Biochemistry of the mitotic spindle

Fluctuation of the Ca-sequestering activity of permeabilized sea urchin embryos during the cell cycle

We have followed the sequestration of Ca(2+) by intracellular compartments in sea urchin embryos through the first cell cycles. To gain biochemical access to these compartments, the embryos were permeabilized by brief exposure to an intense electric field. Sequestration was determined as the retention of tracer, (45)Ca, after filtration of aliquots on Millipore filters. The permeabilized cells sequester Ca(2+) at a constant rate for at least 20 min, with the following characteristics: (i) ATP is required. (ii) Sequestration occurs at Ca(2+) levels corresponding to those estimated in vivo. (iii) The Ca(2+) concentration dependence of sequestration and its insensitivity to mitochondrial poisons imply that the activity derives from a single, nonmitochondrial transport system. The Ca(2+)-sequestering activities of embryos that are permeabiized at successive stages of the first cell cycle (one-cell stage) progressively increase to 5 times the initial level. The rate of sequestration is maximal during telophase and, in some populations of zygotes, is nearly as great throughout prophase. Over the course of the second cell cycle (two-cell stage), the activity undergoes a 2-fold oscillation that bears the same temporal relationship to mitosis as the previous fluctuation.

Visualization of the Ca-transport system of the mitotic apparatus of sea urchin eggs with a monoclonal antibody

Monoclonal antibodies have been obtained to components of Ca(2+)-sequestering vesicles from the endoplasmic reticulum of HeLa cells by isolating hybridomas that were generated by the in vitro immunization of lymphocytes followed by fusion with plasmocytoma cells. One of these monoclonal antibodies specifically labels punctate structures which appear in the mitotic apparatus of sea urchin eggs at the beginning of prophase and disappear upon the completion of cytokinesis. The antibody inhibits the Ca(2+) uptake of the membrane system in vitro. It reacts with one 46-kDa protein out of the complex protein mixture from the membrane fraction. We take all this as evidence that in fact a specific Ca(2+)-transport system is part of the mitotic apparatus, that such a system is very conserved, and that it is most probably derived from the endoplasmic reticulum.

Centrosome structure and function is altered by chloral hydrate and diazepam during the first reproductive cell cycles in sea urchin eggs

This paper explores the mode of action of the tranquillizers chloral hydrate and diazepam during fertilization and mitosis of the first reproductive cell cycles in sea urchin eggs. Most striking effects of these drugs are the alteration of centrosomal material and the abnormal microtubule configurations during exposure and after recovery from the drugs. This finding is utilized to study the mechanisms of centrosome compaction and decompaction and the dynamic configurational changes of centrosomal material and its interactions with microtubules. When 0.1% chloral hydrate or 350-750 microM diazepam is applied at specific phases during the first cell cycle of sea urchin eggs, expanded centrosomal material compacts at distinct regions and super-compacts into dense spheres while microtubules disassemble. When eggs are treated before pronuclear fusion, centrosomal material aggregates around each of the two pronuclei while microtubules disappear. Upon recovery, atypical asters oftentimes with multiple foci are formed from centrosomal material surrounding the pronuclei which indicates that the drugs have affected centrosomal material and prevent it from functioning normally. Electron microscopy and immunofluorescence studies with antibodies that routinely stain centrosomes in sea urchin eggs (4D2; and Ah-6) depict centrosomal material that is altered when compared to control cells. This centrosomal material is not able to reform normal microtubule patterns upon recovery but will form multiple asters around the two pronuclei. When cells are treated with 0.1% chloral hydrate or 350-750 microM diazepam during mitosis, the bipolar centrosomal material becomes compacted and aggregates into multiple dense spheres while spindle and polar microtubules disassemble. With increased incubation time, the smaller dense centrosome particles aggregate into bigger and fewer spheres. Upon recovery, unusual irregular microtubule configurations are formed from centrosomes that have lost their ability to reform normal mitotic figures. These results indicate that chloral hydrate and diazepam affect centrosome structure which results in the inability to reform normal microtubule formations and causes abnormal fertilization and mitosis.

