Mechanics of the nucleus

Abstract

The nucleus is the distinguishing feature of eukaryotic cells. Until recently, it was often considered simply as a unique compartment containing the genetic information of the cell and associated machinery, without much attention to its structure and mechanical properties. This article provides compelling examples that illustrate how specific nuclear structures are associated with important cellular functions, and how defects in nuclear mechanics can cause a multitude of human diseases. During differentiation, embryonic stem cells modify their nuclear envelope composition and chromatin structure, resulting in stiffer nuclei that reflect decreased transcriptional plasticity. In contrast, neutrophils have evolved characteristic lobulated nuclei that increase their physical plasticity, enabling passage through narrow tissue spaces in their response to inflammation. Research on diverse cell types further demonstrates how induced nuclear deformations during cellular compression or stretch can modulate cellular function. Pathological examples of disturbed nuclear mechanics include the many diseases caused by mutations in the nuclear envelope proteins lamin A/C and associated proteins, as well as cancer cells that are often characterized by abnormal nuclear morphology. In this article, we will focus on determining the functional relationship between nuclear mechanics and cellular (dys-)function, describing the molecular changes associated with physiological and pathological examples, the resulting defects in nuclear mechanics, and the effects on cellular function. New insights into the close relationship between nuclear mechanics and cellular organization and function will yield a better understanding of normal biology and will offer new clues into therapeutic approaches to the various diseases associated with defective nuclear mechanics.

Figure 1

Orthoview of mouse embryo fibroblast immunofluorescently labeled for lamin B1 and imaged with an Nikon A1 confocal microscope. The micrograp shows the disk-shaped nuclear morphology in the adherent fibroblast cultured on a fibronectin coated cover slip.

Figure 2

Transmission electon micrograph of a hepatocyte nucleus showing the major structural elements of the nucleus. Adapted from NUS Histonet, WWW Electronic Guide to Histology for Medicine and Dentistry, online at http://www.med.nus.edu.sg/ant/histonet/txt/tacsem/tac01.sem.html. TEM image courtesy of Dr. P. Gopalakrishnakone.

Figure 3

Schematic overview of nuclear structure and nuclear cytoskeletal coupling. (Figure taken from Dahl KN, Ribeiro AJS, and Lammerding J, Circ Res 2008 (102): 1307–1318).

Figure 4

Comparison of A-type and B-type lamins ectopically expressed in Xenopus oocyte nuclei. (A) Scanning electon micrograph of membrane arrays covered by lamin B2 filaments. (B) Transmission electon micrograph (TEM) section of an isolated oocyte nucleus expressing lamin B2. (C) Lamin A filaments. Filaments that are arranged in layered bundles are indicated by small arrows; they surround nuclear pore baskets (large arrrows). (D, E) TEM section of oocyte expressing lamin A (D) and control (E). The lamina is hardly visible in the control nuclei (E) but forms a thick electron-dense layer in oocytes expressing lamin A (D), which leaves the nuclear pores clear (arrows). (Figure assembled from panels taken from Goldberg et al. J Cell Sci 2007 (121): 215–221).

Figure 5

Changes in fibroblast morphology in response to strain. Mouse subcutaneous tissue was stretched ex vivo (A, B) or maintained without stretch (D, E), subsequently fixed and stained with phalloidin (red) and SYTOX (green), and imaged by confocal microscopy. Panels (A) and (D) are composite projections of image stacks containing 20 optical sections taken at 1-μm intervals. Panels (B) and (E) are projections of relevant optical sections containing the cells indicated by arrowheads in (A) and (D), respectively. Scale bars, 40 μm. (C) and (F): Outlines of the cell bodies in (B) and (E). (Figure taken from Langevin at al. Am J Physiol Cell Physiol (2005). 288: C747–C756).

Figure 6

Transmission electron micrographs of condrocytes embedded in agarose constructs held unstrained (A–C) or subjected to 20% compression (D–F). All micrographs were taken at the same magnification (Scale bar=2 μm). The direction of the applied strain is indicated by the horizontal arrow in (D) and (E) and a crossed circle in (F). (Figure taken from Lee at al. J Biomechanics. 2000. 33: 81–85).

Figure 7

Overview of experimental techniques to probe nuclear mechanics. (A) Micropipette aspiration on isolated nuclei (top) or in intact cells after cytoskeletal disruption (bottom). (B) Atomic force microscopy. (C) Substrate strain. (D) Active microrheology. (E). Passive microrheology.

Figure 8

Distinct differences in the deformation of the elastic laminar and the viscoelastic nuclear interior during micropipette aspiration. (A) Tracking a bleached reagion within GFP-H2B labeled chromatin reveals chromatin flow within the pipette. (B) Deformation of the GFP-lamin A labeled lamina stretched into the micropipette, showing elastic stretch of the lamina (C) Nucleoli slowly follow chromatin toward the nuclear tip. Scale bar for (A–C): 3 μm. (D, E). Fibroblast with nuclear lamina labeled by GFP-lamin A before (D) and during (E) micropipette aspiration. The fluorescence gradient suggests thinning of the elastic nuclear lamina towards the tip. (Panels (A–C) taken from Pajerowski et al. Proc Nat Acad Sci 2007. 104: 15619–15624 Panels (D, E) taken from Rowat, Lammerding and Ipsen Biophys J 2006. 91: 4649–4664)

Figure 9

Defects in nuclear morphology in neutrophils from blood smears of individuals with Pelger-Huet anomaly (PHA) labeled with Wright-Giemsa stain. (A) Normal neutrophil with lobulated nucleus. (B) Heterozygous PHA neutrophil with a bilobed nucleus, taken from the mother of the homozygous individual. (C) Heterozygous PHA neutrophil with a bilobed nucleus, taken from the father of the homozygote. (D) Homozygous PHA neutrophil showing an ovoid nucleus with chromatin clumping. Scale bar for light micrographs: 10 μm. (E–G) Transmission electron micrographs of normal human neutrophil nucleus with three apparent lobes and extensive peripheral heterochromatin (E), heterozygous PHA neutrophil with a bilobed nucleus taken from the father of the homozygote (F), and ovoid nucleus from a homozygous PHA granulocyte exhibiting extensive heterochromatin redistribution (G). Scale bar for electron micrographs: 1 μm (Figure taken from Hoffmann et al. Chromosoma. 2007. 116: 227–235)

Figure 10

Abnormasl nuclear mechanics in lamin A/C-deficient fibroblasts. (A) Nucleus of wild-type (Lmna+/+) fibroblast before strain (red) and during application of 22% substrate strain (yellow), revealing only minimal nuclear deformation. (B) Lamin A/C-deficient (Lmna−/−) nucleus before strain (red) and during 19% substrate strain (yellow). Scale bars: 10 μm. (C) Nuclear strain as a function increases linearly with applied membrane strain, but is significantly larger in Lmna−/− fibroblasts. Dashed lines represent linear regression of the data for each cell type. (D) Maximal normalized nuclear strain (i.e., nuclear strain devided by the applied membrane strain) is significantly increased in Lmna−/− fibroblasts, indicating decreased nuclear stiffness. (Figure taken from Lammerding et al. J Clin Invest. 2004. 113: 370–378.)