Mass enucleation of cultured animal cells

Nanomechanical properties of enucleated cells: contribution of the nucleus to the passive cell mechanics

Background

The nucleus, besides its functions in the gene maintenance and regulation, plays a significant role in the cell mechanosensitivity and mechanotransduction. It is the largest cellular organelle that is often considered as the stiffest cell part as well. Interestingly, the previous studies have revealed that the nucleus might be dispensable for some of the cell properties, like polarization and 1D and 2D migration. Here, we studied how the nanomechanical properties of cells, as measured using nanomechanical mapping by atomic force microscopy (AFM), were affected by the removal of the nucleus.

Methods

The mass enucleation procedure was employed to obtain cytoplasts (enucleated cells) and nucleoplasts (nuclei surrounded by plasma membrane) of two cell lines, REF52 fibroblasts and HT1080 fibrosarcoma cells. High-resolution viscoelastic mapping by AFM was performed to compare the mechanical properties of normal cells, cytoplasts, and nucleoplast. The absence or presence of the nucleus was confirmed with fluorescence microscopy, and the actin cytoskeleton structure was assessed with confocal microscopy.

Results

Surprisingly, we did not find the softening of cytoplasts relative to normal cells, and even some degree of stiffening was discovered. Nucleoplasts, as well as the nuclei isolated from cells using a detergent, were substantially softer than both the cytoplasts and normal cells.

Conclusions

The cell can maintain its mechanical properties without the nucleus. Together, the obtained data indicate the dominating role of the actomyosin cytoskeleton over the nucleus in the cell mechanics at small deformations inflicted by AFM.

Centrifugal Displacement of Nuclei in Adherent Cells to Study LINC Complex-Dependent Mechanisms of Homeostatic Nuclear Positioning

The positioning of the nucleus is critical for key cellular processes including division, migration, and differentiation. Traditional approaches to understanding the functions and mechanisms of nuclear positioning have relied upon cellular systems in which nuclei move in response to stimuli or developmental programs and use molecular or pharmacological perturbations of nuclear and cytoskeletal elements. Here, we describe a complimentary approach to perturbing nuclear position in adherent cells using centrifugal force and how this may be used to understand LINC complex mechanisms of homeostatic nuclear positioning.

Centrosome defines the rear of cells during mesenchymal migration

Taking advantage of the strong polarity of cells migrating along micropatterned lines, combined with computational modeling and microsurgery, we found that the centrosome must be localized toward the rear of a cell, likely for controlling the distribution of tail formation signals. This discovery clarifies a long-standing controversy in cell biology.

The importance of centrosome in directional cell migration has long been recognized. However, the conventional view that centrosome determines cell’s front, based on its often-observed position in front of the nucleus, has been challenged by contradictory observations. Here we show that centrosome defines the rear instead of the front, using cells plated on micropatterned adhesive strips to facilitate directional migration. We found that centrosome is always located proximal to the future rear before polarity is established through symmetry breaking or reversed as the cell reaches a dead end. In addition, using microsurgery to alter the distance of centrosomes from cells’ ends, we show that centrosomal proximity is predictive of the placement of the rear. Removal of centrosome impairs directional cell migration, whereas the removal of nucleus alone makes no difference in most cells. Computer modeling under the framework of a local-enhancement/global-inhibition mechanism further demonstrates that positioning of rear retraction, mediated by signals concentrated near the centrosome, recapitulates all the experimental observations. Our results resolve a long-standing controversy and explain how cells use centrosome and microtubules to maintain directional migration.

ScII: an abundant chromosome scaffold protein is a member of a family of putative ATPases with an unusual predicted tertiary structure

Here, we describe the cloning and characterization of ScII, the second most abundant protein after topoisomerase II, of the chromosome scaffold fraction to be identified. ScII is structurally related to a protein, Smc1p, previously found to be required for accurate chromosome segregation in Saccharomyces cerevisiae. ScII and the other members of the emerging family of SMC1-like proteins are likely to be novel ATPases, with NTP-binding A and B sites separated by two lengthy regions predicted to form an alpha-helical coiled-coil. Analysis of the ScII B site predicted that ScII might use ATP by a mechanism similar to the bacterial recN DNA repair and recombination enzyme. ScII is a mitosis-specific scaffold protein that colocalizes with topoisomerase II in mitotic chromosomes. However, ScII appears not to be associated with the interphase nuclear matrix. ScII might thus play a role in mitotic processes such as chromosome condensation or sister chromatid disjunction, both of which have been previously shown to involve topoisomerase II.

