Biology subjects

Dickinson, R. B.

Publications and source records attributed to Dickinson, R. B..

3 recordsLinked to original sources

bioRxiv2022

A predictive unifying explanation for nuclear shapes based on a simple geometric principle

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). Here we establish a simple geometric principle of nuclear shaping: the excess surface area of the nucleus (relative to that of a sphere of the same volume) permits a wide range highly deformed nuclear shapes under the constraints of constant surface area and constant volume, and, 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. We demonstrate this principle by predicting limiting nuclear shapes (i.e. with smooth lamina) in various cell 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. We also show that the lamina surface tension 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 lamina surface area is the key determinant of nuclear shapes, and that nuclear shapes can be determined purely by the geometric constraints of constant (but excess) nuclear surface area and nuclear volume, not by the magnitude of the cytoskeletal forces involved.

biophysics↗

bioRxiv2022

Loss of mechanical homeostasis drives epithelial acinar eversion

Epithelial cells lining a gland and cells grown in a soft extracellular matrix polarize with apical proteins exposed to the lumen and basal proteins in contact with the extracellular matrix. Alterations to polarity, including an apical-out polarity, occur in human cancers. While some aberrant polarity states may result from altered protein trafficking, recent observations of an extraordinary tissue-level inside-out unfolding suggest an alternate pathway for altered polarity. Because mechanical alterations are common in human cancer including an upregulation of RhoA mediated actomyosin tension in acinar epithelia, we explored whether perturbing mechanical homeostasis could cause apical-out eversion. Acinar eversion was robustly induced by direct activation of RhoA in normal and tumor epithelial acini, or indirect activation of RhoA through blockage of {beta}1 integrins, disruption of the LINC complex, oncogenic Ras activation, or Rac1 inhibition. Furthermore, laser ablation of a portion of the untreated acinus was sufficient to induce eversion. Analyses of acini revealed high curvature and low phosphorylated myosin in the apical cell surfaces relative to the basal surfaces. A vertex-based mathematical model which balances tension at cell-cell interfaces revealed a five-fold greater basal cell surface tension relative to the apical cell surface tension. The model suggests that the difference in surface energy between the apical and basal surfaces is the driving force for acinar eversion. Our findings raise the possibility that a loss of mechanical homeostasis may cause apical-out polarity states in human cancers.

biophysics↗

bioRxiv2022

Lamin A/C mediated invaginations in the nuclear surface allow the nucleus to pass unimpeded through a dense array of fiber-like obstacles

Migrating cells must deform their stiff cell nucleus to move through pores and fibers in tissue. Lamin A/C is known to hinder cell migration by limiting nuclear deformation and passage through confining channels, but its role in nuclear deformation and passage through fibrous environments is less clear. We studied cell and nuclear migration through discrete, closely spaced, slender obstacles which mimic the mechanical properties of collagen fibers. Nuclei bypassed slender obstacles while preserving their overall morphology by deforming around them with deep local invaginations of little resisting force. The obstacles did not impede the nuclear trajectory or cause a rupture of the nuclear envelope. Nuclei likewise deformed around single collagen fibers in cells migrating in 3D collagen gels. In contrast to its limiting role in nuclear passage through confining channels, lamin A/C facilitated nuclear deformation and passage through fibrous environments; nuclei in lamin-null (Lmna-/-) cells lost their overall morphology and became entangled on the obstacles. Analogous to surface tension-mediated deformation of a liquid drop, lamin A/C imparts a surface tension on the nucleus that allows nuclear invaginations with little mechanical resistance, preventing nuclear entanglement and allowing nuclear passage through fibrous environments.

cell biology↗