Local perinuclear calcium signals associated with mitosis-entry in early sea urchin embryos

Using calcium-sensitive dyes together with their dextran conjugates and confocal microscopy, we have looked for evidence of localized calcium signaling in the region of the nucleus before entry into mitosis, using the sea urchin egg first mitotic cell cycle as a model. Global calcium transients that appear to originate from the nuclear area are often observed just before nuclear envelope breakdown (NEB). In the absence of global increases in calcium, confocal microscopy using Calcium Green-1 dextran indicator dye revealed localized calcium transients in the perinuclear region. We have also used a photoinactivatable calcium chelator, nitrophenyl EGTA (NP-EGTA), to test whether the chelator-induced block of mitosis entry can be reversed after inactivation of the chelator. Cells arrested before NEB by injection of NP-EGTA resume the cell cycle after flash photolysis of the chelator. Photolysis of chelator triggers calcium release. TreatmenT with caFfeine to enhance calcium-induced calcium release increases the amplitude of NEB-associated calcium transients. These results indicate that calcium increases local to the nucleus are required to trigger entry into mitosis. Local calcium transients arise in the perinuclear region and can spread from this region into the cytoplasm. Thus, cell cycle calcium signals are generated by the perinuclear mitotic machinery in early sea urchin embryos.

In vitro viability and ultrastructural changes in bovine oocytes treated with a vitrification solution

Abattoir-derived oocytes were exposed to a concentrated cryoprotectant solution (DAP213: 2 M DMSO, 1 M acetamide, 3 M propanediol, and 10% FCS in TCM199) for 1.5 or 5 min at the germinal vesicle (GV) stage or after maturation in vitro (IVM). Their viability was assessed by in vitro fertilization (IVF) and culture (IVC) to blastocysts. To investigate the effect of DAP213 on the ultrastructure, GV and IVM oocytes were processed for transmission electron microscopy (TEM) before (control) or after exposure to the cryoprotectant. DAP213 induced profound ultrastructural modifications to the microvilli and mitochondria, resulted in large vesicle formation, and, most significantly, caused the premature release of the cortical granules (CG). In IVM oocytes exposed to the cryoprotectant for 5 min, exocytosis of CG into the perivitelline space was common and the IVF rate was reduced (P < .05). After exposure for 5 min, GV oocytes displayed clusters of CG comparable to controls, but after IVM-IVF, polyspermy rate was increased (P < .05). Furthermore, treated GV oocytes showed a reduced rate of cleavage and blastocyst formation and an increased percentage of oocytes exhibiting alterations in organelles, whereas the viability and ultrastructure of IVM oocytes treated for 1.5 min was not different from controls. These observations demonstrate that 1) cortical granule kinetics is one of the key elements controlling fertilizability of bovine oocytes treated with cryoprotectant, and 2) GV oocytes are more sensitive to the cryoprotectant than those that have already been matured in vitro.

In vitro fertilization and development of frozen-thawed bovine oocytes

Bovine oocytes were vitrified (V-oocytes) or frozen slowly (S-oocytes) at the germinal vesicle (GV) stage or after maturation in vitro (IVM) and their survival assessed morphologically and also by in vitro fertilization (IVF) and culture. The morphological survival of S-oocytes was 30.7% after freezing at the GV stage and 53.3% after IVM. The corresponding survival rates of V-oocytes were significantly lower, viz. 14.6 and 14.0%, respectively. The fertilization rate of S-oocytes frozen after IVM (51.0%) was lower than that of unfrozen controls (75.8%), but higher than after other treatments. Development continued in 16.0% of the fertilized S-oocytes, compared to 39.4% of control IVF zygotes and 1.6% developed into morulae or blastocysts (4.5% in controls). Only 0.8% of frozen-thawed GV stage oocytes and 4.6% of post-IVM V-oocytes cleaved after IVF and none formed morulae or blastocysts. Transfer of four embryos (two morulae and two blastocysts) derived from post-IVM S-oocytes into a recipient heifer resulted in pregnancy and the birth of twin calves.

