Monensin blocks the first meiotic cell division of the Xenopus oocyte: Effect at the nuclear membrane level

The carboxylic ionophore monensin inhibits the meiotic maturation of the Xenopus oocyte. When oocytes are exposed to high concentrations of monensin (10 μM), both progesterone and MPF-induced (maturation-promoting factor-induced) maturations are blocked. Lower doses of monensin (1–10 μM) do not inhibit the formation or amplification of MPF activity in the oocyte cytoplasm; however, breakdown of the nuclear envelope does not occur. These observations show that monensin, which is known to abolish intracellular proton gradients, interferes with the mechanism of the breakdown of the nuclear envelope induced by MPF.     

Cytokeratin phosphorylation, cytokeratin filament severing and the solubilization of the maternal mRNA Vg1

During meiotic maturation, the cortical cytokeratin filament system of the Xenopus oocyte disappears (Klymkowsky, M. W., and L. A. Maynell. 1989. Dev. Biol. 134:479). Here we demonstrate that this disappearance results from the severing of cytokeratin filaments into a heterogenous population of oligomers, with S- values ranging from 12S and greater. Cytokeratin filament severing correlates with the hyperphosphorylation of the type II cytokeratin of the oocyte. Both the severing of cytokeratin filaments and cytokeratin hyperphosphorylation are reversed by treatment with cycloheximide. These data suggest that fragmentation of cytokeratin filaments is controlled, at least in part, by the phosphorylation of the type II cytokeratin, and that the cytokeratin kinase activity responsible is biosynthetically labile. Cytokeratin filaments have been suggested to anchor the maternal mRNA Vg1 to the vegetal cortex of the oocyte (Pondel, M., and M. L. King. 1988. Proc. Natl. Acad. Sci. USA. 85:7216). By injecting fractions containing active maturation promoting factor or a purified, mutant cyclin protein, we find that the bulk of the Vg1 mRNA in the oocyte can be solubilized under conditions that block the fragmentation of cytokeratin filaments, and that the fragmentation of cytokeratin filaments itself leads to the solubilization of only a minor fraction of the Vg1 mRNA. Thus, at best, cytokeratin filaments directly anchor only a minor fraction of the Vg1 mRNA in the oocyte. Moreover, factors distinct from maturation promoting factor appear to be required for the complete solubilization of Vg1 mRNA during oocyte maturation.     

Polar asymmetry in the organization of the cortical cytokeratin system of Xenopus laevis oocytes and embryos

We have used whole-mount immunofluorescence microscopy of late-stage Xenopus laevis oocytes and early embryos to examine the organization of their cortical cytokeratin systems. In both mature oocytes and early embryos, there is a distinct animal-vegetal polarity in cytokeratin organization. In mature (stage-VI) oocytes, the cytokeratin filaments of the vegetal region form a unique, almost geodesic network; in the animal region, cytokeratin organization appears much more variable and irregular. In unfertilized, postgerminal vesicle breakdown eggs, the cortical cytokeratin system is disorganized throughout both animal and vegetal hemispheres. After fertilization, cytokeratin organization reappears first in a punctate pattern that is transformed into an array of oriented filaments. These cytokeratin filaments appear first in the vegetal hemisphere and are initially thin. Subsequently, they form bundles that grow thicker through the period of first to second cleavage, at which point large cytokeratin filament bundles form a loose, fishnet-like system that encompasses the vegetal portion of each blastomere. In the animal region, cytokeratin filaments do not appear to form large fibre networks, but rather appear to be organized into a system of fine filaments. The animal-vegetal polarity in cytokeratin organization persists until early blastula (stage 5); in later-stage embryos, both animal and vegetal blastomeres possess qualitatively similar cytokeratin filament systems. The entire process of cytokeratin reorganization in the egg is initiated by prick activation. These observations indicate that the cortical cytoskeleton of Xenopus oocytes and early embryos is both dynamic and asymmetric.     

