Lamellipodin functions upstream of the Scar/WAVE complex to regulate lamellipodia formation and cell migration.
The image shows the colocalization (yellow) of endogenous Lpd (green fluorescence) and Scar/WAVE1 (red fluorescence) in B16F1 mouse melanoma cells. Image courtesy of Dr Matthias Krause, King's College London, UK.
Not all lamellipodia are created equal: some contain a highly branched actin network, protrude slowly, and are persistent, whereas those with longer, less branched actin filaments are more dynamic. But behind every lamellipodium lies polymerized actin, actin-binding proteins and their regulators, and a protein called lamellipodin (Lpd), and Law et al. now provide new insight into how Lpd controls lamellipodium formation and cell migration.
The Scar/WAVE complex recruits the Arp2/3 complex to the leading edge to regulate actin branching, whereas filament elongation is mediated by Lpd engaging Ena/VASP. But the absence of Ena/VASP has a much less profound effect on lamellipodia than does depleting Lpd, which severely impairs lamellipodia formation, implying that Lpd might regulate additional actin cytoskeleton effectors. Law et al. postulated that the Scar/WAVE complex could be one such candidate, and set about testing their hypothesis.
After showing that Lpd colocalized with the Scar/WAVE complex, they demonstrated that Lpd bound directly not only to one of Scar/WAVE’s five subunits, Abi1, but also to active Rac. The association with Rac promoted the Lpd–Scar/WAVE interaction, implicating Lpd as a new Rac effector.
Because Scar/WAVE and Arp2/3 both regulate cell migration, Law et al. investigated Lpd’s role in this process. Cells depleted of Lpd moved slowly and randomly compared to wild-type cells, and showed reduced direction in scratch wound assays. Conversely, overexpressing Lpd conferred increased speed. But whereas this phenotype depended on the Lpd–Scar/WAVE interaction, blocking the interaction of Lpd with Ena/VASP did not reduce the speed.
Generating Lpd-knockout mice proved to be a challenge for Law et al., with the majority of mice dying at birth. However, those that survived showed a ‘white belly spots’ phenotype, suggestive of a neural crest/melanoblast migration defect. Turning to the Xenopus laevis system to directly study neural crest cell migration, the authors found that, consistent with their hypothesis, Lpd and the Scar/WAVE complex, through Abi1, cooperated to regulate neural crest migration in a cell-autonomous manner. Closer inspection of cells from neural crest explants confirmed that Lpd functions upstream of the Scar/WAVE complex to regulate lamellipodia size as well as migratory attributes.
Finally, the research team investigated whether Lpd regulated the Scar/WAVE complex in Drosophila melanogaster border-cell migration, during which a cluster of epithelial cells moves collectively towards the oocyte. The fly orthologue of Lpd, Pico, interacted both physically and genetically with Scar. Notably, not only did the depletion of pico or Scar abrogate border-cell migration, but so, too, did overexpressing pico, by inducing rear-facing protrusions that caused the border-cell cluster to tumble prematurely. Importantly, though, depleting Scar restored the normal distribution of protrusions in pico-overexpressing cells.
Law et al. therefore present a wealth of data indicating a role for Lpd in regulating the Scar/WAVE complex to generate, through the Arp2/3 complex, a highly branched actin network that supports stable lamellipodia protrusion and, consequently, both individual and collective cell migration. Lpd’s association with Ena/VASP, on the other hand, might facilitate a change in direction by favouring the formation of more dynamic lamellipodia.
ORIGINAL RESEARCH PAPER
Law, A.-H. et al. Lamellipodin and the Scar/WAVE complex cooperate to promote cell migration in vivo.
J. Cell Biol., published online 18 November 2013
doi 10.1083/jcb.201304051 | Article