 the tip of the sidefins contains 20 to 30 filaments 5 run in diameter and between these and the axoneme 20 to 30 tubular elements 15 to 20 nm in diameter. Type 2 possesses a dense cytoplasm and a few tubular elements 10 nm in diameter disposed at the tip of the sidefins. Type 3 contains a cytoplasm which is not dense and in which we found polysaccharides and 1 to 8 tubular elements forming a palisade which lines the plasma membrane at the tip of the sidefins. We interpret these three types as three successive stages in the organization of the flagellum during spermiogenesis. Type 3 corresponds to the spermatic flagellum. These 10-nm-diameter tubules do not have the same chemical composition as the microtubules. Elements of the cytoskeleton serve as a support for the sidefins.     

Immunocytochemical study of tubulin in the 9+‘1’ sperm axoneme of a monogenean (Platyhelminthes), Pseudodactylogyrus sp.

The spermatozoon of the monopisthocotylean monogenean Pseudodactylogyrus sp. (a gill parasite of eels) has a single axoneme showing a 9+‘1’ pattern, a nucleus and a mitochondrion, but has no cortical microtubules. This species thus provides a very simple model for the study of tubulin in the 9+‘1’ axonemes of the Platyhelminthes, in contrast with digenean sperm which have a more complex spermatozoon with two such axonemes and cortical microtubules. Indirect immunofluorescence labelling of tubulin shows that the elongating spermatids, initially lying in all directions in the early stages, are arranged as parallel elements in further stages. The number of spermatids in an isogenic group could also be precisely counted and equals 32. Nuclear labelling with fluorescent dyes shows that the nuclei, first located in the common mass of the spermatids, later elongate and migrate into the growing spermatids, and that the nucleus is located in the central part of the mature spermatozoon, with the two extremities devoid of nucleus. Labelling with antibodies directed against acetylated, tyrosinated, and polyglutamylated tubulin gave positive results, thus indicating that these post-translational modifications of tubulin are present in the axoneme of spermatids and spermatozoa of monopisthocotylean monogeneans.     

Microtubules in the microspikes and cortical cytoplasm of isolated cells

In thin sections through microspikes extending from the surface of isolated cells, a core has been seen which may contain microtubular elements. The differences between these and microtubules seen elsewhere in the cytoplasm are attributed to their rapid growth and exposed location which make them especially vulnerable to injury by preparative treatment. In support of this view it is shown that cytoplasmic microtubules may be altered or even destroyed by exposing the cells to changes in osmotic pressure. Associated with these straight microtubules in the cytoplasm were also found solid microfilaments. The form of these components and their location and alignment in portions of cells which are under tension or in motion suggest that they function in the structural support of the cell and its microspikes and in the transmission of tension in the cytoplasm. A second type of microtubule, smaller in diameter and tortuous in form, was also seen in certain cells and is presumed, from its shape, to have little to do with cytoplasmic support.     

ELECTRON MICROSCOPIC OBSERVATIONS ON TUBULAR STRUCTURES INVOLVED IN CRYSTAL FORMATION IN THE RED ALGA PLEONOSPORIUM SQUARRULOSUM (HARV.) ABBOTT1

Hexagonal or angular crystalline inclusions in Pleonosporium (Naeg.) Hauck vegetative cells were examined using electron microscopy. Ultrastructural analysis reveals that the inclusions initially contain tubular elements resembling microtubules but, with continued differentiation, are transformed into rod containing crystals. The tubular structures initially measure 25 nm in diameter. Scattered tubules become arranged in a parallel and alternate pattern and undergo subsequent enlargement to approximately 29 nm. Following enlargement, each tubule apparently disaggregates into rods that form a crystal having hexagonally arranged rod-like subunits. It is suggested that these tubules may represent microtubules and the resultant crystals are composed of tubulin.     

Ultrastructure of the testes and spermatogenesis in the mite Anystis baccarum

The epithelial lining of testes in Anystis baccarum is glandular and produces a secretory product necessary to form spermatophores. The main stages of spermatogenesis occur in the lumen of the testis in groups of synchronously developing sister cells. Spermatogonia and late spermatids are encircled by glandular cells. Reorganization of developing spermatids is typical of the trombidiform mites and includes formation of the acrosomal complex, cytoplasm elimination, disappearance of the nuclear envelope and formation of invaginations of plasmalemma. The chromatin material condensation is not followed by the entire chromatin body formation. In mature spermatoza, dense chromatin strands (80b nm in diameter) lie along the cell in the peripheral layer of the cytoplasm. Mature spermatozoa lack axonema or any protrusions. A layer of microtubules, visible underneath the outer membrane, may serve for sperm movement in the female genital duct. The acrosomal complex consists of acromal granule, acrosomal filament and subacrosomal substance. This, as well as two aggregates of typical mitochondria, looks plesiomorphic.     