Host cell factors controlling vimentin organization in the Xenopus oocyte

To study vimentin filament organization in vivo we injected Xenopus oocytes, which have no significant vimentin system of their own, with in vitro-synthesized RNAs encoding Xenopus vimentins. Exogenous vimentins were localized primarily to the cytoplasmic surface of the nucleus and to the subplasma membrane "cortex." In the cortex of the animal hemisphere, wild-type vimentin forms punctate structures and short filaments. In contrast, long anastomosing vimentin filaments are formed in the vegetal hemisphere cortex. This asymmetry in the organization of exogenous vimentin is similar to that of the endogenous keratin system (Klymkowsky, M. W., L. A. Maynell, and A. G. Polson. 1987. Development (Camb.). 100:543-557), which suggests that the same cellular factors are responsible for both. Before germinal vesicle breakdown, in the initial stage of oocyte maturation, large vimentin and keratin filament bundles appear in the animal hemisphere. As maturation proceeds, keratin filaments fragment into soluble oligomers (Klymkowsky, M. W., L. A. Maynell, and C. Nislow. 1991. J. Cell Biol. 114:787-797), while vimentin filaments remain intact and vimentin is hyperphosphorylated. To examine the role of MPF kinase in the M-phase reorganization of vimentin we deleted the conserved proline of vimentin's single MPF-kinase site; this mutation had no apparent effect on the prophase or M-phase behavior of vimentin. In contrast, deletion of amino acids 19-68 or 18-61 of the NH2-terminal "head" domain produced proteins that formed extended filaments in the animal hemisphere of the prophase oocyte. We suggest that the animal hemisphere cortex of the prophase oocyte contains a factor that actively suppresses the formation of extended vimentin filaments through a direct interaction with vimentin's head domain. During maturation this "suppressor of extended filaments" appears to be inactivated, leading to the formation of an extended vimentin filament system.     

Breakdown of starfish ovarian follicle induced by maturation-promoting factor

Immature starfish oocytes are surrounded by envelopes consisting of follicular cells. These cells adhere to each other and to the oocyte, immobilizing the latter within the ovary. When isolated oocytes in their follicles are treated with 1-methyladenine (1-MeAde), germinal vesicle breakdown (GVBD) and follicular envelope breakdown (FEBD) occur simultaneously. The 1-MeAde acts on the oocyte surface to produce a maturation-promoting factor (MPF) in the cytoplasm, which brings about GVBD. In the present study, MPF was found to induce FEBD as well as GVBD when injected into immature oocytes with their follicles in Asterina pectinifera. Although GVBD was induced by MPF in the presence of cytochalasin D, this drug prevented MPF-induced FEBD, and each follicular cell remained in situ on the surface of the oocyte. However, desmosomes connecting the processes of the follicle cell with the oocyte surface were disrupted following MPF injection even in the presence of cytochalasin D, and the processes became detached from the oocyte. FEBD occurred in these oocytes when cytochalasin D was removed, resulting in the formation of a small follicular clump by microfilament-mediated contraction of the follicle cells. These results show that FEBD is not brought about by the direct action of 1-MeAde but by the action of MPF. Therefore, in starfish, spawning as well as oocyte maturation is directly triggered by MPF produced under the influence of 1-MeAde.     

The C. elegans Myt1 ortholog is required for the proper timing of oocyte maturation

Maturation promoting factor (MPF), a complex of cyclin-dependent kinase 1 and cyclin B, drives oocyte maturation in all animals. Mechanisms to block MPF activation in developing oocytes must exist to prevent precocious cell cycle progression prior to oocyte maturation and fertilization. This study sought to determine the developmental consequences of precociously activating MPF in oocytes prior to fertilization. Whereas depletion of Myt1 in Xenopus oocytes causes nuclear envelope breakdown in vitro, we found that depletion of the Myt1 ortholog WEE-1.3 in C. elegans hermaphrodites causes precocious oocyte maturation in vivo. Although such oocytes are ovulated, they are fertilization incompetent. We have also observed novel phenotypes in these precociously maturing oocytes, such as chromosome coalescence, aberrant meiotic spindle organization, and the expression of a meiosis II post-fertilization marker. Furthermore, co-depletion studies of CDK-1 and WEE-1.3 demonstrate that WEE-1.3 is dispensable in the absence of CDK-1, suggesting that CDK-1 is a major target of WEE-1.3 in C. elegans oocytes.     

