Biophysical Assays to Probe the Mechanical Properties of the Interphase Cell Nucleus: Substrate Strain Application and Microneedle Manipulation
Summary
We present two independent, microscope-based tools to measure the induced nuclear and cytoskeletal deformations in single, living adherent cells in response to global or localized strain application. These techniques are used to determine nuclear stiffness (i.e., deformability) and to probe intracellular force transmission between the nucleus and the cytoskeleton.
Abstract
In most eukaryotic cells, the nucleus is the largest organelle and is typically 2 to 10 times stiffer than the surrounding cytoskeleton; consequently, the physical properties of the nucleus contribute significantly to the overall biomechanical behavior of cells under physiological and pathological conditions. For example, in migrating neutrophils and invading cancer cells, nuclear stiffness can pose a major obstacle during extravasation or passage through narrow spaces within tissues.1 On the other hand, the nucleus of cells in mechanically active tissue such as muscle requires sufficient structural support to withstand repetitive mechanical stress. Importantly, the nucleus is tightly integrated into the cellular architecture; it is physically connected to the surrounding cytoskeleton, which is a critical requirement for the intracellular movement and positioning of the nucleus, for example, in polarized cells, synaptic nuclei at neuromuscular junctions, or in migrating cells.2 Not surprisingly, mutations in nuclear envelope proteins such as lamins and nesprins, which play a critical role in determining nuclear stiffness and nucleo-cytoskeletal coupling, have been shown recently to result in a number of human diseases, including Emery-Dreifuss muscular dystrophy, limb-girdle muscular dystrophy, and dilated cardiomyopathy.3 To investigate the biophysical function of diverse nuclear envelope proteins and the effect of specific mutations, we have developed experimental methods to study the physical properties of the nucleus in single, living cells subjected to global or localized mechanical perturbation. Measuring induced nuclear deformations in response to precisely applied substrate strain application yields important information on the deformability of the nucleus and allows quantitative comparison between different mutations or cell lines deficient for specific nuclear envelope proteins. Localized cytoskeletal strain application with a microneedle is used to complement this assay and can yield additional information on intracellular force transmission between the nucleus and the cytoskeleton. Studying nuclear mechanics in intact living cells preserves the normal intracellular architecture and avoids potential artifacts that can arise when working with isolated nuclei. Furthermore, substrate strain application presents a good model for the physiological stress experienced by cells in muscle or other tissues (e.g., vascular smooth muscle cells exposed to vessel strain). Lastly, while these tools have been developed primarily to study nuclear mechanics, they can also be applied to investigate the function of cytoskeletal proteins and mechanotransduction signaling.
Protocol
Discussion
Substrate strain assay
Strain application has been successfully used by us and other groups to study induced nuclear deformations in cells subjected to mechanical stress and to investigate the contribution of specific nuclear envelope proteins to nuclear stiffness.4-8 The advantage of this technique is that it probes mechanical properties of living nuclei in their normal cellular and cytoskeletal environment and that the substrate strain application resembles physiological load applica…
Disclosures
The authors have nothing to disclose.
Acknowledgements
This work was supported by the National Institutes of Health (R01 HL082792 and R01 NS059348) and the Brigham and Women’s Hospital Cardiovascular Leadership Group Award.
Materials
| Name of Reagent | Company | Catalogue number |
| Fibronectin | Millipore | FC010 |
| MitoTracker Red FM and Green FM | Invitrogen | M22425 and M-7514 |
| Hoechst 33342 | Invitrogen | H3570 |
| Hank’s Buffered Salt Saline | Invitrogen | 14185 |
| Phenol free, DMEM | Invitrogen | 21063 |
| Fetal bovine serum | Aleken Biologicals | FBSS500 |
| Penicillin/Streptomycin | Sigma | P0781-100ML |
| Borosilicate Glass with filament | Sutter Instrument | BF100-78-10 |
| Gloss/Gloss non-reinforced silicone sheeting, 0.005″ | Specialty Manufacturing Inc. | |
| Dulbecco’s Phosphate Buffered Saline | Invitrogen | 14200 |
| 35 mm glass bottom culture dishes (FluoroDish) | World Precision Instruments, INC | FD35-100 |
| Braycote 804 Vacuum Grease | Spi supplies | 05133A-AB |
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