A New, More Complex Role for a Major Component of Cell Migration

By Eric Sauter

In a new study, published as the cover article in the November 6, 2007 issue of the journal Developmental Cell, scientists at The Scripps Research Institute have shed new light on a previously recognized contributor to the process of cell migration, and added an entirely new level of understanding of how this complex process works in living cells. 

Led by Gary Bokoch, a Scripps Research professor in the Department of Immunology, and Gaudenz Danuser, an associate professor in the Department of Cell Biology, the new study suggests that the actin regulatory protein cofilin regulates cell protrusion in ways beyond those previously recognized.

Protrusion of the cell leading edge is a basic component of the cell migration process, with growing actin filaments serving as the means to extend the front of the cell via a complicated interaction between the lamellipodium, a cytoskeletalactin network at the highly mobile edge of the cell, and the lamella, another zone of actin that extends from near the leading edge towards the cell interior. These two actin networks, which differ in their molecular composition and actin dynamic properties, work in tandem to move the cell forward.

"Three new discoveries in the study are significant," Bokoch said. "First, in terms of signaling pathways, our studies emphasize the Rac1/Pak1 pathway is a key regulator of the two actin networks. Second, we establish a novel function of cofilin—that it acts as an organizing force behind the interaction of these networks. Finally, while other published studies show the positive effect that local regulation of cofilin has within the cell, our study demonstrates that cell-wide, global regulation of cofilin through the Pak1 pathway can also have a negative impact on the process of leading edge protrusion."

Using quantitative fluorescent speckle microscopy, immunofluorescence, and electron microscopy for their in vivo study, the scientists showed that while globally enhanced cofilin activity increases the turnover of F-actin, this does not necessarily lead to high cell protrusion rates. In fact, it may destabilize the mechanical relationship between the two networks necessary for cell migration.

"Protrusion of the leading edge of migrating epithelial cells requires precise regulation of the two F-actin networks," Bokoch said. "Cofilin defines a central switch in the balance of these two processes depending on the location, extent and time scale of its activation. Such global versus local regulation of cofilin by Pak1-mediated signals plays a critical role in determining the effect of cofilin as a positive or negative promoter of cell protrusion."

The Bigger Picture

These new findings, which provide a far more detailed understanding of the processes involved in F-actin-mediated cell protrusion and motility than had previously been available, have broad implications in terms of the science of cell biology itself.

"We see more and more that these [regulatory] pathways are not as linear as cell biologists have generally believed," said Danuser.  "And the mechanics of force generation by cellular machines, which in the end is the sole determinant of what we observe as movements, has mostly been ignored by cell biologists interested in the regulation of cell migration. But cell biologists are not to be blamed for this. It is only now with the revolution of light microscopic imaging that we can begin to understand how chemical signals affect the structure and dynamics of force generating machines in time and space. Our findings are just one more example of how new technologies offer fresh insights into phenomena that turn out to be far more complicated than originally hypothesized."

The new study shows that increased levels of active cofilin markedly affect the organization of the leading edge actin networks—and that the key difference may lie between local and global regulation via the Rac1/Pak1 signaling pathway.  This expanded view of cofilin has evolved along with the growing recognition that cofilin has multiple biochemical activities in vitro, and the new study establishes that these activities have biological consequences for cell motility in vivo.  The study also carries with it the possibility that cofilin may turn out to be an important therapeutic target.

"In terms of disease," Bokoch said, "there is a lot of evidence that cofilin is important in both neuronal development and in cancer metastasis. However, because F-actin is important in regulating such basic processes as cell division, as well as playing structural and dynamic roles in cells, cofilin can affect many different aspects of normal physiology and pathological cell function."

Despite the importance of these new findings, the mechanisms that establish and maintain the actin networks' distinct characteristics, and the specific upstream signals involved in co-regulating the dynamics of the two networks remain largely unknown.

"One of our tasks in the near future will be to reconcile the two competing views of cofilin function by having the local model tested at the same level of detail as our global model," Danuser said. "Ultimately, I think the global regulation will prove to be more important—because if something goes wrong in the cell it's probably a global effect."

Bokoch pointed out another critical element of the new study. Key to its success was the combined efforts of three separate laboratories at Scripps Research, each contributing different and indispensable expertise.  "This study is a great example of scientific cooperation," he said. "We contributed our knowledge of the signaling network, Clare Waterman-Storer's lab developed the techniques to visualize the F-actin dynamics, while Gaudenz Danuser's group provided the means to quantitatively interpret the data. Without these unique contributions on the part of each group, we could not have done the study."

Other authors of the study, Cofilin Activity Downstream of Pak1 Regulates Cell Protrusion Efficiency by Organizing Lamellipodium and Lamella Actin Networks, include: Violaine Delorme, Matthias Machacek, and Céline DerMardirossian of The Scripps Research Institute; Karen L. Andersonand Dorit Hanein of The Burnham Institute for Medical Research; Torsten Wittmann of the University of California, San Francisco; and Clare Waterman-Storer, previously of Scripps Research and now of the National Heart, Lung, and Blood Institute. For more information, see
http://www.developmentalcell.com/content/article/abstract?uid=PIIS1534580707
003309.

The study was supported by the National Institutes of Health, the American Heart Association, and the Swiss National Science Foundation.

Send comments to: mikaono[at]scripps.edu

The new study suggests that the actin regulatory protein cofilin regulates cell protrusion in ways beyond those previously recognized. Click for image details.