Reorganization of actin filaments by ADF/cofilin is involved in formation of microtubule structures during Xenopus oocyte maturation

We examined the reorganization of actin filaments and microtubules during Xenopus oocyte maturation. Surrounding the germinal vesicle (GV) in immature oocytes, the cytoplasmic actin filaments reorganized to accumulate beneath the vegetal side of the GV, where the microtubule-organizing center and transient microtubule array (MTOC-TMA) assembled, just before GV breakdown (GVBD). Immediately after GVBD, both Xenopus ADF/cofilin (XAC) and its phosphatase Slingshot (XSSH) accumulated into the nuclei and intranuclear actin filaments disassembled from the vegetal side with the shrinkage of the GV. As the MTOC-TMA developed well, cytoplasmic actin filaments were retained at the MTOC-TMA base region. Suppression of XAC dephosphorylation by anti-XSSH antibody injection inhibited both actin filament reorganization and proper formation and localization of both the MTOC-TMA and meiotic spindles. Stabilization of actin filaments by phalloidin also inhibited formation of the MTOC-TMA and disassembly of intranuclear actin filaments without affecting nuclear shrinkage. Nocodazole also caused the MTOC-TMA and the cytoplasmic actin filaments at its base region to disappear, which further impeded disassembly of intranuclear actin filaments from the vegetal side. XAC appears to reorganize cytoplasmic actin filaments required for precise assembly of the MTOC and, together with the MTOC-TMA, regulate the intranuclear actin filament disassembly essential for meiotic spindle formation.     

Sea urchin oocytes possess elaborate cortical arrays of microfilaments, microtubules, and intermediate filaments

Extensive arrays of microfilaments, microtubules and cytokeratin-type intermediate filaments were detected in the cortex of Strongylocentrotus droebachiensis oocytes using fluorescently labeled antibodies on both cortex and whole mount preparations. All three filament systems undergo dramatic structural reorganization during meiotic maturation of the egg. Microfilaments form a dense meshwork within the cortex of the oocyte. After meiosis, the filaments rearrange and shorten, resulting in a more loosely organized network. Both cortical microtubules and microtubules associated with a microtubule-organizing center are observed within the oocyte. After meiosis, the number and length of the cortical microtubules gradually diminish. A microtubule organizing center is found situated between the germinal vesicle and the plasma membrane in many oocytes. A network of filaments extends from the microtubule organizing center and radiates peripherally toward the germinal vesicle, presumably marking the animal pole. Cytokeratin-like intermediate filaments form a reticular network within the oocyte cortex, then solubilize during meiosis. In whole mounts of oocytes there is a single focal center of cytokeratin staining from which filaments radiate. Indirect immunofluorescence experiments, using anti-tubulin and anti-cytokeratin antibodies simultaneously, reveal the intermediate filament focal center to be localized within the microtubule organizing center. These results demonstrate the presence of a complex cortical cytoskeleton in premeiotic eggs of the sea urchin, Strongylocentrotus droebachiensis.     

Co-Expression of Cytokeratins and Vimentin in Sheep Cumulus-Oocyte Complexes. Alteration of Intermediate Filament Distribution by Acrylamide

The association between germ cells and somatic granulosa cells persists throughout the growth of the oocyte by means of foot processes of the cumulus corona cells that cross the zona pellucida. During meiotic maturation important nuclear and cytoplasmic events occur in cumulus-oocyte complex suggesting implication of cytoskeletal elements. Immunoblotting analysis of cytoskeletal proteins of the cumulus cells revealed the presence of vimentin polypeptide and of at least two cytokeratin polypeptides. Using immunofluorescence techniques on cryostat sections through frozen tissue, we provided evidence for the presence of cytokeratins of the simple epithelial type in addition to vimentin in sheep cumulus cells. These two types of intermediate filaments were localized throughout the cytoplasm and especially in the foot processes which cross the zona pellucida. The contact area between the two cell types was also labelled with the antibodies. Acrylamide treatment of cumulus-oocyte complexes involved a drastic disorganization of the intermediate filament network and triggered the isolation of the oocyte from its cumulus cells. This isolation resulted in resumption of meiosis. From these results it appears that intermediate filaments could participate in the process of gap junction loss and indirectly in the control of meiosis resumption.     

Cover Image,Volume 234, Number 10, October 2019

Cytoskeleton which includes microtubule and actin filaments plays important roles during mammalian oocyte maturation. In the present study, we showed that protein kinase C mu (PKC mu) was one potential key molecule which affected cytoskeleton dynamics in mouse oocytes. Our results showed that PKC mu expressed and localized at the poles of the spindle during oocyte maturation, and PKC mu expression reduced in the oocytes from 6-month-old mice or 24 hr in vitro culture. We knocked down the expression of PKC mu in oocytes using morpholino injection to explore the relationship between PKC mu and subcellular structure defects. The loss of PKC mu reduced oocyte maturation competence, showing with decreased polar body extrusion rate and increased rate of symmetric division. Further analysis indicated that PKC mu decrease caused the spindle organization defects, and this could be confirmed by the decreased tubulin acetylation level. Moreover, we found that PKC mu affected the phosphorylation level of cofilin for actin assembly, which further affected cytoplasmic actin distribution and spindle positioning. In summary, our data indicated that PKC mu is one key factor for oocyte maturation through its roles on the spindle organization and actin filament distribution.     