Centrophilin: a novel mitotic spindle protein involved in microtubule nucleation

A novel protein has been identified which may serve a key function in nucleating spindle microtubule growth in mitosis. This protein, called centrophilin, is sequentially relocated from the centromeres to the centrosomes to the midbody in a manner dependent on the mitotic phase. Centrophilin was initially detected by immunofluorescence with a monoclonal, primate-specific antibody (2D3) raised against kinetochore-enriched chromosome extract from HeLa cells (Valdivia, M. M., and B. R. Brinkley. 1985. J. Cell Biol. 101:1124-1134). Centrophilin forms prominent crescents at the poles of the metaphase spindle, gradually diminishes during anaphase, and bands the equatorial ends of midbody microtubules in telophase. The formation and breakdown of the spindle and midbody correlates in time and space with the aggregation and disaggregation of centrophilin foci. Immunogold EM reveals that centrophilin is a major component of pericentriolar material in metaphase. During recovery from microtubule inhibition, centrophilin foci act as nucleation sites for the assembly of spindle tubules. The 2D3 probe recognizes two high molecular mass polypeptides, 180 and 210 kD, on immunoblots of whole HeLa cell extract. Taken together, these data and the available literature on microtubule dynamics point inevitably to a singular model for control of spindle tubule turnover.

Epidemiologic and toxicologic evidence for chronic health effects and the underlying biologic mechanisms involved in sub-lethal exposures to acidic pollutants

Since the 1880s, a disparate and extensive literature has evolved examining the biologic effects of acidification on cells. More recently, effects on the health of human and other species of acidic agents contained, for example, in pollutants have been suggested, particularly relating to long-term exposures. This paper provides a review of the epidemiologic and toxicologic evidence concerning health effects--particularly carcinogenicity--attributable to sub-lethal acid exposure. Underlying biologic mechanisms that explain adverse health outcomes include pH modulation of toxicity for a number of xenobiotics (including carcinogens, genotoxins, and teratogens), and low-pH-induced changes of cells involving, for example, alterations in mitotic and enzyme regulation. More focused research is recommended to test the relationship between long-term exposures to acidic agents (with a consequent lowered cellular pH) and various health effects.

Vitrification of mouse oocytes results in aneuploid zygotes and malformed fetuses

Vitrification of mouse oocytes adversely affected the subsequent developmental potential of embryos and fetuses derived from the fertilization of such oocytes after thawing. Only 5% of oocytes vitrified formed viable fetuses on the 15th day of gestation as compared to 47% in the controls. The incidence of chromosomally aneuploid zygotes, derived from cryopreserved oocytes, was approximately threefold higher than the controls irrespective of whether the oocytes were cryopreserved by vitrification or DMSO slow-freezing. Malformed fetuses were obtained from oocytes that had been vitrified as well as those that had been exposed to vitrification solutions only, whereas no malformed fetuses were obtained in oocytes slow-frozen by DMSO or fresh controls--thus demonstrating that the exposure of oocytes to the vitrification chemicals was responsible for the fetal malformations. The data in this study suggest that the vitrification technique should be cautiously applied to human oocyte cryopreservation. Furthermore, the data also demonstrate that the exposure of female gametes to carcinogenic and/or teratogenic chemicals may result in malformed embryos when such oocytes are subsequently fertilized.

Identification of a 52-kD calmodulin-binding protein associated with the mitotic spindle apparatus in mammalian cells

A pool of 10 calmodulin-binding proteins (CBPs) was isolated from Chinese hamster ovary (CHO) cells via calmodulin (CaM)-Sepharose affinity chromatography. One of these ten isolated CBPs with a molecular mass of 52 kD was also found to be present in isolated CHO cell mitotic spindles. Affinity-purified antibodies generated against this pool of isolated CBPs recognize a single 52-kD protein in isolated CHO cell mitotic spindles by immunoblot analysis. Immunofluorescence examination of CHO, 3T3, NRK, PTK-2, and HeLa cells resulted in a distinct pattern of mitotic spindle fluorescence. The localization pattern of this 52-kD CBP directly parallels that of CaM in the spindle apparatus throughout the various stages of mitosis. Interestingly, there was no association of this 52-kD CBP with cytoplasmic microtubules. As is the case with CaM, the localization pattern of the 52-kD CBP in interphase cells is diffuse within the cytoplasm and is not associated with any discrete, cellular structures. This 52-kD CBP appears to represent the first mitotic spindle-specific calmodulin-binding protein identified and represents an initial step toward the ultimate determination of CaM function in the mitotic spindle apparatus.

Mechanistic aspects on chemical induction of spindle disturbances and abnormal chromosome numbers

Work on the chemical induction of spindle disturbances and abnormal chromosome numbers, and work on the composition and biochemistry of the spindle are reviewed. Some early investigations have shown that there is an unspecific mechanism for chemical induction of spindle disturbances. This mechanism is based on the interaction of compounds with cellular hydrophobic compartments. Some compounds act differently and are more active than predicted from their lipophilic character. Selected compounds of that kind and their possible mechanisms of action are discussed. Changes in sulfhydryl and ATP levels, oxidative damage of membranes and impaired control of cytoplasmic Ca2+ levels are discussed in this context.