F-Actin and Tubulin During Spermatogenesis in Gamasid Mites (Acari: Parasitina)

Abstract F-actin and tubulin behaviour was investigated using fluorescence probes and electron microscopy in the course of spermatogenesis in two gamasid mites, Porrhostaspis lunulata Müller (Parasitidae) and Pergamasus truatellus Athias-Henriot (Pergamasidae). In spermatogonia and primary spermatocytes of both species, the proteins were localized mainly in the intercellular bridges and, in lesser quantities, in the cytoplasm. Overall, actin was present along the plasma-lemmal contact sites of the gonial cells. At the beginning of spermatid elongation, actin could be detected in two regions: in perinuclear cytoplasm and under the plasmalemma. Subplasmalemmal actin, visible as threads running along acrosome-adhering protrusions of the nuclear envelope, is supposedly located within the electron-dense material filling the subacrosomal gap. Tubulin was found on both sides of each actin thread; its location was consistent with two sets of microtubules adhering to the inner acrosomal membrane. Their involvement in acrosome shaping is suggested. As spermatid elongation terminated, the previous pattern of proteins disappeared. In Pergamasus, however, actin emerged briefly near the centrifugal ends of spermatids (granular bodies zone). In spermatocyte-containing cysts, actin and tubulin fluorescence (more pronounced in Porrhostaspis) was associated with intercellular junctions between the cyst cells. In both species, diffuse actin fluorescence was also detected in the cytoplasm of cyst cells assembling elongated spermatids; the reaction was intensified at the end of the elongation process, when the cytoplasm of cyst cells aggregated around the centripetal ends of spermatids.     

Electron microscopic observations of an unusual cytoplasmic inclusion in the calcium-starved white mutant (LU887 × LU897) of Physarum polycephalum

Electron micrographs of Physarum polycephalum microplasmodia (LU887 × LU897) reveal cytoplasmic inclusions that appear “striated” at low magnifications; at higher magnifications these exhibit a structure that we have interpreted as microtubule bundles. The light and dark regions in the inclusions are due to the affinity of some microtubules for osmic acid; these appear to have dense regions while other microtubules remain electron lucent. The diameters of the microtubules are about 32–33nm; the subunits forming the tubule walls measure about 8–9nm in diameter. The diameter measurements are slightly larger than the dimensions assigned to vertebrate microtubules (28nm); however, the diameter of the subunits in the microtubule wall measures about 8–9nm which is essentially the same measurement reported for vertebrate tubulin dimers.     

HURP wraps microtubule ends with an additional tubulin sheet that has a novel conformation of tubulin

HURP is a newly discovered microtubule-associated protein (MAP) required for correct spindle formation both in vitro and in vivo. HURP protein is highly charged with few predicted secondary and tertiary folding domains. Here we explore the effect of HURP on pure tubulin, and describe its ability to induce a new conformation of tubulin sheets that wrap around the ends of intact microtubules, thereby forming two concentric tubes. The inner tube is a normal microtubule, while the outer one is a sheet composed of tubulin protofilaments that wind around the inner tube with a 42.5° inclination. We used cryo-electron microscopy and unidirectional surface shadowing to elucidate the structure and conformation of HURP-induced tubulin sheets and their interaction with the inner microtubule. These studies clarified that HURP-induced sheets are composed of anti-parallel protofilaments exhibiting P2 symmetry. HURP is a unique MAP that not only stabilizes and bundles microtubules, but also polymerizes free tubulin into a new configuration.     

Presence of macrotubules and their induction by colchicine in grasshopper spermatids

Ultrastructural studies show that groups of tubular structures of about 45 nm. in diameter appear in the cytoplasm of grasshopper spermatids. These tubules, or macrotubules, appear to have a fuzzy coat or to be twisted, and in some cases their bundles attain a length of 10 um. When grasshoppers were treated with colchicine, spermatid microtubules which composed the manchette are depolymerized and a large number of macrotubules appears. The relationship between these two types of tubules is discussed.     