Protein phosphorylation and oocyte maturation : II. Inhibition of starfish oocyte maturation by intracellular microinjection of protein phosphatases 1 and 2A and alkaline phosphatase

Oocyte maturation (meiosis re-initiation) in starfish is induced by the natural hormone 1-methyladenine (1-MeAde). Following hormonal stimulation of the oocyte, an intracellular Maturation Promoting Factor (MPF) appears in the cytoplasm which triggers nuclear envelope breakdown and maturation divisions. Microinjection of pure preparations of the catalytic subunits of protein phosphatases 1 and 2A inhibits 1-MeAde-induced maturation in a dose-dependent manner. Calmodulin-dependent protein phosphatase 2B is inefficient. Maturation induced by mimetics of 1-MeAde, such as dithiothreitol (DTT), methylglyoxal-bis(guanylhydrazone) (MGBG), 8-hydroxyeicosatetraenoic acid (8 HETE) and arachidonic acid (AA) is also inhibited by these protein phosphatases. In all cases inhibition can be reversed by increasing the concentration of 1-Me-Ade or of mimetic. Alkaline phosphatase also inhibits maturation in a dose-dependent way and in a reversible manner. Microinjection of protein phosphatase is still effective when preformed long after the end of the hormone-dependent period, and can even be effective a few minutes before the breakdown of the nuclear envelope. No detectable MPF activity is found in 1-MeAde-treated phosphatase-injected oocytes. However, microinjection of phosphatase 2A simultaneously with MPF (obtained from 1-MeAde-treated donors) does not result in inhibition. These results constitute direct evidence for the necessity of an elevated level of phosphorylated proteins for MPF activity and maturation. The mode of action of 1-MeAde in inducing starfish oocyte maturation is discussed in relation to protein phosphorylation.     

Immuno-electron microscopical identification of the two types of intermediate filaments in established epithelial cells

The intermediate filament systems of the established epithelial cell lines HeLa and PtK₂ have been characterized by electron microscopy using indirect immunoferritin labelling. The results provide a direct ultrastructural confirmation of the proposal based on indirect immunofluorescence microscopy, that vimentin and cytokeratin fibrils constitute two distinct 10 nm filament systems in much of the cell body. In agreement both with classical histology and with the finding that cytokeratins are typically present in many epithelial tissues, demosome-attached 10 nm filaments (tonofilaments) were found to be of the cytokeratin type. Vimentin, but not cytokeratin filaments were translocated into juxtanuclear caps after exposure of the cells to colcemid. Regions of the cytoplasm where the two distinct systems form mixed bundles were identified and both side-by-side arrangements and the occurrence of vimentin fibers in a sheath-like structure around a cytokeratin filament core are described. Our results emphasize that the two systems interact but differ in their organization and control.     

Aggregation and loss of cytokeratin filament networks inhibit golgi organization in liver-derived epithelial cell lines

Intermediate filaments are one of the three major cytoskeletons. Some roles of intermediate filaments in cellular functions have emerged based on various diseases associated with mutations of cytokeratins. However, the precise functions of intermediate filament are still unclear. To resolve this, we manipulated intermediate filaments of cultured cells by expressing a mutant cytokeratin. Arginine 89 of cytokeratin18 plays an important role in intermediate filament assembly. The expression of green fluorescent protein-tagged cytokeratin18 arg89cys induced aggregations and loss of the intermediate filament network composed of cytokeratins in liver-derived epithelial cells, Huh7 and OUMS29, but only induced the formation of cytokeratin aggregates and did not affect the intermediate filament network of endogenous vimentin in HEK293. The expression of this mutant affected the distribution of Golgi apparatus and the reassembly of Golgi apparatus after perturbations by nocodazole or brefeldin A in both Huh7 and OUMS29, but not in HEK293. Our data show that loss of the original intermediate filament network, but not the existence of cytokeratin aggregates, induces redistribution of the Golgi apparatus. The original intact intermediate filament network is necessary for the organization of Golgi apparatus.     

H-ras(val12) induces cytoplasmic but not nuclear events of the cell cycle in small Xenopus oocytes.