Metabolic inhibitors block anaphase A in vivo

Anaphase in dividing guard mother cells of Allium cepa and stamen hair cells of Tradescantia virginiana consists almost entirely of chromosome-to-pole motion, or anaphase A. Little or no separation of the poles (anaphase B) occurs. Anaphase is reversibly blocked at any point by azide or dinitrophenol, with chromosome motion ceasing 1-10 min after application of the drugs. Motion can be stopped and restarted several times in the same cell. Prometaphase, metaphase, and cytoplasmic streaming are also arrested. Carbonyl cyanide m-chlorophenyl hydrazone also stops anaphase, but its effects are not reversible. Whereas the spindle collapses in the presence of colchicine, the chromosomes seem to "freeze" in place when cells are exposed to respiratory inhibitors. Electron microscope examination of dividing guard mother cells fixed during azide and dinitrophenol treatment reveals that spindle microtubules are still present. Our results show that chromosome-to-pole motion in these cells is sensitive to proton ionophores and electron transport inhibitors. They therefore disagree with recent reports that anaphase A does not require a continuous supply of energy. It is possible, however, that anaphase does not directly use ATP but instead depends on the energy of chemical and/or electrical gradients generated by cellular membranes.

Calcium restriction prolongs metaphase in dividing Tradescantia stamen hair cells

Agents that lower extracellular calcium concentration (EGTA) or modulate calcium transport (lanthanum or D600) have been applied to dividing stamen hair cells of Tradescantia and analyzed for their ability to change the following: (a) the time required to progress from nuclear envelope breakdown to the onset of anaphase (metaphase transit time), (b) the time required to progress from anaphase to the initiation of the cell plate, and (c) the rate of chromosome motion in anaphase. Control cells complete metaphase in 32 min, initiate a cell plate in 19 min, and display a chromosome motion rate of 1.45 micron/min. If cells are treated with a calcium-EGTA buffer (pCa 8) for 4 h, the metaphase transit time is increased to 53 min without any change in the time of cell plate formation or the rate of chromosome motion. Lanthanum and D600, under conditions in which their access to the plasmalemma has been facilitated by pretreating the cells with cutinase, also markedly extend metaphase and in several instances permanently arrest cells. Lanthanum, however, produce little or no change in cell plate initiation or the rate of chromosome motion. Microscopic observations of the mitotic apparatus in calcium-stressed cells reveal normal chromatin condensation and metaphase progression. Chromosomes partly untwine but remain attached at their kinetochores. It is suggested that a flux of calcium, derived from the extracellular compartment, may cause the final splitting of sister chromosomes and trigger the onset of anaphase. However, once anaphase has begun, chromosome motion and cell plate initiation proceed normally even under conditions of extracellular calcium restriction.

A calcium-rich intraspindle membrane system in spermatocytes of wolf spiders

The meiotic spindle of spermatocytes of two wolf spiders contains a highly organized system of ER-like membranes. In cells observed ultrastructurally at early prometaphase, these membranes completely invest each bivalent and are present in the periphery of the spindle in association with the centrosomes. By metaphase each bivalent and its kinetochore fibers are completely encased in a tube of this membrane. We have treated living spermatocytes with the permeant, fluorescent-chelate probe, chlorotetracycline (CTC) to determine whether or not the intraspindle membrane system is rich in associated Ca2+. Spider testes were dissected into PIPES-buffered saline containing 200 microM CTC and were kept in this solution for 10 min. Autofluorescence controls were prepared by incubation in saline without CTC, and nonspecific effects of CTC were assessed by incubation for 10 min in 200 microM oxytetracycline (OTC). Neither unstained nor OTC-treated spermatocytes emit significant fluorescence. In contrast, CTC treatment yields bright, punctate fluorescence, which coincides with the distribution of the mitochondria. The plasma membrane is only weakly fluorescent, while the nuclear envelope exhibits prominent fluorescence. The chromosomes are not fluorescent during prophase, but after nuclear envelope breakdown, they become outlined by dim, but distinct fluorescence. As spindle formation commences, the CTC signal from the intraspindle membrane system becomes strong. In some cells, thin lines of CTC fluorescence are apparent in the metaphase half spindle; this fluorescence pattern mimics the distribution of the intraspindle membrane system and suggests that it is rich in associated Ca2+. We suggest that the intraspindle membrane system functions in the regulation of cytosolic Ca2+ during meiosis through sequestration of the cation.

