Date of Award
Doctor of Philosophy (PhD)
Cell & Molecular Biology
Rho signaling is a conserved mechanism for generating forces through activation of contractile actomyosin. How this pathway is tuned to produce different morphologies of cells and tissues is poorly understood. In the Drosophila embryonic epithelium, I investigated how Rho signaling controls force asymmetries to drive morphogenesis. Specifically, I studied a distinctive morphogenetic process termed “alignment”. This process of coordinated cell shape changes results in a unique cell geometry of rectilinear cells connected by aligned cell-cell contacts. I found that this rearrangement is initialized by contractility of actomyosin cables that elevate the local tension along aligning interfaces. Curiously, I find that hours after establishing the alignment, this cell geometry is stabilized independent of actomyosin at the end of embryogenesis. This suggests that there are alternate mechanical bases for maintaining the aligned cell geometry in the steady state.
My data show that polarization of two branches of Rho signaling, Rho Kinase (ROK) and Diaphanous (Dia), is responsible for the formation of these cables. Constitutive activation of these Rho effectors causes aligning cells to instead invaginate. This observation suggests that moderation of Rho signaling is essential to producing the aligned geometry. Therefore, I tested for feedback interactions in the pathway that could fine-tune Rho signaling. I discovered that F-actin exerts negative feedback on multiple nodes in the pathway. In contrast, Myo-II does not feedback to the Rho pathway. However, inhibiting ROK caused an upregulation in Rho activity. This shows that ROK has a Myo-II independent function in regulating the Rho pathway. Taken together, this work suggests that multiple feedback mechanisms factor into the regulation of Rho signaling, which may account for the versatility of Rho in diverse morphogenetic processes.
Preliminarily, I also find a requirement for a regulator of Rac-Arp 2/3-mediated actin polymerization, pointing towards cooperation and crosstalk between branched actin and linear actin promoting pathways. This may allow for a balance of different mechanical forces that can generate the aligned geometry.
This thesis work lays down a foundation for understanding how the activity of contractile actomyosin and small GTPase signaling be modified to suit numerous morphogenetic processes.
Ong, Katy Lauren, "Multiple Feedback Mechanisms Fine-Tune Rho Signaling To Regulate Morphogenetic Outcomes" (2019). Publicly Accessible Penn Dissertations. 3223.
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