Microinjection of H-ras(val12) protein into fully grown Xenopus oocytes has been shown to induce meiotic maturation. In the present study, mRNA encoding the mutant ras protein was injected into both fully grown (stage 6) and growing (stage 4) oocytes. The mRNA induced nuclear breakdown in stage 6 oocytes, as expected. However, the mRNA induced neither nuclear breakdown nor maturation promoting factor when injected into stage 4 oocytes. Instead, the response in stage 4 oocytes included an activation pulse of calcium, cortical granule breakdown, elevation of the vitelline envelope, and abortive cleavage furrows, all of which are characteristics of the activation response in mature eggs. In addition, the injected mRNA led to increased rates of endogenous protein synthesis and the migration of subcortical organelles into the oocyte interior. These observations are discussed relative to the suggestion that oncogenic ras protein leads to an increase in both diacylglycerol and inositol trisphosphate, which then regulate the various cytoplasmic events described.     

Intermediate filaments in the developing type II cell of fetal monkey lung

Anticytokeratin monoclonal antibody was used to study epithelial cell development in fetal monkey lungs taken from animals of different ages. It is well established that the overall maturity of fetal lung depends greatly on the maturation of type II epithelial cells in the alveolus. In this study, we have correlated the cytokeratin phenotype of mammalian epithelial cells with pneumocyte maturation. We show that differentiation and maturation of the type II cell is related to intermediate filament expression. Twenty-four fetal monkeys (Macaca nemestrina) were delivered by cesarean section at a gestational age of 135-140 days (term = 168 days) and divided into two groups. One group of animals was sacrificed during the first 3 hr of life, and the other group was maintained in incubators for 92-120 hr. Anticytokeratin monoclonal antibody recognizes only alveolar type I and type II epithelial cells. In the first 3 hr of life, the cytokeratin was localized only at the alveolar surface and at the cytoplasmic periphery of the type II cells of these premature animals. However, at the age of 92-120 hr, the epithelia in the lungs reacted more intensely than they did during the first 3 hr. Electron microscopy revealed and confirmed that the type II cells were matured and abundant intermediate filaments appeared in the cytoplasm. The filaments appeared to form either aggregates or parallel filament bundles and few were closely associated with the lamellar bodies. In the immature type II cells at 0-3 hr of life, few intermediate filaments could be localized in the cytoplasm, and no parallel filament bundle was observed, though many appeared in the 92-120 hr lungs. This suggests that the intermediate filaments have a functional significance in the development and maturation of the type II cell. The location and stability of keratin filaments in type II cells may confer the structural strength necessary for cells covering a free surface in the alveoli during lung maturation.     

The receptor tyrosine phosphatase Dlar and integrins organize actin filaments in the Drosophila follicular epithelium.

BACKGROUND: Regulation of actin structures is instrumental in maintaining proper cytoarchitecture in many tissues. In the follicular epithelium of Drosophila ovaries, a system of actin filaments is coordinated across the basal surface of cells encircling the oocyte. These filaments have been postulated to regulate oocyte elongation; however, the molecular components that control this cytoskeletal array are not yet understood. RESULTS: We find that the receptor tyrosine phosphatase (RPTP) Dlar and integrins are involved in organizing basal actin filaments in follicle cells. Mutations in Dlar and the common beta-integrin subunit mys cause a failure in oocyte elongation, which is correlated with a loss of proper actin filament organization. Immunolocalization shows that early in oogenesis Dlar is polarized to membranes where filaments terminate but becomes generally distributed late in development, at which time beta-integrin and Enabled specifically associate with actin filament terminals. Rescue experiments point to the early period of polar Dlar localization as critical for its function. Furthermore, clonal analysis shows that loss of Dlar or mys influences actin filament polarity in wild-type cells that surround mutant tissues, suggesting that communication between neighboring cells regulates cytoskeletal organization. Finally, we find that two integrin alpha subunits encoded by mew and if are required for proper oocyte elongation, implying that multiple components of the ECM are instructive in coordinating actin fiber polarity. CONCLUSIONS: Dlar cooperates with integrins to coordinate actin filaments at the basal surface of the follicular epithelium. To our knowledge, this is the first direct demonstration of an RPTP's influence on the actin cytoskeleton.     

Ultrastructural changes during nuclear maturation in Bufo arenarum oocytes.