The ultrastructure of an intraspindle membrane system in meiosis of spider spermatocytes

An extensive system of membranes was found in the spindles of spermatocytes of two wolf spider species, Lycosa georgicola and L. rabida. Serial section reconstructions of this membrane system revealed that each meiotic bivalent is encased in a tube of membrane, which encloses both kinetochore microtubule bundles and approaches to within a few microns of the centriolar complex. The membrane tube is open at the polar ends. The membrane composing the tube is doubled and resembles smooth endoplasmic reticulum (ER). No evidence of nuclear pore complexes has been found in the intraspindle membrane system, but typical pores are present on the nuclear envelope of prophase cells. The membrane tubes are fenestrated and microtubules sometimes penetrate these fenestrae. Besides its possible function in the regulation of chromosome movement, the intraspindle membrane system may participate in the nonrandom segregation of the sex chromosomes at meiosis in these spiders.

Endoplasmic reticulum whorls as a source of membranes for early cytaster formation in parthenogenetically stimulated sea urchin eggs

Sea urchin eggs exposed to a continuous hypertonic treatment rapidly form many concentric whorls of endoplasmic reticulum (ER) during the pre-activation period of the parthenogenetic development. These whorls, however, are only a temporary configurational alteration of ER which begin to break up just prior to egg activation. The conversion back to normal vesicles and lamellae occurs not only concurrently with the appearance of early cytastral areas, but also frequently in close association with the formation of these membranous areas. It is revealed here that membrane elements from disrupting whorls may become incorporated into adjacent, developing clear areas, early cytastral areas, and that this ER constitutes an initial major source of membranes for these early astral areas. Having previously suggested that the actual formation of ER whorls occurs in direct response to released intracellular calcium in hypertonic stressed eggs, the new findings, along with other related data and correlations, further suggest that whorl disruption and the formation of associated astral areas can be correlated with a corresponding decrease in the concentration of this released calcium in the cytoplasm.

Increased free Ca2+ levels delay the onset of mitosis in fertilized and artificially activated eggs of the sea urchin

Increasing free Ca2+ in the cytoplasm of fertilized and NH4OH- or A23187-activated eggs of Strongylocentrotus purpuratus and Lytechinus variegatus resulted in significant delays in the onset of mitosis. The increased Ca2+ levels were induced by addition of the calcium ionophore A23187 to suspensions of fertilized and activated eggs, or by fertilizing NH4OH-activated eggs. The delays may represent a temporary inhibition of the activation and activities of the mitotic proteins. It is postulated that relatively high free Ca2+ levels in the cytoplasm prevents activation of the mitotic proteins. Active Ca2+ sequestration is therefore suggested as an essential factor in the onset and progression of early mitosis.

Ethanol-induced chromosomal abnormalities at conception

Preliminary findings have indicated that mouse eggs exposed briefly in vivo or in vitro to a dilute solution of ethanol activate parthenogenetically. Cytogenetic analysis of the first-cleavage chromosomes of haploid parthenogenetic embryos indicated that up to 20% of this population were aneuploid as a result of non-disjunction. Anaesthetics also can induce parthenogenesis of rodent eggs, and in studies using anaesthetics, colchicine and colcemid, abnormal chromosome segregation and heteroploidy of rodent embryos have been observed. I now report that when recently mated female mice are given a dilute solution of ethanol by mouth, non-disjunction can be induced in the female-derived, but apparently not in the male-derived, chromosome set of fertilized eggs. Taken together, these findings suggest that ethanol consumption (as well as exposure to other 'spindle-acting' agents) at the time of conception may be the cause of certain types of chromosomal defects commonly observed in human spontaneous abortions.

