Nuclear movements and nuclear actin in bilobed nuclei of Amoeba proteus

Nucleus-associated actin in Amoeba proteus

The presence, spatial distribution and forms of intranuclear and nucleus-associated cytoplasmic actin were studied in Amoeba proteus with immunocytochemical approaches. Labeling with different anti-actin antibodies and staining with TRITC-phalloidin and fluorescent deoxyribonuclease I were used. We showed that actin is abundant within the nucleus as well as in the cytoplasm of A. proteus cells. According to DNase I experiments, the predominant form of intranuclear actin is G-actin which is associated with chromatin strands. Besides, unpolymerized actin was shown to participate in organization of a prominent actin layer adjacent to the outer surface of nuclear envelope. No significant amount of F-actin was found in the nucleus. At the same time, the amoeba nucleus is enclosed in a basket-like structure formed by circumnuclear actin filaments and bundles connected with global cytoplasmic actin cytoskeleton. A supposed architectural function of actin filaments was studied by treatment with actin-depolymerizing agent latrunculin A. It disassembled the circumnuclear actin system, but did not affect the intranuclear chromatin structure. The results obtained for amoeba cells support the modern concept that actin is involved in fundamental nuclear processes that have evolved in the cells of multicellular organisms.

Cytoskeleton organization during the cell cycle in two stramenopile microalgae, Ochromonas danica (Chrysophyceae) and Heterosigma akashiwo (Raphidophyceae), with special reference to F-actin organization and its role in cytokinesis

F-actin organization during the cell cycle was investigated in two stramenopile microalgae, Ochromonas danica (Chrysophyceae; UTEX LB1298) and Heterosigma akashiwo (Raphidophyceae; NIES-6) using FITC-phalloidin. In the interphase cell of O. danica, F-actin bundles were localized forming a network structure in the cortical region, which converged from the anterior region to the posterior, whereas in the interphase cell of H. akashiwo, F-actin bundles were observed forming a network structure in the cortical region without any polarity. In both O. danica and H. akashiwo, at the initial stage of mitosis the cortical F-actin disappeared, and then during cytokinesis assembly of an actin-based ring-like structure occurred in the cell cortex in the plane of cytokinesis. The ring-like structure initiated from aster-like structures was composed of F-actin in both O. danica and H. akashiwo. Different from animal cells, later stages of cytokinesis of O. danica seemed to be promoted by microtubules, although the early stages of cytokinesis progressed with a constriction of the ring-like structure, whereas cytokinesis of H. akashiwo was apparently completed by constriction of the cell mediated by the F-actin ring, as in animal cells.

CORTICAL F-ACTIN REORGANIZATION AND A CONTRACTILE RING-LIKE STRUCTURE FOUND DURING THE CELL CYCLE IN THE RED CRYPTOMONAD, PYRENOMONAS HELGOLANDII(1)

Cortical F-actin reorganization during the cell cycle was observed in Pyrenomonas helgolandii U. J. Santore (SAG 28.87) for the first time in Cryptophyta using fluorescein-isothiocyanate (FITC)-phalloidin staining. In interphase, a number of F-actin bundles were observed as straight lines running parallel to the long axis of the cell on the cell cortical region. They extended from an F-actin bundle that runs along the margin of the vestibulum. Although the F-actin bundles running parallel to the long axis of the cell disappeared during anaphase, they gradually reappeared in telophase. By contrast, the F-actin bundle along the vestibulum margin remained visible during cytokinesis and dynamically changed following the enlargement of the vestibulum, suggesting that F-actin was involved in the mechanism of vestibulum enlargement. F-actins were not found in the cytoplasmic and nucleoplasmic regions throughout the cell cycle. In addition, a contractile ring-like structure appeared at the cleavage furrow during cytokinesis. Treatment with cytochalasin B and latrunculin B significantly inhibited the formation of cleavage furrow, resulting in forming an abnormal cell with two nuclei, suggesting that cytokinesis in P. helgolandii is controlled by the contractile ring-like structure constituted of F-actin.

Actin dynamics in Amoeba proteus motility

We studied the distribution of the endogenous Arp2/3 complex in Amoeba proteus and visualised the ratio of filamentous (F-actin) to total actin in living cells. The presented results show that in the highly motile Amoeba proteus, Arp2/3 complex-dependent actin polymerisation is involved in the formation of the branching network of the contractile layer, adhesive structures, and perinuclear cytoskeleton. The aggregation of the Arp2/3 complex in the cortical network, with the exception of the uroid and advancing fronts, and the spatial orientation of microfilaments at the leading edge suggest that actin polymerisation in this area is not sufficient to provide the driving force for membrane displacement. The examined proteins were enriched in the pinocytotic pseudopodia and the perinuclear cytoskeleton in pinocytotic amoebae. In migrating amoebae, the course of changes in F-actin concentration corresponded with the distribution of tension in the cell cortex. The maximum level of F-actin in migrating amoebae was observed in the middle-posterior region and in the front of retracting pseudopodia. Arp2/3 complex-dependent actin polymerisation did not seem to influence F-actin concentration. The strongly condensed state of the microfilament system could be attributed to strong isometric contraction of the cortical layer accompanied by its retraction from distal cell regions. Isotonic contraction was limited to the uroid.

Reversible changes in size of cell nuclei isolated from Amoeba proteus: Role of the cytoskeleton

Micrurgically isolated interphasal nuclei of Amoeba proteus, which preserve F-actin cytoskeletal shells on their surface, shrink after perfusion with imidazole buffer without ATP, and expand to about 200% of their cross-sectional area upon addition of pyrophosphate. These changes in size may be reproduced several times with the same nucleus. The shrunken nuclei are insensitive to the osmotic effects of sugars and distilled water, whereas the expanded ones react only to the distilled water, showing further swelling. The shrinking-expansion cycles are partially inhibited by cytochalasins. They are attributed to the state of actomyosin complex in the perinuclear cytoskeleton, which is supposed to be in the rigor state in the imidazole buffer without ATP, and to dissociate in the presence of pyrophosphate. Inflow of external medium to the nuclei during dissociation of the myosin from the perinuclear F-actin may be due to colloidal osmosis depending on other macromolecular components of the karyoplasm.Key words: Amoeba proteus, isolated nuclei, perinuclear cytoskeleton.

Peripheral nuclear matrix actin forms perinuclear shells

Perinuclear actin shells have been reported in a variety of organisms. The shells have been identified by staining perinuclear material with fluorescently-labelled phalloidin, but have not been localized to a specific subcellular compartment at the ultrastructural level. We show here that the shells of 3T3 cells lie in the peripheral nuclear matrix. Nuclear shells and matrix actin in other parts of the nucleus are not usually detected by immunohistochemical staining because they are inaccessible to antibodies or to phalloidin. Immunohistochemical detection of nuclear actin is only possible during its deposition at the end of mitosis, or in interphase nuclei that have been extracted with detergent, digested with nucleases and washed with high salt buffers.