In vitro formation of different tubulin polymers from purified tubulin of Ehrlich ascites tumor cells

Preparations of cycled tubulin from Ehrlich ascites tumor cells contain several acessory proteins; once or twice cycled microtubule preparations are usually composed of fibers 10 nm in diameter, but lack vimentin. Highly purified tubulin consists of α- and β-tubulin and a minor component which was identified by peptide mapping as a second β-chain. This pure tubulin is able to form in vitro at low concentrations (1 mg protein/ml) fibers of about 10 nm width, and at higher concentrations (3.5 mg protein/ml) normal microtubules.     

Identification of the major 68,000-dalton protein of microtubule preparations as a 10-nm filament protein and its effects on microtubule assembly in vitro.

The major 68,000-dalton protein present in cycled microtubule preparations from bovine brain can be isolated in a rapidly sedimenting fraction consisting of filaments 10 nm in diameter. This 68,000-dalton protein remains in the filament fraction after gel filtration, phosphocellulose chromatography, or salt extraction of microtubule protein. Microtubule protein devoid of 10-nm filaments contains ring structures under depolymerizing conditions, and it polymerizes into microtubules with a characteristically low critical concentration, although all of the 68,000-dalton protein has been removed from it. When cycled microtubule protein is subjected to chromatography on phosphocellulose, the tubulin fraction (PC-tubulin) assembles into microtubules only at concentrations greater than 2 mg/mL. The other fraction, eluted from phosphocellulose at high ionic strength, contains the major 68,000-dalton protein and can be further resolved into two components by centrifugation. The supernatant, which consists mainly of high molecular weight microtubule-associated proteins, stimulates low concentrations of PC-tubulin to assemble. The pellet contains all of the 68,000-dalton protein, consists of 10-nm filaments, and does not stimulate assembly of PC-tublin. Boiling of purified filaments, however, releases several proteins, including the 68,000-dalton protein, and these released proteins stimulate the assembly of PC-tubulin. The morphology and protein composition of the filaments isolated from microtubule preparations by these techniques are very similar to those of mammalian neurofilaments. These results suggest that the major 68,000-dalton protein in cycled microtubule preparations, which may correspond to tubulin assembly protein [Lockwood, A.H. (1978) Cell 13, 613--627], is a constituent of neurofilaments.     

水蕨精子发生过程中中心体蛋白和微管蛋白的免疫荧光定位

采用透射电镜技术和免疫荧光标记技术对水蕨精子发生的超微结构以及中心体蛋白和微管蛋白在精子发生过程中的动态表达进行了观察。研究发现：（1）生毛体分化早期周围有放射状微管分布,这与线粒体向生毛体的聚集有关。（2）免疫荧光观察表明,中心体蛋白仅定位于生毛体、基体和鞭毛带上,自生毛体至基体阶段呈现明亮的荧光标记,在核塑形、鞭毛形成至精子成熟阶段,中心体蛋白荧光标记随着鞭毛的发生而逐渐减弱,至游动精子阶段中心体蛋白荧光标记信号几乎消失。（3）微管蛋白早期荧光标记与中心体蛋白标记形相同,在生毛体、鞭毛带、基体等运动细胞器上呈现明亮荧光标记,但微管蛋白随着鞭毛的发生其荧光标记越来越强。从二者的时空表达特征可以推断,中心体蛋白主要是运动细胞器的组织者,而非这些运动细胞器的结构成分,其功能是参与或负责中心粒、基体和鞭毛的发生。     

Radioiodination of brain tubulin with Bolton-Hunter reagent

Radio-iodination of tubulin can be achieved by Bolton-Hunter reagent both in the absence and presence of microtubule associated proteins. Specific radioactivities as high as 400 C_i/mmole tubulin dimer can be obtained, i.e. an average of 0.2 molecule of reagent is bound per molecule of tubulin. About 80 % of the [¹²⁵I]- labelled tubulin keeps its ability to assemble in microtubules and polymerizes with the same critical concentration as the native tubulin, which makes the method adequate for preparing tracer tubulin useful for in vivo and in vitro studies. Both α and β subunits are labelled, 60 % of the radiolabel being bound to the β subunit.     

Neurotubule assembly at substoichiometric nucleotide levels using a GTP regenerating system

Tubulin derived from cold depolymerized bovine microtubules has been gel filtered to obtain a tubulin preparation with only 3% of the tubulin dimers containing exchangeable [³H]-guanine nucleotide. In the presence of acetyl-P and bacterial acetate kinase, this preparation polymerizes to form microtubules which are morphologically indistinguishable from microtubules formed in the presence of excess GTP. The extent of microtubule formation at substoichiometric nucleotide levels using the GTP regenerating system exceeds the extent of assembly obtained with excess GTP. It is concluded that the exchangeable guanine nucleotide site can be virtually unoccupied in intact neurotubules and this finding indicates that GDP can “catalyze” tubule assembly in the presence of a GTP regenerating system.     