During progesterone-induced nuclear maturation the oocytes of Bufo arenarum undergo a series of nuclear and cytoplasmic changes. The breakdown of heterocellular communications between the follicular cell projections and the oocyte microvilli, and the consequent enlargement of the perivitelline space, were observed at the animal pole. The more evident cytoplasmic feature during nuclear maturation comprised the gathering of glycogen granules in clusters, some phagocytosed by empty vesicles. With respect to the location of these vesicles, some were observed in close proximity to the oolemma and others were freely suspended in the perivitelline space, extruded from the oocyte. Other visible events were the disruption of the annulate lamellae, the formation of an elaborate cortical endoplasmic reticulum and the rearrangement of the cortical granules in a monolayer immediately beneath the oolemma together with aggregates of endoplasmic reticulum cisternae. Our results show that during nuclear maturation the nuclear oocyte changes include a flattening of the spherical oocyte nucleus, its migration towards the surface of the animal pole, the disappearance of the nucleoli and the dissolution of the nuclear envelope.     

Organization of cytokeratin bundles by desmosomes in rat mammary cells

In a rat mammary epithelial cell line, LA-7, cytokeratin bundles recognized in immunofluorescence by a monoclonal antibody (24B42) disappear after trypsinization of cultures and are gradually reformed after replating. We have followed the time course of cytokeratin filament reappearance by growing cells in low calcium medium (0.1 mM) which prevents desmosome formation, and then shifting to high calcium (1.8 mM) to start the process. By fixing the cells at various intervals and staining them in immunofluorescence for 24B42 cytokeratin and for desmosomal proteins, we found that cell to cell contact and desmosome formation are prerequisites for keratin filament formation in these cells. EGTA treatment, by disassembling desmosomes, causes the cytokeratin filaments to disappear and the 24B42 protein to pass into a soluble form in this cell line, as ascertained by 100,000 g fractionation and immunoenzymatic assay. Cycloheximide treatment also causes cytokeratin filaments to disappear, indicating that protein synthesis is needed for normal filament maintenance. In another related cell line (106A-10a) and in HeLa cells, trypsinization and EGTA exposure do not cause a complete loss of 24B42 immunofluorescence, although distinct filaments disappear, indicating the presence in these cells of different organizing centers, besides desmosomes, for cytokeratin bundle formation. LA7 cells therefore seem to have a cytokeratin system strictly dependent on the presence of desmosomes, which act as an organizing center for filament assembly. 106A-10a cells (also rich in desmosomes) and HeLa cells (showing instead a reduced number of desmosomes) have a cytokeratin system partially or totally independent from that of desmosomes, with different organizing centers.     

Maturation-promoting factor induces nuclear envelope breakdown in cycloheximide-arrested embryos of Xenopus laevis

We have studied the effect of maturation-promoting factor (MPF) on embryonic nuclei during the early cleavage stage of Xenopus laevis development. When protein synthesis is inhibited by cycloheximide during this stage, the embryonic cell cycle arrests in an artificially produced G2 phase-like state, after completion of one additional round of DNA synthesis. Approximately 100 nuclei can be arrested in a common cytoplasm if cytokinesis is first inhibited by cytochalasin B. Within 5 min after injection of MPF into such embryos, the nuclear envelope surrounding each nucleus disperses, as determined histologically or by immunofluorescent staining of the nuclear lamina with antilamin antiserum. The breakdown of the nuclear envelope occurs at levels of MPF comparable to or slightly lower than those required for oocyte maturation. Amplification of MPF activity, however, does not occur in the arrested egg as it does in the oocyte. These results suggest that MPF can act to advance interphase nuclei into the first events of mitosis and show that the nuclear lamina responds rapidly to MPF.     

Transient change of organization of vimentin filaments during mitosis as demonstrated by a monoclonal antibody

A monoclonal antibody specific for vimentin is described which, by immunofluorescence and immunoelectron microscopy, decorates fibrillar and/or granular structures in mitotic and early postmitotic cells but does not react with vimentin filaments of interphase stages of various cultured cells (rat vascular smooth muscle-derived cell line RVF-SM; SV40-transformed human fibroblasts; bovine kidney epithelial cells of line MDBK). These observations indicate that the organization of vimentin filaments varies during the cell cycle, undergoing a perimitotic change of filament organization. These changes of vimentin filaments are described in relation to those reported for cytokeratin filaments of various epithelial and carcinoma cells. The possible functional implications of filament protein rearrangements both during the cell cycle and in cell differentiation processes are discussed.