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First published online 25 August 2004
doi: 10.1242/jcs.01357

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The nuclear envelope lamina network has elasticity and a compressibility limit suggestive of a molecular shock absorber

¹ Department of Chemical and Biomolecular Engineering, 220 South 33rd Street, University of Pennsylvania, Philadelphia, PA 19104-6393, USA
² Department of Cell Biology, Johns Hopkins University, 3400 North Wolfe Street, Baltimore, MD 21205-2105, USA

View larger version (94K): [in a new window]   Fig. 1. Nuclear membranes and lamina detach from bulk nucleoplasm during swelling. (A) Two unswollen nuclei were imaged at 4x magnification by bright-field microscopy and stained with YO-PRO to visualize DNA/RNA in the nucleoplasm. The nucleoplasm in the unswollen nuclei completely fills the nucleus. Fluorescent antibodies against B-type lamins show the peripheral lamina of a different nucleus. (B) A single swollen nucleus was imaged by bright-field and fluorescence microscopy to reveal DNA/RNA in the nucleoplasm with YO-PRO (green) and lamins (red). The nuclear envelope, with attached lamina, detaches from the constant-size nucleoplasm during swelling. (C) A swollen nucleus imaged with bright-field optics (left) and with fluorescent phalloidin to visualize F-actin (right). Extensive actin polymerization as seen here is an artifact of nuclear isolation. Scale bars: 250 µm.

View larger version (89K): [in a new window]   Fig. 2. Progressive aspiration of a swollen Xenopus oocyte nucleus. The Xenopus oocyte nucleus was labeled with the membrane dye FM 4-64 and progressively aspirated into a micropipette. Each image (1-9) was taken after rapid equilibration with a set imposed pressure. Note the membrane wrinkles near the pipette entrance (white arrow) consistent with shear resistance of a solid-like network. The distance between the bracketed fluorescence spots increases, thus showing local envelope stretching of nearly 300%. Scale bar: 250 µm.

View larger version (48K): [in a new window]   Fig. 3. Micropipette aspiration of swollen and unswollen nuclei. (A) Bright-field and DNA/RNA (labeled with YO-PRO) fluorescence images of either unswollen or swollen nuclei aspirated into a micropipette (inner radii Rp=98 µm and 139 µm, respectively). The unswollen nucleus shows deformation of the nucleoplasm coincident with the nuclear envelope. The nucleoplasm of the swollen nucleus remains external to the micropipette. (B) Representative data of Laplace tension, a function of incremental aspiration pressure, versus aspirated length of unswollen () and of swollen () nuclei yield similar slopes (see Table 1). Aspiration is reversible since decreasing the aspiration pressure () follows the same trend as increased pressure (,).

View larger version (22K): [in a new window]   Fig. 4. Reversibility and kinetics of nuclear swelling. (A) After swelling finished in a time (teq), the relative surface area expansion () could be reversed with addition of dextrans (50 mg/ml). Swollen nuclei are also significantly deflated by addition of 5% Triton X-100 detergent () as water and solutes flow through the perforated nucleus. The nuclei remain visibly intact, nonetheless. (B) Swelling (of four nuclei) shows an initial linear increase in with equilibration at teq=500 seconds. (C) Swelling was systematically reduced by addition of either 500 kDa dextran () or 500 kDa dextran sulfate (), but concentrations of dextrans >50 mg/ml do not compress nuclei relative to their original size.

View larger version (11K): [in a new window]   Fig. 5. Estimated nuclear envelope tension versus surface area expansion . The tensions are calculated from kinetic swelling data and suggest that swollen nuclear envelopes are under increasing tension. The slope gives an approximate membrane dilation modulus, K.

View larger version (87K): [in a new window]   Fig. 6. Simulations of lamin rearrangement during nuclear swelling. The native nuclear lamina is in a `compressed' state that expands to the flat, orthogonal network seen in electron micrographs of Xenopus oocyte lamina (Aebi et al., 1986). The compressed native state may contain domains of differing lamin interactions. Simulations are extended from those of Tessier et al. (Tessier et al., 2003).

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