Expression of microtubule networks in normal cells, transformed cells, and their hybrids

Microtubules play an important role in several cellular functions including cellular architecture and chromosome movement in cell division. Tubulin which polymerizes to form mictobules can be purified to homogeneity and used to raised antisera. Antisera prepared against porcine or chicken tubulin reacts well with mammalian tubulin. We have examined normal and transformed cells of mouse and human origin for microtubules by indirect immunofluorescence methods. Extensive networks of microtubules (MN) are easily detectable in normal and some transformed cells. The fixation procedure employed and the morphology and the cellular attachment properties seem to determine the ease of detection of MN in these cells. Cells derived from tumors and exhibiting several transformed phenotypes contained MN comparable to those of normal cells. Hybrids between transformed mouse cells and normal human cells were examined. They showed a variability in morphology, but all contained MN. These hybrids exhibited several transformed phenotypes. We conclude that in the cell lines we have examined there is no correlation between the transformed phenotypes and the organization of tubulin.     

Localization of tektin filaments in microtubules of sea urchin sperm flagella by immunoelectron microscopy

Extraction of doublet microtubules from the sperm flagella of the sea urchin Strongylocentrotus purpuratus with sarkosyl (0.5%)-urea (2.5 M) yields a highly pure preparation of "tektin" filaments that we have previously shown to resemble intermediate filament proteins. They form filaments 2-3 nm in diameter as seen by negative stain electron microscopy and are composed of approximately equal amounts of three polypeptide bands with apparent molecular weights of 47,000, 51,000, and 55,000, as determined by SDS PAGE. We prepared antibodies to this set of proteins to localize them in the doublet microtubules of S. purpuratus and other species. Tektins and tubulin were antigenically distinct when tested by immunoblotting with affinity-purified antitektin and antitubulin antibodies. Fixed sperm or axonemes from several different species of sea urchin showed immunofluorescent staining with antitektin antibodies. We also used antibodies coupled to gold spheres to localize the proteins by electron microscopy. Whereas a monoclonal antitubulin (Kilmartin, J.V., B. Wright, and C. Milstein, 1982, J. Cell Biol. 93:576-582) decorates intact microtubules along their lengths, antitektins labeled only the ends of intact microtubules and sarkosyl-insoluble ribbons. However, if microtubules and ribbons attached to electron microscope grids were first extracted with sarkosyl-urea, the tektin filaments that remain were decorated by antitektin antibodies throughout their length. These results suggest that tektins form integral filaments of flagellar microtubule walls, whose antigenic sites are normally masked, perhaps by the presence of tubulin around them.     

The microtubular system and posttranslationally modified tubulin during spermatogenesis in a parasitic nematode with amoeboid and aflagellate spermatozoa

Using transmission electron microscopy and immunologic approaches with various antibodies against general tubulin and posttranslationally modified tubulin, we investigated microtubule organization during spermatogenesis in Heligmosomoides polygyrus, a species in which a conspicuous but transient microtubular system exists in several forms: a cytoplasmic network in the spermatocyte, the meiotic spindle, a perinuclear network and a longitudinal bundle of microtubules in the spermatid. This pattern differs from most nematodes including Caenorhabditis elegans, in which spermatids have not microtubules. In the spermatozoon of H. polygyrus, immunocytochemistry does not detect tubulin, but electron microscopy reveals two centrioles with a unique structure of 10 singlets. In male germ cells, microtubules are probably involved in cell shaping and positioning of organelles but not in cell motility. In all transient tubulin structures described in spermatocytes and spermatids of H. polygyrus, detyrosination, tyrosination, and polyglutamylation were detected, but acetylation and polyglycylation were not. The presence/absence of these posttranslational modifications is apparently not stage dependent. This is the first study of posttranslationally modified tubulin in nematode spermatogenesis. Mol. Reprod. Dev. 49:150–167, 1998. © 1998 Wiley-Liss, Inc.     

Polarity of microtubular structures in manchette-like formations: possible role of the "11th filament".

