Developing GHz Acoustic Methods for Imaging and Measuring the Mechanical Properties of Individual Cells

Cells migrate by attaching themselves to their surroundings and generating traction forces by rearranging their internal structure. The effectiveness of these tractions depend on the relative rigidity of the cells internal structure (its cytoskeleton) and that of the surrounding material. This can be probed by measuring the elastic properties (stiffness) of a cell and how these properties vary across the cell during migration. There is some evidence that as cells transform from the normal to the cancerous state, their stifness is reduced, and as these cells become progressively more invasive the stiffness is reduced further. Thus by developing methods to probe the mechanical properties of cells we can investigate some fundamental aspects of cell biology and cancer cell biology. In this proposal we will investigate the use of ultra-high frequency acoustic imaging to study the mechanical properties of individual cells. In an acoustic microscope specimens are imaged by acoustic waves in the frequency range 100 MHz - 1 GHz. Image contrast occurs from local differences in the speed of sound, which in turn is a function of the elastic stiffness of the material being imaged. Thus, by measuring the contrast of images of cells at a range of acoustic frequencies it is possible to determine local mechanical properties with a spatial resolution of around 1 micron. However, for this technique to be applicable to the study of the biochemical and physical processes that occur during cell migration, it is necessary to further develop the technique to allow the rapid acquisition of data through images taken every few seconds. The technique will be demonstrated through the investigation of the migration of cell lines that can be controlled by altering one of the proteins used during the migration process thus allowing us to compare cells that migrate rapidly with those that are more static. We will also work in collabnoration with Cancer Research UK who will provide examples of cell lines of known invasive capability for us to characterize their mechanical properties.

The scanning acoustic microscope (SAM) provides a method of characterising the elastic properties of materials with a spatial resolution close to 1 micron if GHz acoustic radiation is used. We have already demonstrated that this technique can be used to determine the mechanical properties of soft biological tissue, here we propose to extend its use to the characterization of the elastic stiffness of individual living cells in vitro. We will exploit a frequency contrast mechanism that we have used successfully on thin histological sections and extend it to allow both temporal and spatial resolution of cell elastic stiffness. The mechanics of living cells has attracted increasing attention and the ability to characterise the local stiffness of cells during processes such as cell migration is of considerable interest to cell biologists. There is also evidence in the literature that cell stiffness changes significantly as cells transform to cancer cells and further changes occur during metastasis and invasion. SAM is a non-contact image based technique and as such has advantages over more conventional atomic force microscopy probes of cell mechanical properties. By investigating series of images it is also possible to look at the evolution of cell properties with time allowing a study of dynamic processes such as cell migration. This will be demonstrated by characterising cell lines withh different focal adhesion properties that show a range of migration behaviour. We will also collaborate with researchers in Cancer Research UK to use the technique to characterize cell lines of known invasive characteristics.