Penn Engineers: Cells Require Gene Expression Feedback to Keep Moving
By Lauren Salig
When cells move throughout the body, they do so by dragging themselves, using molecular “arms” to pull themselves closer to where they need to be while unlatching themselves from the area they’re moving away from. In a recent study, Penn Engineers looked at a few mechanobiological factors that help regulate cells’ migration towards their destination, providing new insight into the gene expression feedback loops that keep them from getting stuck.
The research was led by Joel Boerckel, Assistant Professor of Orthopaedic Surgery in the Perelman School of Medicine and in Bioengineering in Penn Engineering, and bioengineering graduate student Devon Mason. Co-authors include bioengineering graduate student Joseph Collins and researchers from the University of Notre Dame, Indiana University and Purdue University.
The study was published in the Journal of Cell Biology.
In their study, Boerckel and Mason were seeking clarity on the role of transcription — the process through which DNA is copied into mRNA that is later used to create proteins — in sustained cell motility.
“A lot of historical evidence suggests that cells don’t need transcription in order to migrate, but there’s also some evidence to say they do,” says Mason. “Some people have suggested that once the migratory machinery proteins are there, cells don’t require new gene expression, that cells can do any task for which they’ve already produced proteins. What we found is you need continuous production of regulatory proteins for the cytoskeleton of the cell to remain dynamic and to maintain movement.”
The researchers studied the movements of endothelial cells, which line blood vessels, over the course of 8 hours. They used a method called a “monolayer wound assay” that forces cells to migrate in a particular direction. In this study, cells were moving towards an artificially manufactured “wound,” but in the body, cells might move for a number of reasons, including to construct tissue, to repair injuries or to fight bacteria.
In the first part of their study, Boerckel and Mason inhibited transcription across the board: cells could not copy any new mRNA to create more proteins. They found that the cells were able to begin moving, but they eventually slowed down and then got stuck before reaching the artificial wound they were headed towards.
“It’s not that the cells don’t know which direction to go, but rather that they anchor in place and can’t let go,” says Boerckel.
This anchoring effect occurs when transcription is turned off due to a lack of regulation of two important factors in cell motility: cytoskeleton remodeling and focal adhesion maturation. The cytoskeleton of a cell allows it to have structure and to move about its environment, while focal adhesions are the cell’s connections to the extracellular matrix, together forming the “arms” that pull it along or keep it in place. Without proper regulation of these factors, a cell will eventually become unable to keep moving.
To get a better idea of transcription’s role in regulating these factors, Boerckel and Mason looked at two proteins, YAP and TAZ. These proteins are activated by a cell’s cytoskeleton and help regulate cell movement by impacting gene expression.
“YAP and TAZ are transcription coactivators that can’t bind to DNA by themselves, so they have to bind to other transcription factors. We started using YAP and TAZ because we wanted to very narrowly focus on one set of transcription regulators instead of looking at transcription as a whole. Global transcription inhibition is like taking a hammer to the cell; it’s a very coarse approach,” says Mason.
When the researchers depleted cells of YAP and TAZ in the second part of their study, cells experienced the same anchoring issue, confirming those two proteins have some role in maintaining cell motility. Interestingly, the researchers’ initial hypothesis about why cell motility halted without YAP and TAZ’s regulation turned out to be wrong.
“For cells to move, they need to form new focal adhesions at the front and release them at the back. When we first found that YAP/TAZ depletion stopped cells from being able to migrate, we thought maybe it was because trailing adhesions don’t disassemble,” says Boerckel. “But it’s not a defect in regulated disassembly of those adhesions. We found that, without YAP/TAZ, there’s no check on cytoskeletal maturity, resulting in more and more focal adhesions until the cell gets stuck.”
This study found that YAP and TAZ play a key role in adjusting gene expression for cytoskeletal maturity and focal adhesions to create the right conditions for sustained cell movement. Because YAP and TAZ are activated by the cytoskeleton and simultaneously regulate the cytoskeleton, the researchers propose that YAP and TAZ are part of an important feedback process that changes how genes are expressed throughout cell movement.
This study lays a framework for understanding the importance of transcription in maintaining cell motility, but the researchers believe this concept could be expanded to help scientists address a number of research questions.
“Other directions we could take this include looking at disease, like angiogenesis associated with cancer or even cancer cell metastasis itself,” says Boerckel. “We’re also interested in understanding development. For an embryo to develop, you have to have mobilization of progenitor cells to every part of the body. How does that happen?”
This project was supported in part by the American Heart Association through grant 16SDG31230034, the National Institutes of Health’s National Center for Advancing Translational Sciences through grant UL1TR001108, the National Science Foundation through grant 1435467, CAREER Award 1651385 and the NSF Science and Technology Center for Engineering MechanoBiology, grant CMMI-1548571