How cells move – and how they regulate this

The ability of cells to move and change their positions is essential for various processes in our bodies. Immune cells move to sites of action, epithelial cells migrate during wound closure and neurons wire up their axons for signal transmission over long distances, while in metastasis, cancer cells employ their motile machineries to spread throughout our bodies. A team around Prof. Klemens Rottner heading the Research Group “Molecular Cell Biology” at the Helmholtz Centre for Infection Research (HZI) has now shed new light on the central molecular interactions that accompany the formation of cytoskeletal networks at the forefront of migrating cells. Obtained insights pave the way for defining how such processes are changing in diseases and how they can potentially be modulated in the future. The results have just been published in the scientific journal Nature Communications.

Among the most prominent structures driving cell migration are so called lamellipodia, flat and broad in dimension pushing the cell front forward. Lamellipodia are built of dense networks of polymers mostly comprising the protein actin, and they are dynamically renewed to translate pushing forces into forward movement. But how are these structures regulated at the molecular scale? “The role of the actin binding protein profilin in these processes has been controversial, but our results could largely clarify previous inconsistencies in the literature,” Klemens Rottner says.

Using genome editing by CRISPR/Cas9, the researchers first disrupted various players in the process, alone and in combination. Then they explored how this affected formation and function of lamellipodia as well as the relative distributions of remaining proteins. Doing this, they were able to establish the functional connections between the most relevant players in these structures in a step-wise manner, including the roles of four components as follows:

The actin monomer binding protein profilin (Pfn): Profilin binds the building blocks of actin polymers and prepares them for assembly in polymers, so called actin filaments.
Arp2/3 complex: This complex mediates the branching of actin filaments, therefore promoting the generation of actin networks, such as those found in lamellipodia.
Heterodimeric capping protein (CP): This two-subunit complex blocks rapidly growing actin filament ends, thereby shielding them from further polymerization.
Ena/VASP family polymerases: These proteins promote filament growth through actively accelerating further monomer addition, similar to formins, yet another class of filament polymerases.

“We have succeeded in improving our understanding of how all those lamellipodial components interact with and influence each other – thereby collectively regulating forward movement,” Dr. Yubo Tang says, first author on the study. These interactions operate as follows: Profilin counteracts Ena/VASP and at the same time promotes Arp2/3 complex activity. Ena/VASP and capping protein antagonize each other. Combinations of gene disruptions have been particularly informative: Profilin was still crucial for Arp2/3 complex localization even in the absence of Ena/VASP, but this was not seen for CP. In the absence of Ena/VASP, the impact of profilin on CP vanished. These data showed that profilin influences Arp2/3 complex and CP through distinct and separable molecular pathways.

Taken together, the data establish profilin as master regulator of Arp2/3 complex-dependent actin networks. The results thus resolve several controversial issues: Previous research have either suggested profilin to funnel actin monomers into formin or Ena/VASP-dependent actin structures, and thus away from Arp2/3 complex, while others rather saw the opposite. “We are now clarifying this controversy by clearly showing profilin to operate upstream of Arp2/3 complex in lamellipodia and also how,” Klemens Rottner says. Mathematical modeling done in cooperation by Prof. Martin Falcke from Max Delbrück Center Berlin not only recapitulated all experimental results, but also implied that Ena/VASP and CP have to compete for a common recruitment mechanism in lamellipodia.

The work thus provides decisive insights into the molecular logics of building branched actin networks driving the forces for membrane protrusion. “We understand much better now how these molecular players operate with one another at lamellipodial edges,” Klemens Rottner says. “This mechanistic understanding forms the basis for examining what goes wrong in aberrant migration, ranging from cancer metastasis to cells suffering from infections – and how those processes can potentially be targeted.”

One important open question constitutes the molecular nature of the recruitment structure Ena/VASP and CP are competing for. This will constitute one of many important lines of future investigation in Rottner’s team.

Text: Dr. Christian Heinrich