UC Santa Barbara

Whether it is a single cell organism or a multicellular system, both need to undergo changes in mechanics in order to grow and shape themselves. In this thesis, we will look at the interplay between growth and mechanics, both at the unicellular and multicellular levels. For the single cell organism, we will look at how the mechanics of the cell wall are maintained during the process of tip growth in S. cerevisiae and explore the interplay of cell wall assembly and mechanics during the process of mating projection growth. For the multicellular system, we will characterize how the forces change during the process of growth and invasion in 4T1 breast cancer spheroids.Single celled organisms have various forms of growth, but a common type is called tip growth. In yeast, this mode of growth occurs during mating whereby two cells of different mating types grow towards each other. While the molecular pathways of this process have been documented, it is unclear how the mechanical integrity of the cell wall is maintained under the high internal turgor pressure of the organism during cell wall assembly and expansion. By combining theoretical and experimental approaches, we show that mechanical feedback is necessary to allow mating projection growth to occur in S. cerevisiae. We found that the mechanical feedback is provided by the Cell Wall Integrity pathway, which modulates cell wall assembly depending on mechanical changes in the cell wall and stabilizes mating projection growth. By experimentally perturbing key players of this pathway through genetic deletions of cell wall stress sensors, we were able to test the predictions provided by our theoretical description. Our results show that cell wall assembly and the mechanics of the cell wall must be tightly coordinated via a genetically-encoded mechanical feedback to ensure cell viability during morphogenesis. Multicellular cancer aggregates have been used as model systems to study different aspects of tumorigenesis. Here we will investigate the mechanical aspects of the process of invasion, which is one of the initial steps of metastatic growth. Multicellular aggregates, also called spheroids, have been used extensively as they are more representative of the physiology of a tumor and can be embedded in controlled microenvironments. While the forces that a tumor generates on the surrounding environment have been investigated, the forces within the tumor during invasion have never been explored, mainly due to a lack in technologies enabling direct mechanical measurements in 3D multicellular environments. By utilizing fluorescent cell-sized bioinert droplets as force transducers, we quantify how the mechanics within the tumor change as it begins to invade into a collagen type I matrix. Through the use of this technique and in-house developed software, we were able to show that while supracellular stresses remain low and constant during invasion, cell-scale stresses increase in invading spheroids while stresses in the non-invading spheroids stay relatively constant.