Session Y6: Physics of Development and Disease II

Substrate Curvature Regulates Cell Migration -A Computational Study

Cell migration in host microenvironment is essential to cancer etiology, progression and metastasis. Cellular processes of adhesion, cytoskeletal polymerization, contraction, and matrix remodeling act in concert to regulate cell migration, while local extracellular matrix architecture modulate these processes. In this work we study how stromal microenvironment with native and cell-derived curvature at micron-meter scale regulate cell motility pattern. We developed a 3D model of single cell migration on a curved substrate. Mathematical~analysis of cell morphological adaption to the cell-substrate interface shows that cell migration on convex surfaces~deforms more than on concave surfaces.~ Both analytical and simulation results show~that curved surfaces regulate the cell motile force for cell's protruding front through force balance with focal adhesion and cell contraction. We also found that cell migration on concave substrates~is more~persistent. These results offer a novel biomechanical explanation to substrate curvature regulation of cell migration.   

Cell Motility and Jamming across the EMT

We use single-cell tracking and cell shape analysis to highlight the different roles that cell jamming plays in the behaviour of epithelial vs. mesenchymal mammary breast cell lines (MCF-10A, MDA-MB-231) in 2D adherent culture. An automatic segmentation allows for the evaluation of cell shapes, which we compare to predictions made by the self-propelled vertex (SPV) model [Bi et al, Nat. Phys. 2015]. On top of that, we employ co-cultures to study the emerging demixing behaviour of these cell lines, demonstrating that the mesenchymal MDA-MB-231 cell line forms unjammed islands within the jammed collective.   

Jamming and liquidity in 3D cancer cell aggregates

Traditionally, tissues are treated as simple liquids, which holds for example for embryonic tissue. However, recent experiments have shown that this picture is insufficient for other tissue types, suggesting possible transitions to solid-like behavior induced by cellular jamming. The coarse-grained self-propelled Voronoi (SPV) model predicts such a transition depending on cell shape which is thought to arise from an interplay of cell-cell adhesion and cortical tension. We observe non-liquid behavior in 3D breast cancer spheroids of varying metastatic potential and correlate single cell shapes, single cell dynamics and collective dynamic behavior of fusion and segregation experiments via the SPV model.   

Dynamics of cardiac laminar sheets under shear stress

Cardiac tissue is continuously subjected to mechanical stress in vivo. Cells are both stretched and exposed to shear forces due to the relative movement of myocardial sheets against each other and due to the movement of interstitial fluid between individual cells and cell layers. The effect of shear forces, especially under pathological conditions and at different time scales, is not well understood so far. We apply different degrees of flow-induced shear stress to cardiac monolayers to simulate cardiomyopathy and to cardiomyocytes-fibroblast co-cultures as a model for fibrosis. By combining electric cell-substrate impedance sensing and optical microscopy, we analyze the short- and long-term effect of shear forces on cell-cell connectivity, cell morphology and functionality. At the onset of shear, we observe a decrease in the monolayer-substrate distance and a rapid increase in complex impedance of the cell layer. Moreover, this response is faster when higher shear stresses are applied. We quantify the increment of cell contact area, cell elongation and reorientation along the flow direction. Further, the impedance measurements show an increase in collective beating frequency and cell-cell connectivity with higher shear stresses.   

Effects of Mechanical Coupling Between Cardiomyocytes and Cardiac Fibroblasts on Myocardium

Cardiomyocytes show excitatory responses to stimulation solely by mechanical forces through their stretch-activated ion channels, and can fire action potentials upon mechanical stimulation through a pathway known as mechano-electric feedback. Furthermore, cardiomyocyte (CM) -- cardiac fibroblasts (CF) can couple mechanically through cell-cell junctions. Here we investigated the effects of CM and CF mechanical coupling on myocardial physiology and pathology using a bio-nanoindentered coupled with fast calcium imaging and microelectrode arrays. In order to study mechanical signal transmission, we measured the contractile forces generated by CMs, as well as by CFs that were coupled to the CMs. We observed that CFs were beating with the same frequency but at smaller magnitude compared to CMs, and their contractility was dependent on the substrate stiffness. Our results showed that beating CMs actively stretched neighbouring CFs through the deformation of the substrate the cells were seeded on, which promoted the myocardial contractility through mechanical coupling. The results also revealed that CM contractility was propagated greater on soft substrates than stiff ones. Results of this study could help identify the role of the infarcted tissue stiffness and size on heart failure.   

Nuclear stiffness and chromatin condensation as markers for aggressive prostate cancer

Previous studies have demonstrated that nuclear rheology parameters - stiffness and fluidity - depend on expression levels of nuclear membrane proteins lamin A/C and lamin B. No quantitative nuclear rheology has been studied to compare cancers of different aggressiveness. Our nuclear creep experiments using a microfluidic channel with a narrow constriction show that aggressive prostate cancer cell nuclei have a lower stiffness than benign cell nuclei; thus nuclear stiffness can be easily used as a cancer malignancy marker. We also find that nuclear stiffness and fluidity, contrary to prior claims, do not strongly depend on lamin A/C or B expression levels of prostate cancer cells. Rather, we find that nuclear stiffness depends on chromatin condensation.   

Hypoxia alters the physical properties of the tumor microenvironment.

