Session Y41: Focus Session: Physics of Cancer III -- Imaging

Optimality in the Development of Intestinal Crypts

Intestinal crypts in mammals are comprised of long-lived stem cells and shorter-lived progenies, maintained under tight proportions during adult life. Here we ask what are the design principles that govern the dynamics of these proportions during crypt morphogenesis. We use optimal control theory to show that a stem cell proliferation strategy known as a `bang-bang' control minimizes the time to obtain a mature crypt. This strategy consists of a surge of symmetric stem cell divisions, establishing the entire stem cell pool first, followed by a sharp transition to strictly asymmetric stem cell divisions, producing non-stem cells with a delay. We validate these predictions using lineage tracing and single molecule fluorescent in-situ hybridization of intestinal crypts in newborn mice and find that small crypts are entirely composed of Lgr5 stem cells, which become a minority as crypts further grow. Our approach can be used to uncover similar design principles in other developmental systems.   

Molecular Imaging System for Monitoring Tumor Angiogenesis

In cancer, non-invasive imaging techniques that monitor molecular processes~associated with the tumor angiogenesis could have a central role in the evaluation of novel antiangiogenic and proangiogenic therapies as well as early detection of the disease.~Matrix metalloproteinases (MMP) can serve as specific biological targets for imaging of angiogenesis since expression of MMPs is required for angiogenesis and has been found to be upregulated in every type of human cancer and correlates with stage, invasive, metastatic properties and poor prognosis. However, for most cancers it is still unknown when, where and how MMPs are involved in the tumor angiogenesis [1].~Development of high-resolution, high sensitivity imaging techniques in parallel with the ~tumor models could prove invaluable for assessing the physical location and the time frame of MMP enzymatic acitivity. The goal of this study is to understand where, when and how MMPs are involved in the tumor angiogenesis.~ We will accomplish this goal by following two objectives: to develop a high sensitivity, high resolution molecular imaging system, to develop a virtual tumor simulator that can predict the physical location and the time frame of the MMP activity. In order to achieve our objectives, we will first develop a PAM system and develop a mathematical tumor model in which the quantitative data obtained from the PAM can be integrated.~~So, this work will develop a virtual tumor simulator and a molecular imaging system for monitoring tumor angiogenesis.~1.Kessenbrock, K., V. Plaks, and Z. Werb,~MMP:regulators of the tumor microenvironment.~Cell, 2010.~\textbf{141}(1)   

Characterizing Spatial Organization of Cell Surface Receptors in Human Breast Cancer with STORM

Regulation and control of complex biological functions are dependent upon spatial organization of biological structures at many different length scales. For instance Eph receptors and their ephrin ligands bind when opposing cells come into contact during development, resulting in spatial organizational changes on the nanometer scale that lead to changes on the macro scale, in a process known as organ morphogenesis. One technique able to probe this important spatial organization at both the nanometer and micrometer length scales, including at cell-cell junctions, is stochastic optical reconstruction microscopy (STORM). STORM is a technique that localizes individual fluorophores based on the centroids of their point spread functions and then reconstructs a composite image to produce super resolved structure. We have applied STORM to study spatial organization of the cell surface of human breast cancer cells, specifically the organization of tyrosine kinase receptors and chemokine receptors. A better characterization of spatial organization of breast cancer cell surface proteins is necessary to fully understand the tumorigenisis pathways in the most common malignancy in United States women.   

In Vivo Fluorescence Resonance Energy Transfer Imaging for Targeted Anti-Cancer Drug Delivery Kinetics

We describe an approach for the evaluation of targeted anti-cancer drug delivery {\em in vivo}. The method emulates the drug release and activation process through acceptor release from a targeted donor-acceptor pair that exhibits fluorescence resonance energy transfer (FRET). In this case, folate targeting of the cancer cells is used - 40~\% of all human cancers, including ovarian, lung, breast, kidney, brain and colon cancer, over-express folate receptors. We demonstrate the reconstruction of the spatially-dependent FRET parameters in a mouse model and in tissue phantoms. The FRET parameterization is incorporated into a source for a diffusion equation model for photon transport in tissue, in a variant of optical diffusion tomography (ODT) called FRET-ODT. In addition to the spatially-dependent tissue parameters in the diffusion model (absorption and diffusion coefficients), the FRET parameters (donor-acceptor distance and yield) are imaged as a function of position. Modulated light measurements are made with various laser excitation positions and a gated camera. More generally, our method provides a new vehicle for studying disease at the molecular level by imaging FRET parameters in deep tissue, and allows the nanometer FRET ruler to be utilized in deep tissue.   

Exploring X-Ray Phase-Contrast Imaging using Photon Counting Detectors for Early Breast Cancer Detection

X-ray attenuation-contrast (AC) imaging in the form of digital mammography (DM) is the current gold standard of screening for deep-set cancers like breast cancer. DM creates images based on the attenuation differences between normal and malignant breast tissue, but the extremely low attenuation contrast poses severe challenges for early cancer detection. In order to overcome these limitations posed by AC imaging, there is a growing interest in exploring phase changes in x-rays as they propagate through the tissue. Theoretical estimations show that the x-ray phase difference between normal and malignant breast tissue is three orders of magnitude higher than the corresponding absorption contrast. While x-ray attenuation is determined by the atomic number of the elements forming the tissue, the phase change is determined by the density or refractive index. Due to high energy of x-rays, absolute measurement of phase change is challenging. We will present our efforts to understand x-ray phase contrast in biological tissue using a photon counting detector (TIMEPIX), which is capable of energy and time resolved measurements with very high spatial resolution (about 50 microns). We are exploring novel methods, which will also be clinically feasible to extract phase information using a combination of PCDs and phase retrieval techniques. Phase changes and contrast details of various breast cancer types will also be investigated using the energy resolved measurements obtained using PCDs.   

Uncovering cancer cell behavioral phenotype in 3-D \textit{in vitro} metastatic landscapes

One well-known fact is that cancer cell genetics determines cell metastatic potentials. However, from a physics point of view, genetics as cell properties cannot directly act on metastasis. An agent is needed to unscramble the genetics first before generating dynamics for metastasis. Exactly this agent is cell behavioral phenotype, which is rarely studied due to the difficulties of real-time cell tracking in \textit{in vivo} tissue. Here we have successfully constructed a micro \textit{in vitro} environment with collagen based Extracellular Matrix (ECM) structures for cell 3-D metastasis. With stable nutrition (glucose) gradient inside, breast cancer cell MDA-MB-231 is able to invade inside the collagen from the nutrition poor site towards the nutrition rich site. Continuous confocal microscopy captures images of the cells every 12 hours and tracks their positions in 3-D space. The micro fluorescent beads pre-mixed inside the ECM demonstrate that invasive cells have altered the structures through mechanics. With the observation and the analysis of cell collective behaviors, we argue that game theory may exist between the pioneering cells and their followers in the metastatic cell group. The cell collaboration may explain the high efficiency of metastasis.