Role of luteal cell nucleus in the expression of gonadotropin action

Gonadotropin receptors are not only present in cell membranes, but also in nuclei of bovine and human luteal cells. hCG/hLH can directly regulate several nuclear functions. To further investigate the role of luteal cell nucleus in the expression of gonadotropin action, the effect of enucleation of luteal cells on gonadotropin receptors and gonadotropin response was studied. Luteal cytoplasts were prepared by colchicine treatment of purified whole luteal cells followed by centrifugation at 37 C in a Percoll gradient. The cytoplasts were 85 to 90% pure with a recovery of about 57%. Cytoplasts were viable as determined by trypan blue exclusion (87%) and metabolically competent as determined by 3H-leucine incorporation into proteins. On the day of preparation, the viability and metabolic competency of cytoplasts were similar to control cells, i.e. untreated and colchicine treated whole luteal cells. In addition, cytoplasts and control cells showed a similar decline in number and viability during storage at 4 C. While control cells continue to be metabolically competent, cytoplasts showed a dramatic decline by 48 h of storage at 4 C. Neither the cytoplasts nor control cells degraded 125I-hCG. The kinetics of 125I-hCG association and dissociation, specificity and affinity of binding to cytoplasts were similar to control cells. However, the number of available gonadotropin receptors in cytoplasts was significantly lower than in control cells. Cytoplasts contained lower progesterone levels and more importantly, they could not be stimulated by 10 nM hCG or 10 mM dibutyryl cyclic AMP to produce more progesterone. Controls cells, on the other hand, contained higher progesterone levels and responded to hCG and dibutyryl cyclic AMP stimulation. In summary, removal of nuclei from luteal cells results in a partial loss of gonadotropin receptors and complete loss of steroidogenic response to hCG and dibutyryl cyclic AMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Requirement of cell nucleus for Sindbis virus replication in cultured Aedes albopictus cells

The ability of Sindbis virus to grow in enucleated BHK-21 (vertebrate) and Aedes albopictus (invertebrate) cells was tested to determine the dependence of this virus upon nuclear function in these two phylogenetically unrelated hosts. Although both cell types could be demonstrated to produce viable cytoplasts (enucleated cells) which produced virus-specific antigen subsequent to infection. BHK cytoplasts produced a significant number of progeny virions, whereas mosquito cytoplasts did not. The production of vesicular stomatitis virus in mosquito cells was not significantly reduced by enucleation. That such a host function was not essential for vesicular stomatitis virus growth in insect cells is supported by the observation that the production of this virus by mosquito cells is not actinomycin D sensitive. This result agrees with a previously published report in which it was shown that Sindbis virus maturation in invertebrate cells is inhibited by actinomycin D, indicating a possible requirement for host cell nuclear function (Scheefers-Borchel et al., Virology, 110:292-301, 1981).

Centriole cycle in Chinese hamster ovary cells as determined by whole-mount electron microscopy

In interphase Chinese hamster ovary (CHO) cells, the centrosome is attached to the nucleus very firmly. This nuclear-centrosome complex is isolated as a coherent structure by lysis and extraction of cells with Triton X-100 in a low ionic strength medium. Under these conditions, the ultrastructure of the centrioles attached to the nucleus can be discerned by electron microscopy of whole-mount preparations. The structural changes of the centrioles as a function of the cell cycle were monitored by this technique. Specifically, centriolar profiles were placed into six categories according to their orientation and the length ratio of daughter and parent centrioles. The proportion of centrioles in each category was plotted as a frequency histogram. The morphological changes in the centriole cycle were characterized by three distinguishable events: nucleation, elongation, and disorientation. The progress of centrioles through these stages was determined in synchronous populations of cells starting from S or M phase, in cells inhibited in DNA synthesis by addition of thymidine, and in cytoplasts. The results provide a quantitative description of the events of the centriole cycle. They also show that, in complete cells, nucleation, elongation, and disorientation are not dependent upon DNA synthesis. However, in cytoplasts, although elongation and disorientation occur as in normal cells, nucleation is blocked. Procentriole formation appeared to be inhibited by the removal of the nucleus. We suggest that coordination of centriole replication and nuclear replication may depend upon a signal arising from the nucleus.

Rhabdovirus replication in enucleated host cells

Infection of enucleated TC-7 monkey cells with rabies virus resulted in the synthesis of virus-directed RNA and the production of rabies antigens but not of infectious virus. The yield of infectious vesicular stomatitis virus from enucleated TC-7 cells, on the other hand, was almost as high as that from intact cells. Inhibition of the mitochondrial functions of enucleated cells by treatment with ethidium bromide did not influence the development of rabies antigens or the production of infectious vesicular stomatitis virus.