Study of nuclear shapes of some even nuclei

Nuclear shape and size are cell-type specific. Change in nuclear shape is seen during cell division, development and pathology. The nucleus of Saccharomycescerevisiae is spherical in interphase and becomes dumbbell shaped during mitotic division to facilitate the transfer of one nucleus to the daughter cell. Because yeast cells undergo closed mitosis, the nuclear envelope remains intact throughout the cell cycle. The pathways that regulate nuclear shape are not well characterized. The nucleus is organized into various subcompartments, with the nucleolus being the most prominent. We have conducted a candidate-based genetic screen for nuclear shape abnormalities in S. cerevisiae to ask whether the nucleolus influences nuclear shape. We find that increasing nucleolar volume triggers a non-isometric nuclear envelope expansion resulting in an abnormal nuclear envelope shape. We further show that the tethering of rDNA to the nuclear envelope is required for the appearance of these extensions.

The nucleus is a mechanically stable compartment of the cell that contains the genome and performs many essential functions. Nuclear mechanical components chromatin and lamins maintain nuclear shape, compartmentalization, and function by resisting antagonistic actin contraction and confinement. Studies have yet to compare chromatin and lamins perturbations side-by-side as well as modulated actin contraction while holding confinement constant. To accomplish this, we used nuclear localization signal green fluorescent protein to measure nuclear shape and rupture in live cells with chromatin and lamin perturbations. We then modulated actin contraction while maintaining actin confinement measured by nuclear height. Wild type, chromatin decompaction, and lamin B1 null present bleb-based nuclear deformations and ruptures dependent on actin contraction and independent of actin confinement. Actin contraction inhibition by Y27632 decreased nuclear blebbing and ruptures while activation by CN03 increased rupture frequency. Lamin A/C null results in overall abnormal shape also reliant on actin contraction, but similar blebs and ruptures as wild type. Increased DNA damage is caused by nuclear blebbing or abnormal shape which can be relieved by inhibition of actin contraction which rescues nuclear shape and decreases DNA damage levels in all perturbations. Thus, actin contraction drives nuclear blebbing, bleb-based ruptures, and abnormal shape independent of changes in actin confinement.

Our knowledge and understanding of the structures and mechanisms responsible for nuclear organization are based primarily on research in animal cells. In order to address the lack of understanding of plant nuclear morphology, new research specifically targeting the nuclear organization of plants has begun. However, much of this research is limited to the popular model plant, Arabidopsis thaliana. In order to broaden the base of research in this area, this study included not only A. thaliana, but three other well‐studied plant species: Medicago truncatula (barrel clover), Nicotiana benthamiana (a tobacco relative) and Solanum lycopersicum (tomato). The main objectives of this project are to determine whether there are any differences in the nuclear morphology among these plant species in regards to cell/organ‐specificity in nuclear size and shape. In addition, we investigated whether environmental conditions could affect nuclear shape, using a transgenic A. thaliana line carrying a nuclear‐localized GFP marker. Specifically, we exposed this line to different osmotic environments and observed whether any changes in the nuclear shape or size resulted. From this project, we have shown that differentiated (non‐spherical) nuclear shapes are seen in M. truncatula, N. benthamiana and S. lycopersicum root tips, roots, hypocotyls and leaves. Our preliminary observations suggest that influencing the environmental conditions of A. thaliana does have a visual effect on the nuclear shape and size of the nuclei as the molarity of the environment increases.

Actomyosin stress fibers impinge on the nucleus and can exert compressive forces on it. These compressive forces have been proposed to elongate nuclei in fibroblasts, and lead to abnormally shaped nuclei in cancer cells. In these models, the elongated or flattened nuclear shape is proposed to store elastic energy. However, we found that deformed shapes of nuclei are unchanged even after removal of the cell with micro-dissection, both for smooth, elongated nuclei in fibroblasts and abnormally shaped nuclei in breast cancer cells. The lack of shape relaxation implies that the nuclear shape in spread cells does not store any elastic energy, and the cellular stresses that deform the nucleus are dissipative, not static. During cell spreading, the deviation of the nucleus from a convex shape increased in MDA-MB-231 cancer cells, but decreased in MCF-10A cells. Tracking changes of nuclear and cellular shape on micropatterned substrata revealed that fibroblast nuclei deform only during deformations in cell shape and only in the direction of nearby moving cell boundaries. We propose that motion of cell boundaries exert a stress on the nucleus, which allows the nucleus to mimic cell shape. The lack of elastic energy in the nuclear shape suggests that nuclear shape changes in cells occur at constant surface area and volume.

