Cells Have Feelings Too: How Cells Generate and Respond to Mechanical Cues in Tissues

Abstract

Cells are mechanical units, and their ability to orchestrate forces across a tissue allows them to shape organisms. In adults, cellular forces drive a flux of cell generation and extrusion to replenish tissues. In diseases, such as cancer, the mechanical properties of the tumor drive disease progression. These facets make understanding how cells generate and respond to mechanical signals an important area of study. Epithelial tissues act as barriers that protect organisms from dehydration, infection, and chemical damage. An unanswered question in epithelial biology is how or if epithelial tissues maintain their barrier as distortions in cell shape occur during cell division. To determine if cells maintain the epithelial barrier during cell division, we used a dye penetration assay in Xenopus laevis embryos and found that the epithelial barrier remains intact during cytokinesis. Using confocal microscopy, we determined that the contractile ring that drives cell division is coupled to the structures that adhere cells together, cell-cell junctions. Investigating the molecular dynamics of junctions, I found that adherens junction, but not tight junction, proteins are stabilized at the cleavage furrow. Using inhibitors, I demonstrated that forces from the contractile ring are coupled to adherens junctions, but not tight junctions. Finally, we found forces from the contractile ring recruit Vinculin to adherens junctions to reinforce the junctions during cell division. These results position adherens junctions as the load bearing junctions during cell division, which may be important for maintaining the barrier function in proliferative tissues. Understanding which proteins cells use to produce force and change their mechanical properties is critical for our understanding of development, tissue homeostasis, and disease progression. Previous work from our lab showed that Anillin, which is known to regulate cytokinesis, is also an important regulator of cell-cell junctions. Based on this work, we proposed that Anillin promotes tensile forces on junctions. Here, I tested this hypothesis by using two complementary methods to assess junctional tension in Xenopus laevis embryos. I found that increased Anillin expression correlated with increased Vinculin recruitment to junctions, indicating increased junctional tension; however, increased Anillin expression inversely correlated with junction recoil after laser ablation, consistent with reduced junctional tension. These seemingly contradictory results led us to hypothesize that Anillin transitions the load-bearing actomyosin structures from the junction-associated actomyosin to medial-apical actomyosin. Using immunostaining and live imaging, I showed that Anillin organizes the contractile medial-apical actomyosin network across the apical surface of the cells. We hypothesized that Anillin functions to integrate force transmission throughout a tissue and embryo as a whole by organizing medial-apical actomyosin. Indeed, I found that when Anillin was depleted, tissues lost force transmission capabilities, and this loss of tissue-scale force coordination disrupted embryo-level coordination. Building on this, we measured the stiffness of explanted Xenopus tissue and found that tissue stiffness was reduced when Anillin was depleted. These results are of interest because changes in tissue stiffness are required for developmental morphogenesis and significantly impact cancer prognosis. Our results highlight a new role for Anillin in regulating epithelial mechanics at both the cellular and tissue levels. Together, our findings demonstrate that epithelia are not static structures, but heterogeneous mechanical environments that are continually changing. These findings have laid the groundwork for studies on how junctions mechanically respond to cell division and how Anillin-orchestrated force production impacts development, tissue homeostasis, and disease.