Of all the deaths attributed to cancer, 90{\%} are due to metastasis, or the spread of cancer cells from a primary tumor to distant organs, and treatments that prevent or cure metastasis remain elusive. Emerging data indicate that low oxygen states within a tumor, termed hypoxia, can alter the chemical and physical parameters of the extracellular matrix (ECM), or scaffold of the tumor tissue. These changes generate a microenvironment that may be more conducive for promoting metastasis. During tumor evolution, changes in the composition and the overall content of the ECM reflect both its biophysical and biological properties and these strongly influence the cells properties, such as cellular proliferation and cell motility. The talk will cover how hypoxia arises within normal tissue and also in tumors. We will cover the role of hypoxia in collagen biogenesis which influences compositional changes to the tumor microenvironment and discuss how these changes lead to a stiffer tumor stroma. The challenges in determining the influence of chemical versus physical cues on cancer progression will also be considered.   

Partial wave spectroscopy based nanoscale structural disorder analysis for cancer diagnosis and treatment .

Mesoscopic physics based partial wave spectroscopy (PWS) was recently introduced to quantify nanoscale structural disorder in weakly disordered optical media such as biological cells. The degree of structural disorder $(L_{d} )$, defined as $L_{d} =\langle dn^{2}\rangle \times l_{c} $ is quantified in terms of strength of refractive index fluctuation $(\langle dn^{2}\rangle )$in the system and its correlation length$(l_{c} )$.With nanoscale sensitivity,$L_{d} $has been shown to have potential to be used in cancer diagnostics. In this work, we analyze the hierarchy of different stages of prostate cancer cells by quantifying their intracellular refractive index fluctuations in terms of $L_{d}$ parameter. We observe that the increase in tumorigenicity levels inside these prostate cancer cells results in proportionally higher$L_{d} $values. For a weakly disordered optical media like biological cells, this result suggests that the progression of carcinogenesis or the increase in the tumorigenicity level is associated with increased $\langle dn^{2}\rangle $and/or \quad $l_{c} $values for the samples. Furthermore, we also examined the applicability of $L_{d} $ parameter in analyzing the effect of drug on these prostate cancer cells. In accordance with the hypothesis that the cancer cells which survives the drug, becomes more aggressive, we found increased $L_{d} $values for all the drug resistant prostate cells studied.   

Mucin Production Dynamics at the Surface of Corneal Epithelial Cells

Mucous layers form at the apical surface of many epithelia, protecting tissues from pathogens and environmental wear and damage. Although these layers contain many materials they are primarily composed of mucin glycoproteins, the concentration of which may be physiologically controlled to maintain specific rheological properties and to provide proper lubrication. Nowhere is this truer than at the surface of the eye's corneal epithelium, where the mucous layer must additionally achieve structural integrity to withstand the stresses created by blinking, and remain transparent in order to enable vision. I will present results on the growth dynamics, concentration, and rheology of a model corneal epithelial mucous layer, all of which can be viewed as important parameters at this interface. I will also discuss modulation of the mucous layer's dynamics with variation in environmental conditions.   

Serine, Glycine and One-carbon Metabolism in Colorectal Cancer Cell in Heterogeneous Microenvironment

The up-regulation of serine metabolism associated with one-carbon metabolism has been identified to support cellular biosynthesis and redox maintenance of tumors. The consistently over-expressed one-carbon genes have been targeted for potential drug development. To investigate the biological function of specific enzymes, we had genetic engineered HCT116 cell lines, methylenetetrahydrofolate dehydrogenase (MTHFD) and phosphoglycerate dehydrogenase (PHGDH) deleted cell lines, growing in the artificial microhabitats array with serine and glycine gradient across. The impact of depletion of serine and the blocking of biosynthesis pathway will be shown in terms of cell morphology, proliferation rate, and cell motility. The evolution dynamic and migration rate can also be tracked throughout the experiments.   

A mathematical model for the effects of radiation to the induced cancer in mice

We have been studying biological effects of radiation in terms of mathematical models. There are two main objects that we need to study: mutation and cancer. We proposed the Whack-A-Mole (WAM) model which takes account of the repair effects to study radiation induced mutations. We applied it to the mutation of several species including Drosophila and mice, and succeeded to reproduce the dose and dose-rate dependence of the mutation rates. Here, as a next step, we study the effects of low dose-rate radiation to an induced cancer in mice. In the experiment, they divided their mice in four groups and kept them under constant gamma-ray radiations with different dose rate for each group since the birth. On the 35$^{\mathrm{th}}$ day, chemical carcinogen was given to each mouse and they observed the occurrence and the growth of cancer for one year. Our mathematical model consists of two stages. The first stage describes a multiple-step carcinogenesis and the second stage describes its growth. We assume that the carcinogenesis starts with the chemical carcinogen and that the rate of the following processes depends on the dose rate as it does in the WAM model. We found some irregularities in the data, however, the overall fit is satisfactory.   

Possible Explanation for Cancer in Rats due to Cell Phone Radio Frequency Radiation

Very recently, the National Toxicology Program reported a correlation between exposure to whole body 900 MHz radio frequency radiation and cancer in the brains and hearts of Sprague Dawley male rats. Assuming that the National Toxicology Program is statistically significant, I propose the following explanation for these results. The neurons around the brain and heart form closed electrical circuits and, following Faraday's Law, 900 MHz radio frequency radiation induces 900 MHz electrical currents in these neural circuits. In turn, these 900 MHz currents in the neural circuits generate sufficient localized heat in the neural cells to shift the equilibrium concentration of carcinogenic radicals to higher levels and thus, to higher incidences of cancer.