Microrheology of Breast Cancer Cells
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Erin Baker |
According to the American Cancer Society, breast cancer is the most common form of cancer diagnosed among women in the United States: U.S. women have a staggering 1 in 8 probability of developing breast cancer at some time during their lives [1]. Furthermore, breast cancer constitutes the second most fatal form of cancer for women in the U.S [2]. While breast cancer initiates from a single cell and proliferates at abnormally high rates, it is the cancer's metastatic potential (ability to invade distant sites within the body) that renders it so life-threatening. Given that manifestation of this process is a key predictor of survival rate [1], understanding the factors that influence breast cancer metastasis is particularly important, as they will critically impact the future design and development of appropriate drugs and therapies.
It is known that tumors are stiffer than normal tissue, that cell migration ability is affected by the stiffness of the extracellular matrix (ECM) [3], and that specific cellular chemical cues are associated with metastatic breast cancer. Yet, the relationships among intracellular mechanical properties, ECM stiffness, and metastatic propensity in breast cancer are not understood; quantitative explanation of these phenomena is even more incomplete. In conjunction with the Laboratory for Cellular and Molecular Dynamics, our research aims to determine these relationships by employing a biophysical technique termed multiple particle tracking microrheology (MPTMR) and the numerical method of boundary element modeling (BEM).
MPTMR consists of embedding fluorescent, inert polystyrene spherical probes (beads) of size 100nm to 1.0μm into the cytoplasm of individual living cells (Fig. 1). The cells are then imaged using high resolution confocal microscopy, which generates a time series of 100-1500 images over a period of 10 to 60 seconds. Using image analysis software, position trajectories of the thermally driven bead random walks are created, which can then be used to compute the mean squared displacement (MSD) over all beads as a function of time.
Figure 1: Human prostate cancer cell (PC-3 cell line) embedded with 1μm fluorescent polystyrene beads, along with the random walk of one individual bead.
The MSD is then used to directly compute the cytoplasmic compliance (Γ) as a function of time and transformed to compute the cytoplasmic complex shear modulus (G*) as a function of bead frequency,
from which both the viscous modulus (G") and elastic modulus (G') are directly extracted [4] (Fig. 2).
Figure 2: Elastic (G') and viscous (G") moduli of human prostate cancer cells (PC-3 cell line) obtained from MPTMR
The entire MPTMR process is summarized in Fig. 3.
Figure 3: Multiple particle tracking microrheology (MPTMR) consists of (1) culturing cells, (2) embedding fluorescent beads within the cytoplasm, (3) redepositing cells in desired matrix, (4) imaging bead Brownian motions, (5) creating bead position trajectories, and (6) computing cytoplasmic viscoelastic properties
While MPTMR protocols have been established [5], these techniques have yet to be applied in linking the cytoplasmic mechanical environment of cancer cells to their metastatic propensity. Using MPTMR to characterize the mechanical properties of breast cancer cells of varying metastatic chemical cues within mechanically distinct ECMs will provide critical insight a biomedical problem that has wide-reaching impact.
Experimental results will be complimented with the numerical technique of BEM, which will provide novel computational predictions of cancer cell migration within the ECM. By discretizing cell and ECM fiber surfaces, standard BEM methods can be utilized to model force and velocity distributions across these surfaces to predict a cell's change in shape as it migrates through an ECM. Depending on the forces applied to the cell and the ECM fiber stiffness, it will either migrate through or become trapped within an ECM pore of given size (Fig. 4).
Figure 4: Model of cell migration using BEM. BEM simulates force and velocity fields across the cell and ECM surfaces as the cell attempts to migrate toward and through an ECM pore.
Experimental results and computational predictions will be merged to yield a novel, quantitative system that maps ECM stiffness and metastatic propensity of breast cancer cells to their cytoplasmic viscoelastic properties.
References
- Breast Cancer Facts and Figures 2007-2008, 2007, American Cancer Society
- Cancer Facts and Figures 2007, 2007, American Cancer Society
- Zaman, M.H., L.M. Trapani, A.L. Sieminski, D. Mackellar, H. Gong, R.D. Kamm, A. Wells, D.A. Lauffenburger, and P. Matsudaira, Migration of tumor cells in 3D matrices is governed by matrix stiffness along with cell-matrix adhesion and proteolysis. Proc Natl Acad Sci U S A, 2006. 103(29): p. 10889-94.
- Gardel, M.L., M.T. Valentine, and D. A. Weitz, Microrheology, In: Microscale Diagnostic Techniques K. Breuer (Ed.) Springer Verlag, 2005.
- Weihs, D., T.G. Mason, and M.A. Teitell, Bio-microrheology: A frontier in microrheology. Biophys J, 2006. 91(11): p. 4296-4305.