Measurements and Models of Cell Nuclear Mechanics

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Cells are subject to numerous types of mechanical stress, from forces exerted on the skin to fluid shear within blood vessels. Because these forces can be transmitted to the cell nucleus, which houses the genome, mechanisms to adapt to and dissipate mechanical stress are necessary for cell survival, particularly within the nucleus itself. Importantly, defects in the mechanical properties of the nucleus can compromise cell survival during normal cellular processes, like cell migration, sometimes leading to disease. In addition, a cell's mechanical environment is defined by the tissue in which it resides, and is a key determinant of cell and tissue development. Therefore, it is an essential challenge to understand how a cell's external mechanical environment is communicated to the nucleus, where cell fate is largely determined. Moreover, the mechanical properties of the nucleus must be tuned to its tissue environment, a process that is poorly understood. This project will address these challenging questions through an interdisciplinary approach combining a genetic model organism, live-cell imaging, and biophysical tools. In addition, the supported graduate students will be trained to become the next generation of researchers and educators, who both excel at quantitative approaches and possess the biological sophistication to tackle cutting-edge biological problems. Supported graduate students will participate in Yale's Integrated Graduate Program in Physical and Engineering Biology (IGPPEB), for which the PIs serve on the executive committee, mentor students, and teach program courses. IGPPEB provides training in communication skills, outreach activities, and teaching. Summer research opportunities will also be provided to high school students. This project will test and further develop a physical model for cell nuclear mechanics by combining novel live-cell imaging and force-spectroscopy tools capable of probing the mechanical properties of nuclei at biologically-relevant temporal, spatial and force scales, taking advantage of the genetic model system fission yeast (Schizosaccharomyces pombe). First, it will elucidate how changing the heterochromatin-euchromatin balance can alter the mechanics of the nucleus. Second, it will implement biosensors that directly measure the tension on the chromatin-inner nuclear membrane protein interface in living yeast cells in wild-type cells as well as cells with perturbed nuclear mechanics and/or chromatin states. Third, it will exploit a novel optical tweezers assay capable of applying calibrated force to nuclei in living yeast cells, thereby enabling measurements of nuclear viscoelasticity and chromatin flow in vivo. Throughout these experiments, the new information gleaned will be fed back in to a developing mathematical model of nuclear mechanics, ultimately leading to a comprehensive picture of the mechanisms that define the mechanics of nuclei. Finally, this project will test the ability of the models developed to explain nuclear blebbing, which can drive loss of nuclear integrity in transformed mammalian cells.