Isolation and initial characterization of the mammalian midbody

Midbodies were isolated from synchronized cultures of Chinese hamster ovary (CHO) cells and their protein composition was studied by means of SDS PAGE. Gels of the midbodies included alpha and beta tubulins as major bands (approximately 30% of the total protein) and approximately 35 other bands, none of which constituted greater than 3.5% of the total protein. Extraction of the isolated midbodies with Sarkosyl NL-30- solubilized the midbody microtubules but left the central, dense matrix zone of the midbody intact. A protein doublet of approximately 115,000 mol wt was retained preferentially by the particulate fraction containing the matrix zones, indicating it to be a component of the matrix. The 115,000 mol wt doublet was also present in gels of isolated mitotic spindles from CHO cells. The overall protein composition of the isolated spindles was very similar to that of the isolated midbodies.

Functional autonomy of monopolar spindle and evidence for oscillatory movement in mitosis

The oscillations of chromosomes associated with a single spindle pole in monocentric and bipolar spindles were analysed by time-lapse cinematography in mitosis of primary cultures of lung epithelium from the newt Taricha granulosa. Chromosomes oscillate toward and away from the pole in all stages of mitosis including anaphase. The duration, velocity, and amplitude of such oscillations are the same in all stages of mitosis. The movement away from the pole in monocentric spindle is rapid enough to suggest the existence of a previously unrecognized active component in chromosome movement, presumably resulting from a pushing action of the kinetochore fiber. During prometaphase oscillations, chromosomes may approach the pole even more closely than at the end of anaphase. Together, these observations demonstrate that a monopolar spindle is sufficient to generate the forces for chromosome transport, both toward and away from the pole. The coordination of the aster/centrosome migration in prophase with the development of the kinetochore fibers determines the course of mitosis. After the breaking of the nuclear envelope in normal mitosis, aster/centrosome separation is normally followed by the rapid formation of bipolar chromosomal fibers. There are two aberrant extremes that may result from a failure in coordination between these processes: (a) A monocentric spindle will arise when aster separation does not occur, and (b) an anaphaselike prometaphase will result if the aster/centrosomal complexes are already well-separated and bipolar chromosomal fibers do not form. In the latter case, the two monopolar prometaphase half-spindles migrate apart, each containing a random number of two chromatid (metaphase) monopolar-oriented chromosomes. This random segregation of prometaphase chromosome displays many features of a standard anaphase and may be followed by a false cleavage. The process of polar separation during prometaphase occurs without any visible interzonal structures. Aster/centrosomes and monopolar spindles migrate autonomously by an unknown mechanism. There are, however, firm but transitory connections between the aster center and the kinetochores as demonstrated by the occasional synchrony of centrosome-kinetochore movement. The data suggest that aster motility is important in the progress of both prometaphase and anaphase in normal mitosis.

Proteins specifically associated with the microtubules of the mammalian mitotic spindle

The molecular species that determine the unique structure and functions of the microtubules in the mitotic spindle are not known. We describe the results of two new approaches to the molecular structure of the spindle. Both approaches rely on detergent-extracted preparations of synchronized populations of cells metabolically labeled with 35S-methionine or 32P-phosphate. In these preparations, the original cellular microtubules are preserved. The microtubule components can be released from the detergent-extracted preparations by selective depolymerization with calcium ions. Alternatively, the microtubules can be stabilized by taxol, freed of chromatin by digestion with DNAase and freed of the surrounding cage of intermediate filaments by further extraction at low ionic strength. Gel electrophoresis of each of these preparations of mitotic microtubules demonstrates that they contain microtubule-associated proteins that we have previously shown to be present in interphase microtubules. They also contain a protein of 150,000 daltons, which is the first mitosis-specific microtubule-associated protein identified in mammalian cells.

A permeabilized cell model for studying cell division: a comparison of anaphase chromosome movement and cleavage furrow constriction in lysed PtK1 cells

After lysis in a Brij 58-polyethylene glycol medium, PtK1 cells are permeable to small molecules, such as erythrosin B, and to proteins, such as rhodamine-labeled FAB, myosin subfragment-1, and tubulin. Holes are present in the plasma membrane, and the mitochondria are swollen and distorted, but other membrane-bounded organelles of the lysed cell model are not noticeably altered. After lysis, the mitotic apparatus is functional; chromosomes move poleward and the spindle elongates. Cells lysed while in cytokinesis will continue to divide for several minutes. Addition of crude tubulin extracts, MAP-free tubulin, or taxol to the lysis medium retards anaphase chromosome movements but does not affect cleavage. On the other hand, N-ethylmaleimide-modified myosin subfragment-1, phalloidin, and cytochalasin B inhibit cleavage but have no effect on anaphase chromosome movements under identical lysis conditions. These results suggest that actomyosin plays no functional role in anaphase chromosome movement in mammalian tissue culture cells and that microtubule depolymerization is a rate-limiting step for chromosome-to-pole movements.