Introduction: Nuclei have characteristic shapes dependent on cell type, which are critical for proper cell function, and nuclei lose their distinct shapes in multiple diseases including cancer, laminopathies, and progeria. Nuclear shapes result from deformations of the sub-nuclear components-nuclear lamina and chromatin. How these structures respond to cytoskeletal forces to form the nuclear shape remains unresolved. Although the mechanisms regulating nuclear shape in human tissues are not fully understood, it is known that different nuclear shapes arise from cumulative nuclear deformations post-mitosis, ranging from the rounded morphologies that develop immediately after mitosis to the various nuclear shapes that roughly correspond to cell shape (e.g., elongated nuclei in elongated cells, flat nuclei in flat cells). Methods: We formulated a mathematical model to predict nuclear shapes of cells in various contexts under the geometric constraints of fixed cell volume, nuclear volume and lamina surface area. Nuclear shapes were predicted and compared to experiments for cells in various geometries, including isolated on a flat surface, on patterned rectangles and lines, within a monolayer, isolated in a well, or when the nucleus is impinging against a slender obstacle. Results and Discussion: The close agreement between predicted and experimental shapes demonstrates a simple geometric principle of nuclear shaping: the excess surface area of the nuclear lamina (relative to that of a sphere of the same volume) permits a wide range of highly deformed nuclear shapes under the constraints of constant surface area and constant volume. When the lamina is smooth (tensed), the nuclear shape can be predicted entirely from these geometric constraints alone for a given cell shape. This principle explains why flattened nuclear shapes in fully spread cells are insensitive to the magnitude of the cytoskeletal forces. Also, the surface tension in the nuclear lamina and nuclear pressure can be estimated from the predicted cell and nuclear shapes when the cell cortical tension is known, and the predictions are consistent with measured forces. These results show that excess surface area of the nuclear lamina is the key determinant of nuclear shapes. When the lamina is smooth (tensed), the nuclear shape can be determined purely by the geometric constraints of constant (but excess) nuclear surface area, nuclear volume, and cell volume, for a given cell adhesion footprint, independent of the magnitude of the cytoskeletal forces involved.

Mutations in the nuclear envelope proteins lamins A and C cause a broad variety of human diseases, including Emery-Dreifuss muscular dystrophy, dilated cardiomyopathy, and Hutchinson-Gilford progeria syndrome. Cells lacking lamins A and C have reduced nuclear stiffness and increased nuclear fragility, leading to increased cell death under mechanical strain and suggesting a potential mechanism for disease. Here, we investigated the contribution of major lamin subtypes (lamins A, C, and B1) to nuclear mechanics by analyzing nuclear shape, nuclear dynamics over time, nuclear deformations under strain, and cell viability under prolonged mechanical stimulation in cells lacking both lamins A and C, cells lacking only lamin A (i.e. "lamin C-only" cells), cells lacking wild-type lamin B1, and wild-type cells. Lamin A/C-deficient cells exhibited increased numbers of misshapen nuclei and had severely reduced nuclear stiffness and decreased cell viability under strain. Lamin C-only cells had slightly abnormal nuclear shape and mildly reduced nuclear stiffness but no decrease in cell viability under strain. Interestingly, lamin B1-deficient cells exhibited normal nuclear mechanics despite having a significantly increased frequency of nuclear blebs. Our study indicates that lamins A and C are important contributors to the mechanical stiffness of nuclei, whereas lamin B1 contributes to nuclear integrity but not stiffness.

We study nuclear potential-energy surfaces, ground-state masses and shapes calculated by use of the Yukawa-plus-exponential macroscopic model and a folded-Yukawa single-particle potential for 4023 nuclei ranging from 16O to 279112. We present an overview of the results in the form of four colour contour diagrams vs. proton number Z and neutron number N. The four diagrams show calculated values of |ϵ2| and ϵ4 at the ground state, ground-state microscopic shell-plus-pairing corrections and the deviations between experimental and calculated masses. The diagrams vividly display the regions of magic and deformed nuclei. In particular, the plot of |ϵ2| vs. Z and N clearly shows the well-known deformed actinide and rare-earth regions and the two new deformed regions around A = 80 and A = 100. The plots indicate differences between the various deformed regions. For instance, there are differences in the magnitude of the deformation and in the character of the transition from spherical to deformed shapes. We discuss extensively the transition from spherical to deformed shapes and study the relation between shape changes and the mass corresponding to the ground-state minimum, and the significance of additional minima in the nuclear potential-energy surface. For a few illustrative cases we discuss the effect of angular momentum on the nuclear shape. The calculated values for the ground-state mass and shape show good agreement with experimental data throughout the periodic system, but some discrepancies remain that deserve further study.

SummaryThe nuclear shape observed after the forced expression of mouse or human protamine 1 (PRM1) in fibroblasts led us to propose the hypothesis that the PRM1 sequence plays an important role in imposing the overall shape of the protaminized nucleus. Comparison of mouse and human PRM1 sequences pointed to cysteines 15 and 29 as potential critical residues in the mouse PRM1 sequence, inducing the characteristic mouse “hooked” nuclear sperm shape. To explore this idea, mice with mutations in PRM1 Cys15 and Cys29 were generated. These mice remained fertile with no significant changes in sperm count or protamine expression levels. However, modifications in sperm head shape were observed. Transmission electron microscopy revealed disrupted chromatin condensation in mutant sperm, with several morphological changes and a remarkable increase in histone retention. Overall, the findings suggest that species-specific PRM1 cysteine residue positions are crucial for nuclear shape determination and histone retention in spermatozoa.

