Mechanical properties of the cell nucleus and the effect
of emerin deficiency
Amy C Rowat 1*, Jan Lammerding 2 and John H Ipsen 3
1 Harvard University
2 Harvard University Medical School
3 University of Southern Denmark
* To whom correspondence should be addressed. E-mail: rowat{at}deas.harvard.edu.
Submitted on April 3, 2006
Revised on May 10, 2006
Accepted on 5 September 2006
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Abstract |
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Nuclear structure and mechanics are gaining recognition as important factors that affect gene expression, development, and differentiation in normal function and disease, yet the physical mechanisms that govern nuclear mechanical stability remain unclear. Here we examined the physical properties of the cell nucleus by imaging fluorescentally labeled components of the inner nucleus (chromatin and nucleoli) and the nuclear envelope (lamins and membranes) in nuclei deformed by micropipette aspiration (confocal imaged microdeformation). We investigated nuclei, both isolated and in intact, living cells, and found that nuclear volume significantly decreased by 60-70% during aspiration. While nuclear membranes exhibited blebbing and fluid characteristics during aspiration, the nuclear lamina exhibited behaviour of a solid-elastic shell. Under large deformations of GFP-lamin A-labeled nuclei, we observed a decay of fluorescence intensity into the tip of the deformed tongue that we interpreted in terms of non-linear, two-dimensional elasticity theory [Rowat et al (2005) J. Roy. Soc. Interface 2:63-9]. Here we applied this method to study nuclear envelope stability in disease and found that mouse embryo fibroblasts lacking the inner nuclear membrane protein, emerin, had a significantly decreased ratio of the area expansion to shear moduli (K/µ) compared to wildtype cells (2.1±0.2 versus 5.1±1.3). These data suggest that altered nuclear envelope elasticity caused by loss of emerin could contribute to increased nuclear fragility in Emery-Dreifuss muscular dystrophy patients with mutations in the emerin gene. Based on our experimental results and theoretical considerations, we present a model describing how the nucleus is stabilized in the pipette. Such a model is essential for interpreting the results of any micropipette study of the nucleus and porous materials in general.
Key Words:
confocal microscopy, elasticity theory, membranes, micropipette aspiration, nuclear envelope
