The role of cellular forces in regulating epithelial morphogenesis and drug delivery
Open Access
- Author:
- Wei, Qiong
- Graduate Program:
- Engineering Science and Mechanics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 16, 2017
- Committee Members:
- Sulin Zhang, Dissertation Advisor/Co-Advisor
Sulin Zhang, Committee Chair/Co-Chair
Bernhard R. Tittmann, Committee Member
Patrick James Drew, Committee Member
Peter J Butler, Outside Member
Yong Wang, Committee Member - Keywords:
- cellular force
drug delivery
wound healing
epithelial morphogenesis - Abstract:
- Epithelial cells, essential building blocks of surfaces of biological organisms, can assemble into cohesive monolayers with rich morphologies on extracellular matrix (ECM). The self-assembly is led by the interplay between a cascade of complex extracellular and intercellular activities that activate by cell mechanotransduction. In the process of cell mechanotransduction, actomyosin motors generate contractile forces in cytoskeletons. For an isolated adherent cell, the contractile force is transmitted to ECM through integrin-mediated focal adhesions, generating the extracellular traction force. For cells in multicellular monolayers, the contractile force is transmitted through both cell-ECM adhesion points and cell-cell adherens junctions, generating the extracellular traction on ECM on the one hand and the long-range intercellular tension within the multicellular sheet on the other. These cellular forces, including intracellular contraction, extracellular traction and intercellular tension, are crucial to the integrity and tensional homeostasis of epithelia. When dysregulated, pathological processes such as tumorigenesis and cancer metastasis can be triggered. In this dissertation, we start with the development of force microscopy techniques that have been used in all later studies for cellular force measurement. With the implementation of these experimental measures, we find that the extracellular traction force and the intercellular tension are dependent on the matrix stiffness as well as the size of an epithelial monolayer. These findings infer the crosstalk between intercellular and extracellular activities in the course of cells sensing mechanical cues in their surroundings, suggesting that the cellular microenvironment may pose controlling factors such as matrix stiffness and geometrical constraints for the tensional homeostasis of epithelial monolayers by steering cellular forces. The force-driven tensional homeostasis is important to the barrier function of epithelial cells. Wound healing, a process of elimination of holes in the epithelia, is a scenario in which forces drive epithelia to reestablish functional barriers and restore tissue integrity. The collective cellular motion, caused by either cell crawling or the purse-string-like pulling of a contractile multicellular actin cable, is widely taken as the major mechanism that accounts for the closing of gaps with different sizes, shapes and distributions of ECM proteins. Here we unveil a novel mechanism by which epithelial cells invade and heal small non-adhesive regions, distinctly different from the previously concluded mechanism. Our findings suggest that the stiffness of the actin ring determines whether a newly proliferating cell can generate intracellular contraction to spread at the wound edge, thereby invading the wound area by consecutive actin ring forming, stiffness sensing and attachment of new cells to the wound front. The failure in generating contractile forces results in the piling up of new cells at the wound edge. This force-driven epithelial morphogenesis provides new mechanistic insights on wound healing. The tensional homeostasis of epithelia, however, also poses challenges in the context of drug delivery. In past three decades, targeted delivery of nanoparticle (NP)-based diagnostic and therapeutic agents to malignant tissues has exclusively relied on conjugation of NPs with ligands that specifically bind to over-expressed receptors on diseased cells, yet theoretical studies have predicted the role of cell surface mechanics on the delivery of nanoparticles. In the context of mechanotransduction, it is known that cellular forces, correlated with cell surface mechanics, can be steered by biophysical cues in cellular microenvironment. By tailoring cellular forces through either modulating the matrix stiffness or cell malignancy levels, we hereby demonstrate that cellular uptake can also be biased by altered stress states at both single cell level and multicellular level, enabling mechanotargeting. Our in vitro delivery study exemplifies how mechanics can be harnessed in the future of drug delivery to improve targeting efficiency.