Chromatin’s physical properties shape the nucleus and its functions

Chromatin and Cytoskeletal Tethering Determine Nuclear Morphology in Progerin-Expressing Cells

The shape and size of the human cell nucleus is highly variable among cell types and tissues. Changes in nuclear morphology are associated with disease, including cancer, as well as with premature and normal aging. Despite the very fundamental nature of nuclear morphology, the cellular factors that determine nuclear shape and size are not well understood. To identify regulators of nuclear architecture in a systematic and unbiased fashion, we performed a high-throughput imaging-based siRNA screen targeting 867 nuclear proteins including chromatin-associated proteins, epigenetic regulators, and nuclear envelope components. Using multiple morphometric parameters, and eliminating cell cycle effectors, we identified a set of novel determinants of nuclear size and shape. Interestingly, most identified factors altered nuclear morphology without affecting the levels of lamin proteins, which are known prominent regulators of nuclear shape. In contrast, a major group of nuclear shape regulators were modifiers of repressive heterochromatin. Biochemical and molecular analysis uncovered a direct physical interaction of histone H3 with lamin A mediated via combinatorial histone modifications. Furthermore, disease-causing lamin A mutations that result in disruption of nuclear shape inhibited lamin A-histone H3 interactions. Oncogenic histone H3.3 mutants defective for H3K27 methylation resulted in nuclear morphology abnormalities. Altogether, our results represent a systematic exploration of cellular factors involved in determining nuclear morphology and they identify the interaction of lamin A with histone H3 as an important contributor to nuclear morphology in human cells.

Nuclear area and shape of epithelial cells were measured in cytological specimens from 10 patients with cystic hyperplasia of the endometrium and four patients with adenomatous hyperplasia. Only specimens from patients with histologically confirmed widespread disease of the endometrium were accepted in the study. The mean nuclear area in cystic hyperplasia was significantly lower than in adenomatous hyperplasia. Results from previous measurements with the same method in normal and malignant conditions were compared with those from hyperplastic conditions. Both cystic and adenomatous hyperplasia differed from normal endometrium but not from malignant conditions. The scatter in values in the different conditions overlapped to such a degree as to make nuclear size of little importance as a diagnostic criterion. There were no differences in nuclear shape between normal, hyperplastic, and malignant conditions.

In the absence of active volume regulation processes, cell volume is inversely proportional to osmolarity, as predicted by the Boyle Van't Hoff relation. In this study, we tested the hypothesis that nuclear volume has a similar relationship with extracellular osmolarity in articular chondrocytes, cells that are exposed to changes in the osmotic environment in vivo. Furthermore, we explored the mechanism of the relationships between osmolarity and nuclear size and shape. Nuclear size was quantified using two independent techniques, confocal laser scanning microscopy and angle-resolved low coherence interferometry. Nuclear volume was osmotically sensitive but this relationship was not linear, showing a decline in the osmotic sensitivity in the hypo-osmotic range. Nuclear shape was also influenced by extracellular osmolarity, becoming smoother as the osmolarity decreased. The osmotically induced changes in nuclear size paralleled the changes in nuclear shape, suggesting that shape and volume are interdependent. The osmotic sensitivity of shape and volume persisted after disruption of the actin cytoskeleton. Isolated nuclei contracted in response to physiologic changes in macromolecule concentration but not in response to physiologic changes in ion concentration, suggesting solute size has an important influence on the osmotic pressurization of the nucleus. This finding in turn implies that the diffusion barrier that causes osmotic effects is not a semi-permeable membrane, but rather due to size constraints that prevent large solute molecules from entering small spaces in the nucleus. As nuclear morphology has been associated previously with cell phenotype, these findings may provide new insight into the role of mechanical and osmotic signals in regulating cell physiology.

Most atomic nuclei exhibit ellipsoidal shapes characterized by quadrupole deformationβ2and triaxialityγ, and sometimes even a pear-like octupole deformationβ3. The STAR experiment introduced a new 'imaging-by-smashing' technique ((STAR Collaboration) 2024Nature63567; Jia 2025Rep. Prog. Phys.88092301) to image the nuclear global shape by colliding nuclei at ultra-relativistic speeds and analyzing outgoing debris. Features of nuclear shape manifest in collective observables like anisotropic flowvnand radial flow via mean transverse momentum[pT]. We present new measurements of the variances ofvn(n = 2, 3, and 4) and[pT], and the covariance ofvn2with[pT], in collisions of highly deformed238U and nearly spherical197Au. Ratios of these observables between the two systems effectively suppress common final-state effects, isolating the strong impact of uranium's deformation. By comparing results with state-of-the-art hydrodynamic model calculations, we extractβ2UandγUvalues consistent with those deduced from low-energy nuclear structure measurements. Measurements ofv3and its correlation with[pT]also provide the first experimental suggestion of a possible octupole deformation for238U. These findings provide significant support for using high-energy collisions to explore nuclear shapes on femtosecond timescales, with implications for both nuclear structure and quark-gluon plasma studies.

Nuclear shapes in the inner crust of a neutron star