Bundles of microtubular structures appear in the cytoplasm of spermatids of the African frog Dicroglossus occipitalis. They are observed in the vicinity of axonemes. Natural tubulin polymerization leads to the formation of hooks on microtubular structures. They can be related to experimentally induced tubulin hooks. The direction of curvature of the hooks allows us to define the polarity of the bundles. This is opposite to the polarity of axonemal microtubules: Bundles and axonemes are antiparallel. Under colchicine action, arch-like microtubular structures are shown to open in the same direction as they lock. This enables us to characterize their opening and locking site: It corresponds to the place of the "11th filament" described in microtubular structures such axonemes. The "11th filament" is thus demonstrated to be the most susceptible to natural opening and to the action of colchicine in microtubular structures.     

Tubulin Polyglycylation: Differential Posttranslational Modification of Dynamic Cytoplasmic and Stable Axonemal Microtubules in Paramecium

Polyglycylation, a posttranslational modification of tubulin, was discovered in the highly stable axonemal microtubules of Paramecium cilia where it involves the lateral linkage of up to 34 glycine units per tubulin subunit. The observation of this type of posttranslational modification mainly in axonemes raises the question as to its relationship with axonemal organization and with microtubule stability. This led us to investigate the glycylation status of cytoplasmic microtubules that correspond to the dynamic microtubules in Paramecium. Two anti-glycylated tubulin monoclonal antibodies (mAbs), TAP 952 and AXO 49, are shown here to exhibit different affinities toward mono- and polyglycylated synthetic tubulin peptides. Using immunoblotting and mass spectrometry, we show that cytoplasmic tubulin is glycylated. In contrast to the highly glycylated axonemal tubulin, which is recognized by the two mAbs, cytoplasmic tubulin reacts exclusively with TAP 952, and the α- and β- tubulin subunits are modified by only 1–5 and 2–9 glycine units, respectively. Our analyses suggest that most of the cytoplasmic tubulin contains side chain lengths of 1 or 2 glycine units distributed on several glycylation sites. The subcellular partition of distinct polyglycylated tubulin isoforms between cytoplasmic and axonemal compartments implies the existence of regulatory mechanisms for glycylation. By following axonemal tubulin immunoreactivity with anti-glycylated tubulin mAbs upon incubation with a Paramecium cellular extract, the presence of a deglycylation enzyme is revealed in the cytoplasm of this organism. These observations establish that polyglycylation is reversible and indicate that, in vivo, an equilibrium between glycylating and deglycylating enzymes might be responsible for the length of the oligoglycine side chains of tubulin.     

Bundling of microtubules by synapsin 1. Characterization of bundling and interaction of distinct sites in synapsin 1 head and tail domains with different sites in tubulin.

Synapsin 1 is a nerve terminal phosphoprotein whose role seems to encompass the linking of small synaptic vesicles to the cytoskeleton. Synapsin 1 can join small synaptic vesicles to neuronal spectrin, microfilaments and microtubules; it can also bundle microtubules and microfilaments. In this paper, the mode of interaction between synapsin 1 and microtubules has been investigated. Bundling is shown to be highly cooperative: the apparent Hill coefficient is 3.06 +/- 0.3, and bundling is half-maximal at 0.63 +/- 0.02 microM. Bundling occurs either when whole synapsin 1 preparations (containing monomers and oligomers) or when monomeric synapsin 1 is added to microtubules. However, it is not clear that synapsin 1 remains monomeric in the presence of microtubules. Synapsin 1-microtubule mixtures contain two types of filament. One type is characterised by microtubules often with synapsin 1 bound to their surface. The other type is composed of filaments of diameter 15 +/- 5 nm. This filament type is granular and made up in part of 14-nm-diameter particles. These dimensions are consistent with their being made up of polymerised synapsin 1. It is possible that microtubules induce the polymerisation of synapsin 1. Synapsin 1 had independent tubulin binding sites in the N-terminal head domain and in the C-terminal tail domain. Whole synapsin 1 can interact with tubulin after it has been digested to remove the tubulin C terminus (des-C-terminal tubulin). The interaction of des-C-terminal tubulin with synapsin 1 appears to be via the head domain, since 125I-des-C-terminal tubulin only shows specific binding to the head domain on gel blots. By contrast intact tubulin binds to both head and tail domains. Binding to the tail domain can be inhibited by a synthetic peptide representing the microtubule-associated protein 2 (MAP2) binding site of class II beta tubulin. These results suggest a model for microtubule bundling by synapsin 1 in which independent sites in the head and tail domains of synapsin 1 cross-link microtubules by interactions with two distinct sites in tubulin.