Cell migration: it’s no walk in the park

From the Cooper Lab, Basic Sciences Division

The authoritative vision of the cell presented in an introductory textbook or a high school lecture is that of a smooth and spherical structure – a simple ball, essentially. And sure, there may be a few cells in our bodies that look like that, but that morphology is far from representative and, most scientists would likely say, is honestly pretty boring. In reality, our bodies are host to a wide range of cellular shapes. And I don’t just mean rectangles and triangles and stars, though those are all there. I mean cells that bulge and ruffle and fan. Fuzzy cells and long skinny cells and cells shaped like intricately branched trees. And those whose shapes are constantly in flux. Within this group, perhaps the most interesting and beautiful is the migrating cell. Ever morphing as it crawls over rugged landscapes, turning and darting and squeezing through holes, the migrating cell appears as an organism unto itself. At the fore is the lamellipodium, a flattened fan of ruffling membrane, expanding ever outward to extend the cell into new territory. From its leading edge, thin filopodial extensions dart out and back like the tongue of a snake, sensing the environment to choose the right path. At the rear, the lagging edge, a lazy-looking tail that appears as though it’s being unceremoniously dragged along, occasionally rushing to catch back up lest it be left behind. And beneath the cell, integrin-based focal adhesions – myriad tiny feet that anchor the cell to its substrate, constantly being set down at the front and picked up at the rear as the cell propels itself forward. It’s a stunning vision of life in action. And a far cry from the simple sphere so universally used to demonstrate the character of the cell.

migrating cell
The actin cytoskeleton of a migration cell showing the lamellipodium (bottom right) and trailing edge (top left). Image courtesy of Burnette et al. (2014) J. Cell Biol. 205:83-96 via a creative commons license.

The various cellular behaviors involved in cell migration must be carefully coordinated to ensure that the process runs smoothly. Dr. Jon Cooper, a professor in the Basic Sciences Division at Fred Hutch and a member of the UW/Fred Hutch Cancer Consortium, is intent on understanding how these behaviors are controlled in normal cells and in cancer cells as they spread through the body. In a new study published in eLife and led by former graduate student Dr. Elizabeth “Liesje” Steenkiste, Dr. Cooper’s lab described the details of signaling circuit that ensures proper control of migratory functions within the cell.

Different components of the migrating cell mutually support each other’s functions, explain the authors. “Integrin engagement stimulates protrusion of a dynamic leading edge lamellipodium. Inside the lamellipodium, rearward flowing actin [clusters] integrins…to form transient structures called nascent adhesions.” In the current work, the group sought to understand the molecular events that control such interactions. “One important adhesion-related signaling protein is Cas…which plays critical roles in lamellipodium protrusion, membrane ruffling, adhesion assembly, [and] adhesion turnover.” Phosphorylation of Cas is crucial to its activity. However, “while the consequence of Cas phosphorylation are well-understood, it is less clear how Cas phosphorylation is regulated.”

The group first sought to clarify the nature of Cas regulation by identifying other proteins that are co-regulated with Cas and with which it might function. They inhibited the complex involved in Cas destruction and performed a proteomic screen for other proteins whose destruction was impaired. One protein identified in this screen was BCAR3, a promising target as it had previously been reported to bind Cas. Digging deeper, they found that the similar mode of regulation of these proteins was no mere coincidence – introducing a mutation into BCAR3 that blocked its binding to Cas also prevented it from being degraded, suggesting that the interaction of these proteins is important for their co-regulation.

To draw further functional connections between BCAR3 and Cas, the group examined whether BCAR3 also regulates cell migration. By knocking it down and examining the behaviors of migrating cells, they found that BCAR3, like Cas, is indeed needed for proper lamellipodial dynamics and cell migration. Once again, blocking BCAR3-Cas binding revealed the close functional relationship between these proteins, as it impaired BCAR3’s ability to regulate cell migration. “These findings raise the question of how BCAR3…and Cas binding cooperate to regulate [cell migration],” the authors noted. They thought perhaps that BCAR3 might recruit Cas to the correct subcellular locations to do its job but, by examining the localization of these proteins, found the opposite to be true – Cas is in fact needed to recruit BCAR3. To round out the puzzle, the authors explain, “we considered that BCAR3 may activate Cas.” Using an antibody specific to Cas’s activated phosphorylation state, they found that in the absence of BCAR3, while Cas still localized to the correct sites, it was no longer activated. As Dr. Cooper describes, the mechanism revealed by this work was a bit perplexing. “Liesje's work revealed a surprisingly arcane mechanism, in which Cas brings BCAR3 to the adhesion sites, BCAR3 is then phosphorylated, and phosphorylated BCAR3 triggers Cas phosphorylation. Each protein is then degraded. It's hard to understand how such a baroque mechanism evolved.”

This work also revealed another intriguing aspect of the logic by which BCAR3/Cas activity is regulated in cells. In examining Y117, the major phosphorylation site on BCAR3, the authors found that it has two roles. “First, Y117 is the main phosphorylation site for BCAR3 down-regulation, second, it is also the main phosphorylation site for BCAR3 function.” This property provides a second form of feedback of this circuit, they explain. “The use of a single phosphorylation site in BCAR3 for activation and degradation ensures reliable negative feedback by the ubiquitin-proteasome system”. Cell migration, it turns out, is beautiful not just for the dynamic cellular movements it involves, but for the precision with which they are controlled.

This work was supported by the National Institutes of Health, the United States Public Health Service, and the Fred Hutchinson Cancer Research Center.

Fred Hutch/UW Cancer Consortium member Jon Cooper contributed to this work.

Steenkiste EM, Berndt JD, Pilling C, Simpkins C, Cooper JA. (2021) A Cas-Bcar3 co-regulatory circuit controls lamellipodia dynamics. eLife 10:e67078.