Studies on the in vivo sensitivity of spindle microtubules to calcium ions and evidence for a vesicular calcium-sequestering system

I microinjected calcium ions into echinoderm eggs during mitosis to determine the calcium sensitivity of microtubules (Mts) in vivo. Spindle birefringence (BR), a measure of the number of aligned Mts in the spindle, is locally, rapidly, and reversibly abolished by small volumes of microinjected CaCl2 (1 mM). Rapid return of BR is followed by anaphase, and subsequent divisions are normal. Similar doses of MgCl2, BaCl2, KCl, NaCl, pH buffers, distilled water, or vegetable oil have no effect on spindle BR, whereas large doses of such agents sometimes cause slow, uniform loss in BR over the course of a minute or more. Of the ions tested, only Sr++ causes effects comparable to Ca++. Ca-EGTA buffers, containing greater than micromolar free Ca++, abolishes BR in a manner similar to millimolar concentrations of injected CaCl2. Caffeine, a potent uncoupler of the Ca++-pump/ATPase of sarcoplasmic reticulum, causes a local, transient depression in spindle BR in the injected region. Finally, injection of potassium oxalate results in the formation of small, highly BR crystals, presumably CA-oxalate, in Triton-sensitive compartments in the cytoplasm. Taken together, these findings demonstrate that spindle Mts are sensitive to levels of free Ca++ in the physiological range, provide evidence for the existence of a strong cytoplasmic Ca++-sequestering system, and support the notion that Mt assembly and disassembly in local regions of the spindle may be orchestrated by local changes in the cytoplasmic free Ca++ concentration during mitosis. An appendix offers the design of a new chamber for immobilizing echinoderm eggs for injection, a new method for determining the volume of the injected solution, and a description of the microinjection technique, which was designed, but never fully described, by Hiramoto (Y. Hiramoto, Exp. Cell. Res., 1962, 27:416-426.).

Dynein-like Mg2+-ATPase in mitotic spindles isolated from sea urchin embryos (Strongylocentrotus droebachiensis)

Two distinctly different ATPases have been reported to be endogenous to the mitotic apparatus: a Mg2+-ATPase resembling axonemal dynein, and a Ca2+-ATPase postulated to be bound in membranes. To examine the nature of the Mg2+-ATPase, we isolated membrane-free mitotic spindles from Stronglylocentrotus droebachiensis embryos by rapidly lysing these in a calcium-chelating, low-ionic-strength buffer (5 mM EGTA, 0.5 mM MgCl2, 10 mM PIPES, pH 6.8) that contained 1% Nonidet P-40. The fibrous isolated mitotic spindles closely resembled spindles in living cells, both in general morphology and in birefringence. In electron micrographs, the spindles were composed primarily of microtubules, free from membranes and highly extracted of intermicrotubular cytoplasmic ground substance. As analyzed by SDS-polyacrylamide gel electrophoresis (SDS-PAGE), the pelleted spindles contain 18% tubulin, variable amounts of actin (2-8%), and an unidentified protein of 55 kdaltons in a constant weight ratio to tubulin (1:2.5). The isolated spindles also contained two polypeptides, larger than 300 kdaltons, that comigrated with egg dynein polypeptides, and ATPase activity (0.02 mumol Pi/mg . min) that closely resembled both flagellar and egg dynein. The spindle Mg2+-ATPase showed a ratio of Ca2+-/Mg2+-ATPase = 0.85, had minimal activity in KCl and EDTA, and cleaved GTP at 35% of the rate of ATP. The Mg2+-ATPase was insensitive to ouabain or oligomycin. The spindle Mg2+-ATPase was inhibited by sodium vanadate but, like egg dynein, was less sensitive to vanadate than flagellar dynein. The spindle Mg2+-ATPase does not resemble the mitotic Ca2+-ATPase described by others. We propose that the spindle Mg2+-ATPase is egg dynein. Bound carbohydrate on the two high-molecular-weight polypeptides of both egg dynein and the spindle enzyme suggest that these proteins may normally associate with membranes in the living cell.